WO2018146174A1

WO2018146174A1 - Monitoring a ups battery

Info

Publication number: WO2018146174A1
Application number: PCT/EP2018/053128
Authority: WO
Inventors: Yannick Maret; Daniel Aggeler; Esa-Kai PAATERO; Franz Zurfluh; Ivan Furlan; Nicola NOTARI
Original assignee: Abb Schweiz Ag
Priority date: 2017-02-09
Filing date: 2018-02-08
Publication date: 2018-08-16

Abstract

The present invention provides a method for monitoring a battery (22) connected to a UPS device (10), the UPS device (10) comprising a central DC link (12), a power supply side AC/DC converter (14), a power supply side DC/DC converter (16), and a load side output converter (18), whereby all converters (14, 16, 18) are connected to the DC link (12), and the DC/DC converter (16) is connected to the battery (22) at its power supply side, comprising the steps of generating at least one current pulse (42) through the battery (22) using the DC/DC converter (16), measuring a voltage and current across the battery (22) as response to the at least one current pulse (42), and determining at least one battery parameter based on the voltage and current measured across the battery (22) as response to the at least one current pulse (42). The present invention also provides a UPS device (10) comprising a central DC link (12), a power supply side AC/DC converter (14), a power supply side DC/DC converter (16), and a load side output converter (18), whereby all converters (14, 16, 18) are connected to the DC link (12), and the DC/DC converter (16) is connected to the battery (22) at its power supply side, wherein the UPS device (10) is adapted to perform the above method.

Description

MONITORING A UPS BATTERY

Technical Field

[0001] The present invention relates to a method for monitoring a battery connected to a UPS device, the UPS device comprising a central DC link, a power supply side AC/DC converter, a power supply side DC/DC converter, and a load side output converter, whereby all converters are connected to the DC link, and the DC/DC converter is connected to the battery at its power supply side.

[0002] The present invention also relates to a UPS device comprising a central DC link, a power supply side AC/DC converter, a power supply side DC/DC converter, and a load side output converter, whereby all converters are connected to the DC link, and the DC/DC converter is connected to the battery at its power supply side, wherein the UPS device is adapted to perform the above method.

[0003] The present invention also relates to a method for monitoring a battery connected to a UPS system, the UPS system comprising multiple UPS devices, which are connected in parallel, whereby each UPS device comprises a central DC link, a power supply side AC/DC converter, a power supply side DC/DC converter, and a load side output converter, whereby all converters are connected to the DC link, and the DC/DC converters of the multiple UPS devices are connected in parallel at their power supply side to the battery.

[0004] The present invention further relates to a UPS system comprising multiple UPS devices, each UPS device comprising a central DC link, a power supply side AC/DC converter, a power supply side DC/DC converter, and a load side output converter, whereby all converters are connected to the DC link, and the DC/DC converters of the multiple UPS devices are connected to the battery at their power supply side, wherein the UPS system is adapted to perform the above method.

Background Art

[0005] Power quality events in electrical installations are an important issue. Power quality events comprise any kind of disturbances of an AC source covering from e.g. sags or failures of a single phase of the AC source up to outages of entire multi-phase AC source. In order to deal with power quality events, uninterruptible power supply devices and uninterruptible power supply systems to provide uninterruptible power supply (UPS) to a load.

[0006] UPS systems typically have a modular setup. Accordingly, multiple UPS devices are provided in parallel. The UPS devices are typically provided as modules, which have a similar or even identical setup to make the UPS-systems scalable and easy to maintain. Hence, an UPS-system can easily be adapted to different loads by adding or removing one or multiple modules. In case of failure, a faulty module can easily be replaced with a working module.

[0007] In this area, on-line static systems are of major importance. A typical UPS

module of such an UPS system comprises an AC/DC converter, also referred to as rectifier, and an output converter, also referred to as inverter. The AC/DC converter and the output converter are inter-connected by a DC link. The DC link has a midpoint reference, a positive reference and a negative reference, which are connected by two capacitances provided in series. Furthermore, the AC/DC converter is connected at a power supply side of the UPS device to an AC source, and the output converter is connected at a load side of the UPS device to a load, which is typically an AC load. Still further, the UPS device comprises an additional DC/DC converter and a DC source, whereby the DC/DC converter connects the DC source to the DC link. The DC source is typically a battery, which is charged via the DC/DC converter from the split DC link. Each UPS device can be connected to an individual battery, or the UPS devices of the UPS system can share a single or multiple batteries. The battery or batteries can be integral part of the UPS device or UPS system, or they can be provided separately. In any case, operation of the UPS device or the UPS system does not change, since this is more a question of definition.

[0008] Each of the converters typically has a setup with multiple switching units. The switching units may comprise controlled and/or uncontrolled switching devices, which are typically provided as semiconductor switching devices enabling rapid power switching. The semiconductor switching devices can e.g. be provided as diodes or transistors, in particular power diodes or power transistors like IGBT or others. It is e.g. known to use semiconductor switching devices based on silicon or wide bang gap devices, which are based e.g. on silicon carbide, GaN or other technologies.

[0009] UPS devices with 'double conversion' operation are robust and provide load

isolated from the AC source and transients provided via the AC source.

Drawback of the 'double conversion' operation are comparatively high losses leading to reduced overall efficiency, since two converters operate continuously and in series for load support, even when no power quality event occurs. Hence, in normal operation, the load is powered by the AC source via the AC/DC converter and the output converter, and in case of failure, the load is powered by the battery via the DC/DC converter and the output converter. The load can be an AC load or a DC load, and the output converter can be a DC/AC converter or a DC/DC converter, respectively.

[0010] Nowadays, UPS efficiency is of increasing importance due to evolving

environmental concerns and costs. Hence, an improvement in UPS efficiency is achieved by operation of the UPS devices and UPS systems in so called 'eco'- mode. In 'eco'-mode, the UPS device or the UPS system comprises a switchable bypass power train, so that the AC source can be directly connected to the load. Very fast control allows reliable transfer to the output converter in the event of an abnormality on the bypass. This is applicable for AC loads. In the case of a DC load, at least one power converter is required in the bypass to convert the AC power into DC power.

[001 1] Hence, in 'eco'-mode, the UPS device or the UPS system, the converters are operated in an operation state, so that they can rapidly take over supply of the load from the battery. Typically, the DC link is supported by free wheel diodes of the converters.

[0012] In such UPS devices and systems, power quality events are an exceptional case, so that the UPS devices and systems area typically not drawing power from the battery. Hence, estimation of battery is rather difficult, since determining battery parameters requires operation of the battery. A battery in stand-by only provides very little valuable information in regard to battery parameters.

[0013] Conventional approaches to determine battery parameters typically generate an excitation current at the battery level by switching a known load in parallel to the battery. This has several drawbacks, since an additional load is required.

Furthermore, full converter operation is required in order to provide the current to the load, which adds for reasonable power losses. Furthermore, energy removed from the UPS device or the UPS system has to be diverted in the load. Such removed energy has to be inserted into the UPS device or the UPS system, i.e. the battery has to be charged after generation of the excitation current.

Furthermore, in case of a power quality event when determining battery parameters, it is difficult to turn the UPS device or the UPS system to load support. [0014] In this context, document US 2016/0291089 A1 refers to methods including determining first and second voltages of a battery before and after a load interval, determining a current through the battery during the load interval, and determining a status of the battery from the determined first and

second voltages and the determined current. The load interval includes an interval, during which a discharge pulse is applied to the battery, and the first and second voltages are voltages, when the battery is open circuited.

The first voltage is determined before the battery transitions from an open- circuited state to a pulse discharge state, and the second voltage is

determined after the battery transitions from the pulse discharge state to the open-circuited state.

[0015] Furthermore, US 2016/0190867 A1 refers to a low-cost hybrid backup

energy system that consists of a battery bank that includes one or more power converters, DC-link capacitors, a compressed air energy storage system, that is composed of an air compressor, one or more gas storage tanks, one or more efficient pneumatic motors, pressure regulators, one or more electrical generators, and one or more power converters, and one integrated master controller. This hybrid system is used to provide

uninterrupted, immediate, and sustained DC energy supply when outages occur to feed power converters AC such as a UPS or similar for long or intermediate durations of time to allow uninterrupted operations in data centers, hospitals, telecommunication stations, and other critical loads. The strategy used in this system reduces the amount of batteries required and at the same time minimizes deeper battery discharges prolonging battery life in comparison to standard backup systems.

[0016] EP 2 846 395 A2 refers to a battery pack including a battery coupled to a load and a charging device, and comprising at least one battery cell, and a battery management unit for controlling charging of the battery from the charging device and discharging of the battery to the load, wherein the battery management unit includes a measuring unit for generating cell voltage data and current data by measuring a cell voltage and a current of the at least one battery cell, a capacity estimating unit for generating current capacity data based on the cell voltage data and the current data, an internal resistance estimating unit for generating current internal resistance data based on the cell voltage data and the current data, and a state of health (SOH) estimating unit for estimating an SOH of the at least one battery cell based on the current capacity data and the current internal resistance data.

Disclosure of Invention

[0017] It is an object of the present invention to provide a method for monitoring a

battery connected to a UPS device as specified above, an UPS device adapted to perform the above method, a method for monitoring a battery connected to a UPS system, and a UPS system adapted to perform the above method, which overcome at least some of the above disadvantages. In particular, it is an object of the present invention to provide a method for monitoring a battery connected to a UPS device as specified above, an UPS device adapted to perform the above method, a method for monitoring a battery connected to a UPS system, and a UPS system adapted to perform the above method, which enable an easy and reliable determination of battery parameters of a battery used in a UPS device of a UPS system.

[0018] This object is achieved by the independent claims. Advantageous embodiments are given in the dependent claims.

[0019] In particular, the present invention provides a method for monitoring a battery connected to a UPS device, the UPS device comprising a central DC link, a power supply side AC/DC converter, a power supply side DC/DC converter, and a load side output converter, whereby all converters are connected to the DC link, and the DC/DC converter is connected to the battery at its power supply side, comprising the steps of generating at least one current pulse through the battery using the DC/DC converter, measuring a voltage and current across the battery as response to the at least one current pulse, and determining at least one battery parameter based on the voltage and current measured across the battery as response to the at least one current pulse.

[0020] The present invention also provides a UPS device comprising a central DC link, a power supply side AC/DC converter, a power supply side DC/DC converter, and a load side output converter, whereby all converters are connected to the DC link, and the DC/DC converter is connected to the battery at its power supply side, wherein the UPS device is adapted to perform the above method.

[0021] The basic idea of the invention is to provide voltage and current measurements of the battery to determine at least one battery parameter, and to use the UPS device and its components to perform the method, whereby UPS refers to uninterruptible power supply. In particular, the DC/DC converter of the UPS device generates short current pulses through the battery and measures the current and voltage, in particular during and after the at least one current pulse. During and after the at least one current pulse, the voltage across each cell and battery block and the current through the same may vary depending on different battery parameters. Accordingly, also the current and the voltage of the entire battery may vary. Hence, the measurement of these values can be used to determine different battery parameters, for example state-of-health (SOH) of the battery may be inferred from the variation of the impedance of the battery or from other parameters that can be extracted from the relationship between current and voltage. Since the measurements can be performed using the DC/DC converter, no hardware modifications are required. All features required for the generation of the at least one current pulse and the measurement of voltage and current across the battery are typically implemented in a state of the Art DC/DC converter, so that no additional hardware is required. This refers in particular to the generation of the at least one current pulse. However, also the measurements can be performed by the DC/DC converter covering the battery as a whole, thereby enabling a low-cost monitoring of the at least one parameter of the battery. No additional hardware is required.

[0022] The load can be an AC load or a DC load, and the output converter can be a DC/AC converter or a DC/DC converter, respectively.

[0023] The current pulses do not need to be square but can have any suitable shape, as long as their frequency content is rich, e.g. pure sinusoidal signals, square wave, PRBS, white noise, or others. However, certain shapes of pulses may be easier to generate than others, and some shapes may improve the signal to noise ratio.

[0024] The pulse current of the current pulse should be sufficiently large in order to

produce a well measurable voltage drop. E.g., a battery with an impedance of 10ΓΠΩ requires a current of a few Amperes in order to produce a voltage change in the range of tens of mV. On the other hand, a single pulse only negligibly charges or discharges the battery, e.g. a 5A/200ms impulse charges/discharges a 30Ah battery by less than 0.001 %. Additionally, the pulse current and the duration of the pulse shall be small enough to not increase the temperature of the battery by more than a fraction of Kelvin. Typically, a pulse current of 5-10 A can be used for monitoring the at least one parameter of a battery having an impedance in the order of tens of milliohms. For monitoring a battery with a single cell, whose impedance is rather lower, e.g. in the range of sub milliohms, an increased current is required for the at least one current pulse.

[0025] Preferably, the amplitude of the current pulse can be adjusted, e.g. according to an impedance of the battery. The advantage is that the voltage drop excited on the battery can be kept above a well-measurable amplitude, since the sensitivity of the battery voltage measurement unit is limited.

[0026] A pulse duration of the current pulse is preferably in the order of hundreds of milliseconds. On the one hand, it should be as short as possible in order not to modify a state of charge of the battery. On the other hand, the pulse should be long enough to capture the slowest transient effects of interest to determine the at least one battery parameter. If one is only interested e.g. in the high-frequency impedance with a frequency of tens of Hertz and more, the pulse duration can be rather short, i.e. less than a few hundred milliseconds.

[0027] The rising edge and falling edge of the current pulse need to be sharp enough in order to have a rich-enough frequency content of e.g. up to a few hundred hertz. In general, the bandwidth of a step signal is given by 0.35/RT, where RT is the rise time of the edge. Accordingly, a bandwidth of e.g. 300 Hz requires a rise time of less than 1 ms.

[0028] A current-voltage relationship with rich enough frequency content enables

estimation of different battery parameters, in particular it enables to determine more than just an impedance of the battery at one frequency. Hence, a complex battery model can be generated and estimation of different battery parameters is supported.

[0029] According to a modified embodiment of the invention the battery comprises

multiple individual strings, which are connected in parallel, and the step of measuring a voltage and current across the battery as response to the at least one current pulse comprises measuring a current for at least one of the multiple individual strings. Each string comprises at least one battery cell or battery block, whereby each battery block typically comprises multiple battery cells. The UPS device comprises measurement means to measure the current through the multiple strings. The overall current over all strings is the current through the DC/DC converter. Hence, when measuring the voltage and current across the one string, at least one battery parameter can be determined in more detail based on the individual string currents and the battery voltage, which applies to all parallel strings. Measuring the string current and the voltage permits e.g. determining a string impedance of an entire string. Tracking the string impedance allows determining if one or more individual cells or cell blocks within a string is faulty. It does not allow identifying the faulty battery cell within the string.

[0030] According to a modified embodiment of the invention, the battery comprises at least one individual string, and the at least one string comprises multiple battery blocks, which are connected in series, and the step of measuring a voltage and current across the battery as response to the at least one current pulse comprises measuring a voltage for at least one of the multiple battery blocks of the at least one string. Hence, detailed voltage measurements can be performed e.g. for individual battery blocks or battery cells. The UPS device comprises measurement means to measure the voltage at the multiple battery cells or battery blocks. By performing an individual measurement on battery block or battery cell level, a finer granularity and better sensitivity are provided. This allows, for example, a detection of faulty battery cells or battery blocks. It is however to be noted that the proposed pulse generation approach can be applied likewise for any of those cases.

[0031] According to a modified embodiment of the invention, the step of generating at least one current pulse through the battery comprises generating at least one charge pulse and/or at least one discharge pulse. Each type of current pulse can be used to determine the at least one battery parameter. Charge current as well as discharge current provide a specific characteristic behavior of the battery.

[0032] According to a modified embodiment of the invention, the step of generating at least one current pulse through the battery comprises generating a current pulse train. The current pulse train comprises a sequence of multiple pulses, which can be different or identical pulses. E.g., the pulse train may comprise alternating discharge and charge pulses, so that a sum of energy transfer between the battery and the DC/DC converter is almost zero. However, the pulses can be different, e.g. to obtain measurement results to identify different parameters of the battery. The current pulse train comprises typically a break between two subsequent pulses, where no current flows between the DC/DC converter and the battery. These breaks can have different length, depending on the effect to be achieved. For example, an effect called„coup de fouet" refers to a phenomenon associated with voltage drop at the beginning of discharge of a battery, in particular a lead acid battery. This effect can be used e.g. to determine a state of health (SOH) of a battery. However, e.g. when trying to determine an impedance of the battery, the„coup de fouet" may lead to erroneous measurements, resulting in an erroneous impedance value. To overcome this problem, several subsequent pulses can be generated in order to chemically excite the battery and to clear effects such as "coup de fouet". In this case, since the„coup de fouet" occurs at the beginning of a discharge of a battery, the current pulse train preferably comprises multiple discharge pulses with only a small gap in between, so that the "coup de fouet" can be cleared, and the measurements can be started after clearing the effect. Furthermore, the parameter estimation becomes more precise as more pulses are measured because more information becomes available to determine the at least one battery parameter. On the other hand, the train of multiple current pulses can lead to a significant discharge of the battery or to an increase of the temperature of the battery, which can result in a change of the at least one parameter of the battery. Hence, the break should be chosen sufficiently long in order to reduce the impact of such parameter changes. In particular, discharge of the battery shall be limited to a fraction of percent points, and temperature change shall be limited to a fraction of Kelvin. This can in general be achieved with the length of the break being much longer than the pulse duration, thereby ensuring effect-independences between the pulses in the frequency band of interest. Furthermore, in order to avoid falsification of the measurement results, the battery shall not be used by the UPS device between the pulses of the pulse train, i.e. no charging or discharging of the battery shall occur between the pulses. However, if the UPS device requires in particular discharging the battery, the current pulse train can be interrupted and a new pule train can be restarted afterwards.

[0033] It should be noted that exciting the battery with a periodic signal of period T_p improves a 'signal-to-noise' ratio for the estimation of the at least one battery parameter at the frequency 1/T_P. E.g. an impedance of the battery can be estimated very simple and robust by the RMS value of the fundamental frequency of the voltage divided by that of the current. However, a large number of periods, i.e. a number of tens, is needed to get good estimation results using this simple approach. However, this approach is preferably used for frequencies of not more than a few tens of Hz, which can be reliably generated using the DC/DC converter.

[0034] According to a modified embodiment of the invention, the step of generating a current pulse train comprises repeatedly generating a current pulse train. Hence, the signal to noise ration of the current and voltage measurements can be improved by averaging the measured signals. In order to improve the signal to noise ratio (SNR), the following approach can be used. In particular, the battery can be repetitively excited with the same time limited input signal, e.g. a train of current pulses or other. At the end of each input signal, the measured response, i.e. the current flowing through the battery, and an output signal, i.e. a voltage of the battery, is added to the response obtained at the end of the previous iterations. Applying N times this procedure, the deterministic parts of the input and output signals increases with a factor N while the random parts of the signals, i.e. noise, only increases with a factor VN, assuming that the noise is white noise. As result, the signal to noise ratio will increase with VN. The train of pulses can be generated every few hours, days or weeks, e.g. depending on the expected rate of change of the at least one battery parameter.

[0035] According to a modified embodiment of the invention, the step of generating at least one current pulse through the battery comprises generating a discharge pulse through the battery, providing the pulse current via the DC/DC converter to the DC link, and supplying energy from the discharge pulse via the output converter to a load. Hence, energy from the discharge pulse is used to power the load connected to the UPS device. This enables performing measurements of battery current and voltage during operation. Preferably, the current from the discharge pulse is provided via the DC/DC converter to the DC link, and a load current is provided via the output converter to the load. Hence, the load current comprises the pulse current and an additional current provided via the AC/DC converter, whereby the two currents together sum up for the load current.

[0036] According to a modified embodiment of the invention, the step of generating at least one current pulse through the battery comprises generating a discharge pulse through the battery, providing the pulse current via the DC/DC converter to the DC link, and storing energy from the discharge pulse in the DC link. The DC link typically comprises energy storage elements, e.g. storage capacitors, which can be used to temporarily store the energy provided from the current pulse. The stored energy can be used for operation of the UPS device. In case the UPS energy is not used, e.g. when the UPS device comprises a bypass and powers the load via the bypass or when the load is switched off, the energy stored in the DC link dissipates due to losses.

[0037] According to a modified embodiment of the invention, the method comprises the additional step of generating at least one charge pulse through the battery fed by energy stored in the DC link via the DC/DC converter. Hence, energy stored in the DC link can be used to generate a current pulse through the battery, and the current pulse can be generated even without drawing energy from the power supply side of the UPS device. Preferably, charge and discharge pulses are generated according to an alternating pattern, so that energy from at least one discharge pulse is first stored in the DC link, and then the energy stored in the DC link is used to feed at least one charge pulse. Such a current pulse train can be generated internally within the UPS device and enables energy efficient current pulse generation.

[0038] According to a modified embodiment of the invention, the UPS device comprises a bypass comprising a bypass switch, whereby the bypass switch is switchable to connect/disconnect the power supply side to/from the load side, and the method comprises the additional step of operating the UPS device by connecting the power supply side to the load side using the bypass switch, whereby the converters are off and the DC-link is maintained to AC peak voltage through the freewheel diodes of the converters, and the method comprises the additional step of operating the output converter to decrease the DC-link voltage prior to the step of generating at least one current pulse through the battery using the DC/DC converter. Hence, the UPS device is operated in an operation mode with low losses, where the converters are merely used to provide power to the load in case of a power quality event. This operation mode is sometimes referred to as eco-mode. However, also other names are used for this operation mode. In eco- mode, the converters are in general not actively controlled and the DC-link is maintained to the AC peak voltage through the freewheel diodes of the converters. Preferably, the output converter is operated to decrease the DC-link. Once the DC-link has reached its low value, the DC link provides sufficient margin e.g. for storing energy from a discharge pulse of the battery. The bypass enables a direct connection of the AC to the load. Very fast control allows reliable transfer to the output converter in the event of a power quality event. This is applicable directly for AC loads. In the case of a DC load, at least one power converter is required in the bypass to convert the AC power into DC power.

[0039] According to a modified embodiment of the invention, the method comprises the additional step of creating a discharge pulse using a voltage window for the DC- link between AC peak and allowed high DC. High DC is typically determined by voltage rating of the capacitors of the DC link. In practice, the voltage window can range e.g. from 320 VDC, which corresponds to nominal AC peak, to about 430VDC for a 450V rated capacitor. Hence, energy is limited by a capacitance of the DC link and its voltage rating. Such current pulses can have e.g. up to 10A based on a voltage swing 100V, a capacitance of 10mF, and a pulse width of 100ms, so that the output converter does not run.

[0040] According to a modified embodiment of the invention, the UPS device comprises an output switching device comprising a silicon controlled rectifier and a contactor, which are connected in parallel to a load side of the output converter, and the method comprises the additional step of lowering the voltage of the DC link to a voltage level below load AC peak using the silicon controlled rectifier. The contactor is an electrically controlled switch used for switching an electrical power circuit, similar to a relay except with higher current ratings. A silicon controlled rectifier (scr) is a very fast switch, which typically operates in a time scale of less than one millisecond. The scr may be based on a semiconductor device, e.g. IGBT or IGCT, which is capable of controlled operation ion this time scale. In a three-phase UPS device, DC link voltage cannot be reduced further by operation of the output converter, as all the free wheel diodes will conduct to maintain the DC link voltage. Hence, a typical output switching device, which merely comprises a contactor to reduce losses, cannot be operated to further reduce the DC link voltage, since its response is too slow, e.g. in the range of 20ms to even more than 50ms. Similar considerations apply to the power supply side. Hence, using the silicon controlled rectifier, the DC link can be further discharged prior to generating a discharge pulse, so that the discharge pulse can have higher energy, based on an increased voltage swing.

[0041] According to a modified embodiment of the invention, the method comprises the additional steps of creating a discharge pulse, and operating the AC/DC converter and/or the output converter to absorb excess energy or a part thereof. Hence, energy from the discharge pulse can be diverted when using the output converter to load or to bypass source, or to both. When using the AC/DC converter, the energy can be diverted to a power supply side, e.g. an AC power supply. Obviously, there is no energy limitation for the pulses.

[0042] According to a modified embodiment of the invention the method comprises the additional step of monitoring parameters influencing the at least one battery parameter. Some battery parameters, e.g. its impedance, usually depend on other parameters, e.g. a battery temperature or a battery charge status. Since the impedance is a slow changing parameter, it is not necessary to track it at fixed and frequent intervals. Therefore, it is of advantage to only estimate the battery parameter when e.g. an ambient temperature enters a given target temperature window. The system thus provides, by design, parameter estimates that are 'independent' of the other parameter, e.g. independent of a temperature and take advantage of day/night cycles. In the case of the charge status as other parameter, it has to be further considered a current flow through the battery, i.e. charging or discharging the battery, warms the battery up. Hence, in addition to the considerations regarding the battery charge status itself, also a recent change of the battery charge status, i.e. a charge or discharge of the battery, may inherently result in a change of temperature of the battery's internal temperature, which further influences measurement of the at least one battery parameter. Preferably, the method is performed under the same conditions, e.g. when the battery is fully charged and has been at rest for a time larger than thermal inertia of the battery.

[0043] The present invention also provides a method for monitoring a battery connected to a UPS system, the UPS system comprising multiple UPS devices, which are connected in parallel, whereby each UPS device comprises a central DC link a power supply side AC/DC converter, a power supply side DC/DC converter, and a load side output converter, whereby all converters are connected to their respective DC link, and the DC/DC converters of the multiple UPS devices are connected in parallel at their power supply side to the battery, comprising the steps of generating at least one synchronized current pulse through the battery commonly using the DC/DC converters of the multiple UPS devices, measuring a voltage and current across the battery as response to the at least one current pulse, and determining at least one battery parameter based on the voltage and current measured across the battery as response to the at least one current pulse.

[0044] The present invention further provides an UPS system comprising multiple UPS devices, each UPS device comprising a central DC link, a power supply side AC/DC converter, a power supply side DC/DC converter, and a load side output converter, whereby all converters are connected to the DC link, and the DC/DC converters of the multiple UPS devices are connected to the battery at their power supply side, wherein the UPS system is adapted to perform the above method. It is known in the Art to use multiple UPS devices, which are connected in parallel, between a power supply side and a load side. The parallel UPS devices enable scalability and serviceability of the UPS system. However, multiple UPS devices can be commonly connected to a single battery. Hence, in order to achieve pulse with required current, a single UPS device cannot be sufficient, and the current pulse has to be generated commonly by multiple UPS devices. It is then important to synchronize the DC/DC converters of the multiple UPS devices, so that the current pulses generated by each UPS device overlap each other with a certain accuracy, thereby forming a single pulse for the battery. If the overlap is not accurate enough, the resulting pulse, i.e. the sum of all individual pulses, can be smeared out, so that its frequency content becomes poor. This problem can be tackled in different ways.

[0045] According to a modified embodiment of the invention, the parallel UPS devices are connected to a control device, which sends a synchronization signal to all UPS devices. The control device can be a control device of the UPS system, or a control device of one of the UPS devices, which takes over the task of controlling at least this aspect of the UPS system. Accordingly, the UPS system requires a communication link, which interconnects the parallel UPS device. Hence, the step of generating at least one synchronized current pulse through the battery commonly using the DC/DC converters of the multiple UPS devices comprises sending a synchronization signal to the multiple UPS devices.

[0046] According to a modified embodiment of the invention, one of the UPS devices is provided as master device, which generates a first pulse in a sequence, i.e. a current pulse train, and the other UPS devices monitor the current and/or voltage of the battery in order to identify this first pulse and synchronize their pulse clock on this first pulse. Preferably, the onset of the pulse is used as time-zero, and the duration may be used as a unit of time. Hence, the step of generating at least one synchronized current pulse through the battery commonly using the DC/DC converters of the multiple UPS devices comprises providing one of the multiple UPS devices as master device, generating a first pulse in a current pulse train using the master device, monitoring the current and/or voltage of the battery in the other UPS devices in order to identify this first pulse, and synchronizing their pulse clock on this first pulse.

[0047] According to a modified embodiment of the invention, the parallel UPS devices commonly synchronize to the output grid or the AC power supply, e.g. based on the zero crossing. Hence, the step of generating at least one synchronized current pulse through the battery commonly using the DC/DC converters of the multiple UPS devices comprises synchronizing the multiple UPS devices to the power supply side or the load side.

Brief Description of Drawings [0048] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0049] In the drawings:

[0050] Fig. 1 shows a schematic view of a double conversion uninterruptible

power supply (UPS) device with bypass according to a first, preferred embodiment of the present invention together with current flows at different locations when generating current pulses,

[0051] Fig. 2 shows a schematic view of a double conversion uninterruptible

power supply (UPS) device with bypass according to a second embodiment of the present invention together with current flows at different locations when generating current pulses,

[0052] Fig. 3 shows a schematic view of a double conversion uninterruptible

power supply (UPS) device with bypass and a silicon controlled rectifier in parallel with a contactor provided at a load side of the UPS device according to a third embodiment of the present invention,

[0053] Fig. 4 shows a timing diagram with two subsequent current pulses of a current pulse train as generated with the UPS device of any of the

UPS devices shown in any of figures 1 to 3,

[0054] Fig. 5 shows a timing diagram with a current pulse train as generated with the UPS device of any of the UPS devices shown in any of figures 1 to 3, together with a voltage during and after the current pulse train, [0055] Fig. 6 shows a schematic diagram of a battery impedance as determined based on the a measurement of voltage and current with of any of the UPS devices shown in any of figures 1 to 3,

[0056] Fig. 7 shows a schematic view of a UPS system with multiple UPS devices of one of the previous embodiments connected in parallel according to a fourth embodiment of the present invention,

[0057] Fig. 8 shows a flow chart of a method for monitoring a battery connected to a UPS device of the first embodiment according to a fifth

embodiment of the present invention,

[0058] Fig. 9 shows a flow chart of a method for monitoring a battery connected to a UPS device of the second embodiment according to a sixth embodiment of the present invention, and

[0059] Fig. 10 shows a flow chart of a method for monitoring a battery connected to a UPS device with multiple UPS devices of the previous embodiments connected in parallel according to a seventh

embodiment of the present invention.

Detailed Description of the Invention

[0060] Fig. 1 shows an uninterruptible power supply (UPS) device 10 according to a first, preferred embodiment of the present invention. The UPS device 10 is a double conversion UPS device 10.

[0061] The UPS device 10 of the first embodiment comprises a central DC link 12, a power supply side AC/DC converter 14, a power supply side DC/DC converter 16, and a load side output converter 18, which is a DC/AC converter 18 in this embodiment. All converters 14, 16, 18 are connected to the DC link 12. The DC link 12 further comprises storage capacitors 13, one of which is shown by way of example in Fig. 1.

[0062] The AC/DC converter 14 is connected to an AC power supply 20, and the DC/DC converter 16 is connected to a battery 22. The DC/AC converter 18 is connected to a load 24. The DC/DC converter 16 comprises measurement means for measuring battery current and battery voltage for the entire battery 22.

[0063] The battery 22 has an impedance in the order of tens to hundreds of milliohms.

The battery 22 comprises multiple strings 26, which are provided in parallel in the battery 22. Each of the strings 26 comprises multiple battery blocks 28, which are connected in series in each string 26 of the battery 22. The impedance of each battery block 28 is in the order of milliohms or tens of milliohms. Each battery block 28 comprises multiple battery cells, which are not individually shown in the figures. The parallel strings 26 have the same setup with the same number of battery blocks 28, each of which has the same number of battery cells.

[0064] The UPS device 10 of the first embodiment further comprises a bypass 30 with a bypass switch 32, which is provided in this embodiment as silicon controlled rectifier, also referred to as scr. The bypass 30 provides a connection between the AC power supply 20 and the load 24, which is provided in parallel to the AC/DC converter 14, the DC link 12, and the DC/AC converter 18.

[0065] The UPS device 10 of the first embodiment also comprises a controller 34, which controls the operation of all controllable components of the UPS device 10, i.e. the AC/DC converter 14, the DC/DC converter 16, the DC/AC converter 18 bypass switch 32. Furthermore, the controller 34 receives measurement results from the current and voltage measurements as performed by the DC/DC converter 16. Further components of the UPS device 10, which are not relevant for understanding the present embodiment, are not shown in Fig. 1 . However, a person skilled in the Art knows how to implement such components as required.

[0066] Below will be discussed method for monitoring the battery 22, which is connected to the UPS device 10, with respect to a fifth embodiment, as shown in Fig. 8. The method can be applied similarly to the UPS devices 10 of other embodiments.

[0067] In step S100, the UPS device 10 is operated in double conversion mode, i.e. the load 24 e.g. is powered through the AC/DC converter 14 and the DC/AC converter 18.

[0068] In step S1 10, a current pulse train 40 of discharge current pulses 42 through the battery 22 is generated by the DC/DC converter 16. The current pulse train 40 comprises a sequence of multiple, almost rectangular current pulses 42, which are identical current pulses 42 in this embodiment. The current pulse train 40 can be seen e.g. in Fig. 5. Furthermore, a sequence of two current pulses 42 is depicted schematically in Fig. 4 to show the characteristics of the current pulse train 40.

[0069] The pulse current of the current pulse 42 is indicated as IP in Fig. 4. The pulse current is sufficiently large in order to produce a well measurable voltage drop, which can be seen by way of example in Fig. 5. Hence, the pulse current is approximately 5-1 OA, and the pulse duration, indicated as to in Fig. 4, is approximately 200ms. Between two current pulses 42 of the current pulse train 40, a break, indicated as to in Fig. 4, is provided, where no current flows between the DC/DC converter 16 and the battery 22. The length of the break is chosen to be longer than the duration of the current pulse 42.

[0070] The rising edge, indicated as t.R in Fig. 4, and falling edge of each current pulse 42 are sharp enough in order to have a rich-enough frequency content of e.g. up to a few hundred hertz. The bandwidth of a step signal is given by 0.35/RT, where RT is the rise time of the edge of the current pulse 42. Accordingly, the rise time of the current pulse 42 is less than 1 ms, resulting in a bandwidth of e.g. 300 Hz.

[0071 ] With the UPS device 10 of the first embodiment, the pulse current is provided via the DC/DC converter 16 to the DC link 12, and the energy from the current pulses 42 is provided via the DC/AC converter 18 to the load 24. Accordingly, as indicated in Fig. 1 , each string 26 provides a current, whereby the current of all strings 26 sums of to a battery current. The DC/AC converter 18 provides a current to the load 24, which comprises the battery current, i.e. the current pulse train 40, together with an additional current provided from the AC power supply 20 via the AC/DC converter 14.

[0072] In step S120, parameters influencing the at least one battery parameter are

monitored. Some battery parameters, e.g. its impedance, usually depend on other, influencing parameters, e.g. a battery temperature or a battery charge status. The influencing parameters can be determined in the environment surrounding the battery 22. More reliable influencing parameters can in general be determined directly at or in the battery 22, e.g. by temperature measurements directly at some cells of the battery 22. Further influencing parameters can be a day/night cycle and/or a charge status of the battery 22. Hence, to determine such influencing parameters, or to exclude impact of such influencing

parameters, the UPS device 10 can e.g. determine a time from the last current through the battery 22 in order to exclude some of the influencing parameters. In case of a detected influence of the influencing parameters on the subsequent measurement in step S130, the method can be interrupted or aborted. Step S120 can also be performed in parallel or after subsequent step S130.

[0073] In step S130, a voltage and current are measured across the battery 22 as

response to the current pulses 42 of the current pulse train 40. The voltage and current are measured for the battery 22 as a whole in the DC/DC converter 16.

[0074] In an alternative embodiment, the UPS device 10 comprises additional measuring means for additionally measuring the current for all parallel strings 26 individually. Furthermore, the UPS device 10 comprises measuring means for additionally measuring the voltage drop for all battery blocks 28. The measuring means can be provided as part of a battery management system, which can be part of the UPS device 10, which can be part of the battery 22, or which can be an independent component.

[0075] As indicated in the flow chart of Fig. 8, method returns to step S1 10 for

generation of a further current pulse train 40 of discharge current pulses 42. Based on the repeated current and voltage measurements, the measured signals are averaged. Accordingly, for each current pulse train 40, the current flowing through the battery 22 and the voltage of the battery 22, are added to the previously measured signals. Applying N times this procedure, the deterministic parts of the current and voltage increases with a factor N while the random parts of the signals, i.e. noise, only increases with a factor VN, assuming that the noise is white noise. As result, the signal to noise ratio will increase with VN. [0076] After N repetitions, in step S140, at least one battery parameter is determined by the controller 34 based on the averaged voltage and current measured across the battery 22 as response to the current pulse train 40. In this embodiment, an impedance of the battery 22 is determined. Fig. 6 indicates impedance values based on the measurements of voltage and current after the frequency rich current pulse trains 40 for a frequency of up to ~40Hz.

[0077] The impedance of the battery 22 correlates with a capacity of the battery 22.

Hence, the impedance serves as indicator for the capacity of the battery 22, which can be determined as one battery parameter.

[0078] There is also a relation between single cell capacity and single cell conductance.

Hence, battery health can be further determined based on these parameters, which can be determined based on voltage and current measurements of the battery 22.

[0079] According to the alternative embodiment, the UPS device 10 performs the

determination of the at least one parameter based on the individual current for the parallel strings 26 and the individual voltage drop for all battery blocks 28. Hence, the impedance can be calculated individually for each battery block 28, so that e.g. a state of health can be determined individually for each battery block 28.

[0080] Fig. 2 shows an uninterruptible power supply (UPS) device 10 according to a second embodiment of the present invention. The UPS devices 10 of the first and second embodiments have the same hardware setup, so that the UPS device 10 of the second embodiment will not be described again in detail. Reference is made to the respective description of the UPS device 10 of the first embodiment.

[0081] The UPS device 10 of the second embodiment differs from the UPS device 10 of the first embodiment in the operation for determining the at least one battery parameter.

[0082] Below will be discussed method for monitoring the battery 22, which is connected to the UPS device 10, with respect to a sixth embodiment, as shown in the flow chart of Fig. 9. The method can be applied similarly to the UPS devices 10 of other embodiments.

[0083] In step S200, the UPS device 10 is operated in eco mode, i.e. the load 24 e.g. is powered from the AC power supply 20 via the bypass 30. Accordingly, the bypass switch 32 is closed to provide a direct connection between the AC power supply 20 and the load 24. The converters 14, 16, 18 are in general not actively controlled and the DC-link 12 is maintained to the AC peak voltage through the freewheel diodes of the converters 14, 16, 18.

[0084] In step S205, the DC/AC converter 18 is operated to decrease the voltage of the DC-link 12 to provide sufficient margin for storing energy from discharge pulses of the battery 22 of subsequent step 210.

[0085] In step S210, a current pulse train 40 of discharge current pulses 42 through the battery 22 is generated by the DC/DC converter 18. The current pulse train 40 comprises a sequence of multiple, almost rectangular current pulses 42, as described above with respect to the UPS device 10 of the first embodiment. Hence, the details in respect to the current pulses 42 and the overall current pulse train 40 apply as discussed with respect to the first embodiment.

[0086] With the UPS device 10 of the second embodiment, the pulse current is provided from the battery 22 via the DC/DC converter 16 to the DC link 12, and stored in the storage capacitors 13 of the DC link 12, as indicated in the upper right diagram indicating the increasing DC link voltage. Excess energy stored in the DC link 12 dissipates over time due to losses.

[0087] Subsequent steps 220 to 240 correspond to steps S 120 to S140, as discussed above with respect to the method of the fifth embodiment. As indicated in the flow chart of Fig. 9, the only difference is that the method returns to step S205 for generation of a further current pulse train 40 of discharge current pulses 42.

[0088] Fig. 3 shows an uninterruptible power supply (UPS) device 10 according to a third embodiment of the present invention. The UPS devices 10 of the first and second embodiments have essentially the same hardware setup, so that the UPS device 10 of the third embodiment will not be described again in detail. Reference is made to the respective description of the UPS device 10 of the first embodiment.

[0089] The UPS device 10 of the second embodiment shows additionally to the UPS device 10 of the first embodiment the bypass switch 32 comprising a bypass contactor 44 and a bypass silicon controlled rectifier 46.

[0090] Furthermore, the UPS device 10 of the third embodiment is connected to two individual AC power supplies 20. One AC power supply is connected to the bypass 30, and the other AC power supply 20 is connected to the AC/DC converter 14. The UPS device 10 comprises a power supply side switch 50, which is provide between the AC power supply 20 and the AC/DC converter 14. Similarly, the UPS device 10 comprises an output switching device 52, which is provide between the DC/AC converter 18 and the load 24. [0091] The output switching device 52 comprises a load side contactor 54 and a load side silicon controlled rectifier 56. The load side contactor 54 and the load side silicon controlled rectifier 56 are arranged in parallel. Hence, in step S205, the voltage of the DC link 12 is lowered to a voltage level below load AC peak.

[0092] A the UPS system 60 according to a fourth embodiment can be seen in Fig. 7,

The UPS system 60 comprises multiple UPS devices 10, which are connected in parallel. The UPS devices 10 can be any UPS device of the first to third embodiment. In the described embodiment, the UPS devices 10 are the UPS devices 10 of the first embodiment.

[0093] As can be further seen in Fig. 7, the UPS system 60 comprises a communication bus 62, which interconnects the UPS devices 10. Furthermore, a user interface 64 is connected to the communication bus 62. The UPS system 60 further comprises an AC power supply bus 66, which interconnects the AC/DC converters 14 of the UPS devices 10. The AC power supply bus 66 is connected to an AC power supply 20. The UPS system 60 still further comprises a DC battery supply bus 68, which interconnects the DC/DC converters 16 of the UPS devices 10. The DC battery supply bus 68 is connected to a battery 22.

Accordingly, the UPS devices 10 are commonly connected to a single battery 22. The UPS system 60 also comprises a load bus 70, which interconnects the DC/AC converters 18 of the UPS devices 10. The load bus 70 is connected to a load 24.

[0094] Subsequently, a method for monitoring the battery 22 connected to the UPS

system 60 according to a seventh embodiment will be discussed with respect to Fig. 10. The method of the seventh embodiment is based on the method of the fifth embodiment, so that the method of the seventh embodiment will not be described again in detail. Reference is made to the respective description of the method of the fifth embodiment.

[0095] In step S300, the UPS devices 10 of the UPS system 60 are operated in double conversion mode, i.e. the load 24 e.g. is powered through the AC/DC converters 14 and the DC/AC converters 18 from the AC power supply 20.

[0096] In step S305, the UPS devices 10 of the UPS system 60 are synchronized.

Hence, one controller 34 of one of the UPS devices 10 sends a synchronization signal to all UPS devices 10 via the communication bus 62.

[0097] In step S310, a synchronized current pulse train 40 of discharge current pulses 42 through the battery 22 is generated by the DC/DC converters 18 of the UPS system 60. Details of the current pulse train 40 are as described above with respect to the fifth embodiment. Hence, according to the seventh embodiment, the pulse current is provided via the DC/DC converters 16 to the respective DC links 12, and the energy from the current pulses 42 is provided via the respective DC/AC converters 18 to the load bus 70, which is connected to the load 24.

Hence, the battery current is split over different UPS devices 10, which commonly provide the current to the load 24.

[0098] In step S320, parameters influencing the at least one battery parameter are

monitored, in accordance with step S120 as discussed above.

[0099] In step S330, a voltage and current are measured across the battery 22 as

response to the current pulses 42 of the current pulse train 40. The voltage and current are measured for the battery 22 as a whole in the DC/DC converters 16 of the UPS system 60. Hence, at least one of the DC/DC converters 16 performs a voltage measurement, and each of the DC/DC converters 16 measures its current. The sum of the current through all DC/DC converters 16 is the battery current in this embodiment. As indicated in the flow chart of Fig. 10, the method returns to step S305 for generation of a further synchronized current pulse train 40 of discharge current pulses 42.

[00100] After N repetitions, in step S340, at least one battery parameter is determined based on the averaged voltage and current measured across the battery 22 as response to the current pulse train 40. In this embodiment, an impedance of the battery 22 is determined.

[00101 ] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to be disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting scope.

Reference signs list

10 uninterruptible power supply device, UPS device DC link

AC/DC converter

DC/DC converter

output converter, DC/AC converter

AC power supply

battery

load

string

battery block

bypass

bypass switch

controller

current pulse train

current pulse

bypass contactor

bypass silicon controlled rectifier

power supply side switch

output switching device

load side contactor

load side silicon controlled rectifier

uninterruptible power supply system, UPS system communication bus

user interface

AC power supply bus

DC battery supply bus

load bus

Claims

1. Method for monitoring a battery (22) connected to a UPS device (10), the UPS device (10) comprising a central DC link (12), a power supply side AC/DC converter (14), a power supply side DC/DC converter (16), and a load side output converter (18), whereby all converters (14, 16, 18) are connected to the DC link (12), and the DC/DC converter (16) is connected to the battery (22) at a power supply side of the DC/DC converter (16), comprising the steps of

generating at least one current pulse (42) through the battery (22) using the DC/DC converter (16),

measuring a voltage and current across the battery (22) as response to the at least one current pulse (42), and

determining at least one battery parameter based on the voltage and current measured across the battery (22) as response to the at least one current pulse (42), wherein

the step of generating at least one current pulse (42) through the battery (22) comprises generating a discharge pulse through the battery (22), providing the pulse current of the current pulse (42) via the DC/DC converter (16) to the DC link (12), and storing energy from the discharge pulse (42) in the DC link (12).

2. Method according to claim 1 , wherein

the battery (22) comprises multiple individual strings (26), which are connected in parallel, and

the step of measuring a voltage and current across the battery (22) as response to the at least one current pulse (42) comprises measuring a current for at least one of the multiple individual strings (26).

3. Method according to preceding claims 1 or 2, wherein

the battery (22) comprises at least one individual string (26), and the at least one string (26) comprises multiple battery blocks (28), which are connected in series, and the step of measuring a voltage and current across the battery (22) as response to the at least one current pulse (42) comprises measuring a voltage for at least one of the multiple battery blocks (28) of the at least one string (26).

4. Method according to any of preceding claims 1 to 3, wherein the step of generating at least one current pulse (42) through the battery (22) comprises generating at least one charge pulse and/or at least one discharge pulse.

5. Method according to any preceding claim, wherein

the step of generating at least one current pulse (42) through the battery (22) comprises generating a current pulse train (40).

6. Method according to any preceding claim, wherein

the step of generating at least one current pulse (42) through the battery (22) comprises generating a discharge pulse through the battery (22), providing the pulse current of the current pulse (42) via the DC/DC converter (16) to the DC link (12), and supplying energy from the discharge pulse (42) via the output converter (18) to a load (24).

7. Method according to any preceding claim, wherein

the method comprises the additional step of generating at least one charge pulse through the battery (22) fed by energy stored in the DC link (12) via the DC/DC converter (16).

8. Method according to any preceding claim, wherein

the UPS device (10) comprises a bypass (30) comprising a bypass switch (32), whereby the bypass switch (32) is switchable to connect/disconnect the power supply side to/from the load side, and

the method comprises the additional step of operating the UPS device (10) by connecting the power supply side to the load side using the bypass switch (32), whereby the converters (14, 16, 18) are off and the DC-link (12) is maintained to AC peak voltage through the freewheel diodes of the converters (14, 16, 18) and

the method comprises the additional step of operating the output converter (18) to decrease the DC-link voltage prior to the step of generating at least one current pulse (42) through the battery (22) using the DC/DC converter (16).

9. Method according to the preceding claim, wherein

the method comprises the additional step of creating a discharge pulse using a voltage window for the DC-link (12) between AC peak voltage and allowed high DC voltage as determined by link cap voltage rating of the DC link (12).

10. Method according to any of the two preceding claims, wherein

the UPS device (10) comprises an output switching device (52) comprising a silicon controlled rectifier (56) and a contactor (54), which are connected in parallel to a load side of the output converter (18), and

the method comprises the additional step of lowering the voltage of the DC link (12) to a voltage level below load AC peak using the silicon controlled rectifier (56).

1 1. Method according to any of the three preceding claims, wherein

the method comprises the additional steps of creating a discharge pulse, and operating the AC/DC converter (14) and/or the output converter (18) to absorb excess energy or a part thereof.

12. Method according to any preceding claim, wherein

the method comprises the additional step of monitoring parameters influencing the at least one battery parameter.

13. Method for monitoring a battery connected to a UPS system, the UPS system

comprising multiple UPS devices (10), which are connected in parallel, whereby each UPS device (10) comprises a central DC link (12), a power supply side AC/DC converter (14), a power supply side DC/DC converter (16), and a load side output converter (18), whereby all converters (14, 16, 18) are connected to their respective DC link (12), and the DC/DC converters (14, 16, 18) of the multiple UPS devices (10) are connected in parallel at their power supply side to the battery (22), comprising the steps of

generating at least one synchronized current pulse through the battery (22) commonly using the DC/DC converters (16) of the multiple UPS devices (10),

14. UPS device (10) comprising a central DC link (12), a power supply side AC/DC converter (14), a power supply side DC/DC converter (16), and a load side output converter (18), whereby all converters (14, 16, 18) are connected to the DC link (12), and the DC/DC converter (16) is connected to the battery (22) at a power supply side of the DC/DC converter (16), wherein

the UPS device (10) is adapted to perform the method according to any of claims 1 to 12.

15. UPS system (60) comprising multiple UPS devices (10), which are connected in

parallel, each UPS device (10) comprising a central DC link (12), a power supply side AC/DC converter (14), a power supply side DC/DC converter (16), and a load side output converter (18), whereby all converters (14, 16, 18) are connected to their respective DC link (12), and the DC/DC converters (16) of the multiple UPS devices (10) are connected in parallel to the battery (22) at their power supply side, wherein the UPS system (60) is adapted to perform the method according to preceding claim 13.