GB2564910A

GB2564910A - Battery management

Info

Publication number: GB2564910A
Application number: GB1712124.5A
Authority: GB
Inventors: Leslie Gordon Hardy John
Original assignee: Intercal Uk Ltd
Current assignee: Intercal Uk Ltd
Priority date: 2017-07-27
Filing date: 2017-07-27
Publication date: 2019-01-30
Anticipated expiration: 2037-07-27
Also published as: WO2019021007A1; GB201712124D0; GB2564910B

Abstract

A battery connection system for connecting a rechargeable battery 500 to a bus 102. The connection between the battery 500 and the bus is provided by two separate connections 503, 504, each providing a unidirectional current path. Current flows out of the battery through one path 503 to the bus to provide power when an alternator 104 or other power source is not supplying power. When the alternator is running or another source of power is available, current flows from the bus to the battery via the other current path 504 during charging. The current flowing into the battery is modulated by a charging module 501 which controls the charging process. The charging module 501 may be controlled by a BMS (Battery Management System) 502. The battery may be a lithium ion (li-ion) battery and may be used to replace a lead acid battery.

Description

The present invention relates to the field of battery management and more particularly to arrangements for managing the charge and discharge of rechargeable battery systems.

Very large numbers of land vehicles, aircraft and water-borne vessels are powered principally or wholly by internal combustion engines which operate in combination with a low voltage DC electrical system to provide ancillary services. These services vary widely depending on the application but commonly include engine starting, lighting, instrumentation and other equipment and ancillary appliances. Similar low voltage DC configurations may be found in other applications requiring electrical services where there is no permanent or reliable access to an electricity supply.

These electrical systems tend to incorporate a rechargeable battery which must be connected to a suitable source of electrical power for charging. The battery types currently most commonly used in these applications are Lead-acid (referred to herein as “PbAc”) or Nickel-Cadmium (referred to herein as “Ni-Cad”). The use of Lithium ion (referred to herein as “Li-ion”) batteries would in many circumstances be desirable for reasons of performance, weight saving or durability. However, their special nature creates difficulties in managing them safely and effectively within conventional configurations.

Li-ion batteries are in widespread use in a range of applications, including as the primary power sources in electric vehicles. As costs have reduced they have also become an attractive potential alternative to PbAc or Ni-Cad for providing ancillary power in motor vehicles, aircraft and waterborne vessels. Li-ion batteries can provide a number of advantages depending upon the application:

• They are smaller and lighter, typically being about a third of the weight of PbAc for a given amp-hour rating. This is a major advantage in aircraft but also has value in land vehicles and boats.

• They do not suffer from significant loss of capacity at high current discharge (the “Peukert effect”), which is a problem with PbAc batteries. This is important for starter motors and other high-power intermittent applications.

• They typically have a far longer service life than PbAc equivalents, providing potentially reduced total lifetime costs, greater reliability and simpler maintenance.

• They are tolerant of habitual undercharging, which can be a common situation in some applications. In contrast, routine undercharging can degrade PbAc cells quickly.

• They are tolerant of long periods of inactivity (for example in recreational water craft used just a few times a year). In contrast PbAc batteries deteriorate relatively quickly if not routinely fully charged.

• They do not suffer from the phenomenon of “memory effect” observed in Ni-Cad batteries that have been incompletely discharged before recharging.

These advantages offer considerable improvements over the alternative options. However, there are some challenges with using Li-ion batteries, notably in ensuring the safe and effective management of the charging and discharging process. A PbAc battery can be charged from simple relatively crude devices without serious concerns for safety or operation of the battery. They tend to be tolerant of large variations in voltage and charging current.

In contrast, Li-ion batteries require smart micro-processor controlled chargers and battery management systems (“BMS”) configured for the specific application. In order to prevent accelerated deterioration in performance, rapid cell failure and even thermal runaway and fire, precautions need to be undertaken to ensure the battery is operated correctly. For example, the cells in a Li-ion battery require positive charge termination and should not be left on a continuous float charge. The charge current needs to be controlled to avoid excessive charge current. Similarly, the charging device needs to ensure that excessive charge voltage is not applied to the cells. Li-ion batteries are also more sensitive to their operating temperature and cells should only be charged if the battery is within a defined temperature range.

In addition to managing the charging of Li-ion cells, they should also be protected from over-discharge and particularly the damaging effects of voltage reversal which can occur in series connected battery packs when individual cells are over discharged even though the battery as a whole is still able to provide charge.

Charging control for Li-ion cells and batteries is typically achieved using a suitable “Constant Current/Constant Voltage” (CC/CV) profile. Figure 1 shows such a typical

CC/CV charge profile. At low states of charge, a controlled constant current is applied (the “CC” phase) to initially replenish the battery. As the charging progresses, the cell voltage increases. Subsequently, the battery/cell voltage reaches a predetermined value and the charging control system switches to a constant voltage profile (the “CV” phase). During this CV phase the current is progressively reduced in order to ensure permissible voltage levels are not exceeded. Once the battery is determined to be fully charged, further charging is stopped.

In order to prevent excessive discharge, control circuitry will also usually be included to isolate the battery during deep discharge before damaging low voltage thresholds are crossed. Some applications, notably in cold climates, will also require control devices to prevent charging outside of an acceptable temperature range.

While it is standard practice to install suitable BMS in applications using 12V and higher Li-ion batteries, special difficulties arise with integrating a BMS into the types of installation described above. For example, if the BMS cannot control contactors to isolate the battery in the event of a voltage or temperature excursion out of the range of desirable values then it is ineffective. If it can control contactors, it will have the ability to take the battery off line suddenly which may damage the alternator and other electrical components. Additionally, Li-ion batteries typically have different temperature limits for charge and discharge. In a conventional installation, the BMS would have to take the battery off line altogether below the minimum charging temperature (say 0^eC) even if the battery would be quite safe supplying power at lower temperatures (e.g. - 20^eC). Furthermore, with the above arrangement, the BMS cannot control the charge profile. With a discharged battery, if the alternator is capable of delivering, for example, 300 Amps, then that is the current that may flow, even if the battery limit is specified to be a lower value, e.g. 30 Amps. Similarly, charging cannot be terminated when the desired level of charge has been reached, as is normal for Li-ion batteries. The BMS cannot prevent charging without taking the battery offline altogether.

Apart from the battery, the electrical power required for the types of ancillary systems described here may be obtained from a number of sources. This is typically provided by an alternator or generator installed in the vehicle or craft, normally powered by an engine but occasionally powered from other sources such as regenerative braking through road wheels. Other on-board power generators of various types including solar panels and small wind turbines may sometimes be used. Additionally, when available, a static external supply may be used such as from the electrical grid or a portable generator.

In a typical configuration, such as in a car, the battery or batteries will be connected permanently to a low voltage bus bar along with an engine driven alternator or other onboard power source. The alternator voltage is typically set slightly above the battery voltage; so for example an ordinary car battery voltage peaks at around 12 Volts, but the alternator is likely to be regulated to around 14 Volts when powered. The excess voltage causes a charging current to flow into the battery. For the commonly used PbAc or NiCad batteries, this is a simple and effective means of ensuring that the battery is recharged and then has a continuous float charge when the engine is running.

Figure 2 shows a simplified schematic view of a typical low voltage DC bus in a car or other road vehicle. The battery and alternator are connected directly to the bus along with the various power consuming devices on the vehicle such as the lighting, wipers, audio devices and window heaters etc., shown in Figure 2. As will be apparent from Figure 2, the permanent connection of the bus to the battery means that whilst the engine is running to power the alternator, or another power source connected to the bus is available, then a float charge will be delivered to the battery.

With PbAc cells, the resulting constant (“float”) charging is beneficial. The cells typically have a small self-discharge and float charging ensures they are always topped up and ready for use. In contrast, leaving a Li-ion battery permanently connected to a DC bus bar at or near its maximum voltage is associated with Lithium plating and/or the growth of dendrites which cause loss of capacity over time, increase the risk of early cell failure and occasionally may lead to thermal runaway of the type periodically reported in cell phones and laptops.

An additional consequence of the arrangement in Figure 2 is that, when the alternator or other power source starts to deliver power to a battery in a discharged state, current will flow into the battery initially at an uncontrolled and potentially excessive rate, limited only by the capacity of the alternator, the small internal impedance of the battery and the minimal resistance of the charging circuit. As the battery becomes charged, the current naturally drops to the final “float charge” level. Figure 3 shows a typical charge profile with an alternator connected to the battery via a bus such as in Figure 2.

Comparison of the typical charge profile using an alternator (Figure 3) with the required charge profile for Li-ion (Figure 1) illustrates the issue that needs to be overcome. In Figure 3 the initial charge current is a function of the alternator characteristics, which is in turn driven by the overall demands of the particular electrical system rather than the requirements of the battery. For example, the substantial equipment loads in a modern car may mandate use of a powerful alternator. Thus a typical mid-sized car may require a 175 Amp alternator and a 62 Amp-hour battery.

This does not present a problem in practice with PbAc cells since they have relatively high internal impedance which serves to limit the charge current; and they can in any event tolerate brief periods of high charge rate at low state of charge. However, this type of uncontrolled charging can readily exceed the maximum allowable charging rate of a Li-ion battery, with potentially serious consequences. For example, the recommended CC-phase current for a 60 Amp-hour Li-ion battery might be 20 Amps, so charging it routinely at 175 Amps will greatly exceed the maximum allowable fast-charge current, particularly if the battery is cold.

A further issue is that the safe functioning of the type of charging regime described in Figure 1 is dependent on the correct operation of the charging device. If alternator runaway occurs (an uncontrolled rise in alternator voltage caused by a component failure), both charge voltage and float charge current can become excessive. For example, a regulator failure or the loss of a ground path might cause the voltage of an alternator in a 12 volt system to rise from around 14 volts to around 17 volts.

In such a case, while this component failure may lead in a conventional installation to destructive venting of a PbAc battery, the consequences are not generally hazardous. Provided the battery box is ventilated, the chances of fire or explosion occurring are minimal. By contrast, such an event with an unprotected Li-ion battery circuit may have severe consequences, since the difference between a normal charge voltage and the onset of accelerated electrolyte breakdown can be as little as 0.2 volts per cell; a more substantial overvoltage for a significant period can lead to venting of inflammable electrolyte and even thermal runaway where exothermic breakdown of the cell produces oxygen and thus a self-sustaining conflagration. This clearly poses a significant risk of damage to the vehicle and injury to the occupants.

A further issue with substituting batteries concerns the management of charging dependent upon the temperature. PbAc cells have a very wide allowable temperature range for charging and discharging, both ranges being substantially the same. This wider range means that in most applications, operating temperature limits do not have to be applied.

Li-ion cells typically have temperature limits for charging that are more restrictive than those for discharge. For example a battery may be usable as a power source down to temperatures of -20^eC, while charging might be prohibited below 0^eC.

In the case of a road vehicle, yacht or aircraft battery left in a cold environment overnight, for example in winter, the unit might be fully capable of cranking a starter motor in the morning while at the same time being below the manufacturer’s minimum charging temperature. In order for a BMS to protect the battery, it must be able to inhibit charging while it is outside the permissible temperature range. If the battery is permanently connected to the bus bar as depicted in Figure 2, this cannot be achieved. The consequences may not be immediate, but there is a heightened risk of early cell failure and even thermal runaway.

Over-discharge can occasionally happen in the types of system described here. Car headlights may be left on overnight, or a boat owner may fail to turn off some small load, leaving it on for extended periods of time. PbAc cells generally survive occasional overdischarge satisfactorily, although with some types of PbAc cells, repeated deep discharge may shorten battery life. A fully discharged PbAc battery may freeze and crack its case but otherwise a fully discharged PbAc battery can normally simply be connected up to a power source, after which it will recover to normal operation. A familiar example is the use of jump leads to start a car with a discharged battery.

Again, Li-ion cells are much more sensitive in that a cell discharged to below about 2 volts may be permanently damaged. A cell within a battery that is discharged down to the point of voltage reversal will almost certainly be destroyed. For this reason, equipment powered by Li-ion batteries will in general require the battery to be isolated or recharged once a given level of discharge is reached.

The above problem poses a challenge to the installation or retro-fitting of Li-ion batteries in applications where PbAc or Ni-Cad batteries are conventionally used. It has been known for Li-ion batteries sold as PbAc replacements simply to consist of a unit containing cells, connectors and terminal posts with no battery management system (BMS) and no way of disconnecting the battery if its limits are exceeded. Such a system is clearly a fire risk and will probably have a short life. If the cells were balanced before assembly and the battery were installed in a temperate environment, charged to a relatively low peak voltage and never seriously over-discharged, the system might last some time without mishap.

In general, proprietary drop-in replacement Li-ion battery packs will as a minimum incorporate a simple Li-ion battery management system, often within the battery casing. This typically will be configured to equalise the charge of all the individual cells in the battery within acceptable voltage limits, and may have an output to shut down an external charger.

In the type of application considered here, installing a BMS without additional components is of limited benefit. There is little the BMS can do other than attempt equalisation and it cannot protect the battery against excessive voltage, over discharge or float charge without the insertion of relays, contactors or power transistors to turn off power.

Some systems incorporate a heavy duty relay controlled by the BMS which can disconnect the battery if the BMS detects an error (Figure 4). This serves to protect the battery, for example in the event of alternator runaway or when the battery is outside the acceptable temperature range.

While this arrangement addresses some risks, there remain several difficulties. If the BMS opens the relay whilst a high current (potentially 100 amps or more) is flowing in the system, this may cause severe voltage spikes which can damage or destroy equipment connected to the system. The alternator and/or other connected equipment tends to rely on the battery acting as a large buffer to smooth out smaller but more frequent voltage spikes arising elsewhere in the system. Running the alternator with the battery disconnected can damage other sensitive electronic equipment such as Engine Control

Units (ECUs). Incorporation of one or more large capacitors could be used to mitigate this but this adds to the cost and weight.

This approach is also not suitable for preventing high charge currents, since if the allowable peak charge current is set below the peak output of the alternator, it will trip whenever the alternator is operating at peak output, preventing the battery from being charged. Similarly, isolating the battery when it is fully charged to prevent float charging would mean that the battery would have to be offline, i.e. disconnected form the bus, for much of the time, which would not be acceptable in many applications.

A possible refinement is to permit the BMS to control the alternator output. In most practical configurations, however, the alternator is not just required to charge the battery. Interfering with the alternator output may compromise the operation of other electrical equipment. Furthermore, many modern alternators have internal regulation which means that implementing this configuration in an existing installation would require replacing the alternator as well as the battery.

Given the limitations of the solutions described above, another recognised approach is to use a supplementary PbAc battery in parallel with the Li-ion battery. This configuration can overcome some of the difficulties discussed above since it enables the BMS and an isolator to disconnect the Li-ion battery when fully charged so as to avoid float charging and also to prevent low temperature charging. The PbAc battery in parallel will act as a buffer which will largely eliminate the issues which would otherwise arise each time the Li-ion battery is disconnected. However, the need for a second battery adds back bulk and weight to the system, negating the gains which may have been a reason for preferring Li-ion batteries in the first place. It also adds complexity and another point of failure into the system. Furthermore, it adds to installation and maintenance costs. Effective control of charging current is also difficult in such an arrangement due to the differing requirements of the two battery technologies.

There is therefore a need for a solution which allows for the use of Li-ion type batteries without the problems described above. It is particularly desirable to provide a solution which can be used in existing installations without needing modification. The ability to be able to substitute a PbAc or other battery in an existing system without needing to modify the system is desirable. The present invention aims to solve or at least ameliorate some of those problems.

Therefore according to the present invention there is provided a battery connection system for connection to a rechargeable battery, the system including: a first connection for connection to said battery; a second connection; a first unidirectional current path arranged to allow current flow in only a first direction between the first connection and the second connection to provide a discharge path; a second unidirectional current path arranged to allow current flow in only a second direction, opposite to the first direction, between the first connection and the second connection to provide a charging path; and a current controller for controlling the current flow through said second unidirectional current path.

The use of separate charge and discharge current paths in combination restores full control to the charging process whilst providing the advantages of having the battery connected for supplying power to a bus. This arrangement allows charging to be positively terminated without taking the battery off line. This also allows charging to be terminated at a low current, avoiding damaging voltage spikes associated with interrupting a large current. Charging can be prevented if the cell temperatures are outside the permitted temperature range, while still enabling the battery to provide power.

The effective peak voltage at the battery terminals can be controlled in circumstances where the alternator rated output voltage exceeds allowable limits for the battery. In this way, the battery is effectively protected in the event of unexpected high voltages such as those caused by alternator runaway.

The charge current can be limited to suit the BMS requirements regardless of alternator output which allows constant current/constant voltage charge management of the required profile to optimise safety and battery life. The battery can still be safely isolated if it reaches a specified discharge voltage limit or if cell temperatures exceed the limits for discharge. The battery may be recharged using a simple proprietary PbAc charger which can safely be connected and left unattended. Similarly, the battery can also be recharged by connecting to other power sources including solar or wind generators provided their voltage and current outputs were adequate.

The first unidirectional current path may include a power diode, a power transistor or other similar switching element to provide a unidirectional current flow, for preventing current flowing in a direction opposite to said first direction. This allows discharge current to flow between the first and second connections whilst preventing a charging current flowing back into the battery.

The second unidirectional current path may include a power diode, a power transistor or other similar switching element to provide a unidirectional current flow, for preventing current flowing in a direction opposite to said second direction. This allows a charging current to flow into the battery whilst preventing a discharge current flowing from the battery.

The second unidirectional current path preferably includes a power transistor controlled by said current controller, for controlling the current flow through said second unidirectional connection in said second direction. The second unidirectional current path may include one or more variable output DC to DC converters, controlled by said current controller, for controlling the current flow through said second unidirectional current path in said second direction. These allow the current to be controlled to control the charging of the battery independent of the voltage on the second connection, which may be connected to an alternator or other power source where the precise output voltage cannot or is not readily controlled.

The present invention may also provide a rechargeable power source including a battery connection system such as that described above in combination with a battery. These may be packaged in the same case or provided as separate elements. However, by providing them in a combined unit, a convenient drop-in replacement for existing PbAc systems can be provided with no modifications required to the rest of the electrical system.

Such a battery is preferably a Li-ion battery.

In the rechargeable power source, there is preferably provided a positive output of the rechargeable power source connected to the second connection of the battery connection system, with the first connection of the battery connection system connected to the positive terminal of the battery. The rechargeable power source is preferably also provided with a negative output connected to a negative terminal of the battery. In this arrangement, discharge current can flow from the battery into the first connection and then out of the second connection.

As an alternative, the rechargeable power source may be provided with a negative output connected to the second connection of the battery connection system, with the first connection of the battery connection system connected to the negative terminal of the battery. Such a rechargeable power source is preferably also provided with a positive output connected to a positive terminal of the battery. In this arrangement, discharge current can flow into the second connection and then out of the first connection and into the battery.

The rechargeable power source may be provided with a capacitor and preferably capacitor control means for selectively connecting the capacitor between the positive power connection and the negative power connection of the rechargeable power source.

The rechargeable power source may further comprise an alternative power source connected to a common power bus in parallel with the rechargeable power source. For example, an alternator may be provided connected to a mechanical power source or a solar or wind generator may provide a power source.

Such alternative power source may produce an unregulated voltage. This may not be ideal for charging batteries where careful control of charging is desirable, such as Li-ion batteries. However with the invention, the charging current can be controlled whilst still leaving the battery connected to a bus or other power providing connection.

The rechargeable power source may include one or more of the elements described above such as the battery and capacitor, which may be provided in a single unit. The unit may also include other elements such as a BMS, a charging module etc. in the same case, to provide the simple drop-in replacement mentioned above.

The current controller may be integrated with the BMS or may operate in conjunction with a separate BMS.

In the above arrangements, the first connection is preferably connected to one of the battery terminals whilst the second connection is preferably connected to a bus or other load/recharging power source connection.

Specific examples of the present invention will now be described in more detail by reference to the following drawings in which:

Figure 1 shows a typical CC/CV charge profile for a Li-ion cell or battery;

Figure 2 shows a schematic layout of a typical low voltage DC bus system in a road vehicle;

Figure 3 shows a typical unregulated charge profile for a conventional cell or battery;

Figure 4 shows one arrangement for protecting a Li-ion battery application;

Figure 5 shows the layout of an embodiment according to the present invention;

Figure 6 shows an example of a charging module;

Figure 7 shows a schematic circuit layout of a bus based system using the charging system of the invention;

Figure 8 shows the layout of an alternative embodiment according to the present invention;

Figure 9 shows an example of an alternative charging module;

Figure 10 shows a further example of a charging module; and

Figure 11 shows a schematic circuit layout of a bus based system using an alternative charging system of the invention.

The arrangement shown in Figure 5 has a modified structure compared to the prior art arrangement of Figure 2. Instead of a single wire connecting one of the battery terminals (the positive terminal in Figure 2) to the bus, the connection between the battery 500 and the bus is provided by two separate connections 503, 504, each providing a unidirectional current path. Current flows out of the battery 500 through one path 503 to the bus 102 to provide power the system when the alternator 104 or other power source is not supplying power. When the alternator is running or another source of power is available, current flows from the bus 102 to the battery 500 via the other current path 504 during charging.

The current flowing into the battery is modulated by a charging module 501 which controls the charging process. The charging module 501 is controlled by the BMS 502. The BMS is shown as a separate unit to the charging module 501 but the charging module and the BMS could be integrated into a single unit. Both units may also be integrated into a single unit along with the battery to provide a simple “drop-in” replacement for a PbAc battery or similar.

Figure 6 shows a simplified example of the charging module 501. The bus connection 504 is selectively switched to the battery connection under the control of a charge controller 601. The charge controller can control the voltage and current provided to the battery, for example by using PWM to control the connection. It may also disconnect the battery from the bus when charging is not required, for example then there is no external power source or the battery is fully charged.

The schematic circuit layout of a bus system using the invention is shown in Figure 7. The alternator 104 is connected to the bus 102 along with ancillary components 103 (103a, 103b, 103c). The diode symbols shown in Figure 7 represent the unidirectional paths to and from the battery to the bus. Whilst diodes may be used, other means or limiting the direction of flow of the current may be used, e.g. power transistors.

This configuration allows the charging module 501 to control key charge parameters including charging voltage, charging current, operating temperature and charge termination, so that the battery is provided with an appropriate charge profile such as that shown in Figure 1. This control is achieved without interfering with the functioning of the alternator or other power source.

The “out” or “discharge” current path 503 allows current to flow from the battery to the bus should the voltage on the bus drop, e.g. if the alternator is no longer providing power. The connection 503 may be configured to also selectively block or regulate current out of the battery as well as into the battery. This may be desirable if the battery is at risk of being discharged more than desirable or if the temperature of the battery is outside the desirable operating range. This would generally be controlled by the BMS.

In this arrangement, the battery 500 is normally connected to the bus via connection 503 providing the benefits of a permanently connected battery but the battery can still be disconnected in extreme circumstances to prevent load current out of the battery, particularly where the battery is heavily discharged.

Adverse consequences can result from variation or ripple on the bus voltage. This may be positive or negative, i.e. with the voltage rising above and below the nominal bus voltage. With the arrangement of this embodiment, the negative component of the voltage ripple can be minimised. If at any instant the bus voltage drops, the battery which is connected via connection 503 is able to supply additional current to the bus to correct the voltage drop, irrespective and independent of whether the charging circuit has connected the bus to the battery at that point.

In a conventional system, positive ripple can also be controlled by directly connecting to the battery and using it to sink excess current. An increase in bus voltage will increase the current into the battery which will help to reduce the additional voltage on the bus. As noted above, it is not desirable to use Li-ion batteries in this way. Connecting a Li-ion battery to the bus in this way, without regulation can lead to large charging currents with undesirable consequences. In this arrangement, the unidirectional connection 503, would prevent current flowing from the bus to the battery and causing damage.

However, to absorb the positive part of any DC ripple, the charging module may be arranged to selectively connect a large capacitor 505 to the bus 102 when required. The capacitor would typically be pre-charged to a voltage close to or equal to the desired bus voltage and connected when the bus voltage rises above the desired voltage. The capacitor is pre-charged to avoid a large voltage differential when it is connected to the bus, which could lead to a large current and potential damage to the switching components or other components on the bus. When it is connected, the capacitor will then tend to limit a rise in voltage on the bus by absorbing current.

Alternatively, the charging module may be arranged to selectively connect a resistor to the bus 102 in order to sink excess current and restrain the bus voltage.

Figure 8 shows an alternative arrangement where the charging module 601 is put in the negative current path. The approach is logically identical to the arrangement of Figure 5 but the charging module 601 is placed between battery 500 and earth rather than between the bus 102 and the battery. With some types of switching device, this may slightly simplify the control electronics.

In this arrangement, the positive terminal of the battery 500 is connected directly to the bus 102. A unidirectional connection 703 is provided between the negative terminal and earth to provide a path for current flow when the battery is providing power to the system. The earth effectively provides the negative bus for the system. In a vehicle such as a car, the earth is provide by the metal chassis and body. This allows the body to be used as the return path so that only a single positive connection is needed to be provided to the ancillary device or alternator for operation.

When the battery is not required to power the system and an alternative power source is available, then the battery may be charged via the connection 704 to the charging module 601. The charging module 601 controls the current and voltage applied to the battery during charging to provide the desired charging profile, such as that set out in claim 1. The charging module 601 operates in a similar way to the charging unit 501.

There are a number of possible methods for implementing the approach illustrated in Figures 5 and 8. For example the charging module 501, 601 may use one or more electronic power switching devices, for example Field Effect Transistors (“FETs”) or Insulated-Gate Bipolar Transistors (“IGBTs’j or an array of such devices in order to modulate or completely shut off current and voltage. The device or devices will function in much the same way that a DC motor controller operates by pulse width modulation (“PWM”).

Figure 9 shows an example of a charging module which uses a MOSFET for controlling current and voltage. These devices are effectively very fast acting electronic switches. The charge controller 701 typically includes a microcontroller which controls the electronic switches to modulate current by switching them rapidly on and off hundreds or even thousands of times a second. The ratio of “on” time to “off” time governs the effective output. As shown in Figure 9, the charging module 601 is arranged between the battery and earth rather than between the bus bar and the battery. In this arrangement, the FETs or alternatively IGBTs are put in the negative current path. The example shown in Figure 9 is based on a MOSFET 703 as a switch or voltage controller. The charge controller 701 would govern the current flow by outputting a PWM control signal to the FET under the control 702 of the BMS. The ratio of on-time to off-time in the pulses would determine the current flow from the battery in order to set the correct charging current.

The complete installation would require additional components (freewheeling diodes, pull-down resistors, capacitors etc.) to prevent damage to components and ensure their proper operation. These have been omitted for clarity since they are standard requirements which do not have any bearing on the principle of operation of the solution described here.

Figure 9 shows an alternative arrangement for the charging module 501, in which the power from an engine-driven alternator could be used to drive a DC-DC converter. Figure 9 shows an overview of a charging module 501 including a DC-DC converter 903. The charging module shown assumes an arrangement similar to that shown in Figure 5 with the charging module provided between the bus and the battery. Figure 9 shows a typical implementation of the charging module 501, again incorporating a low voltage microcontroller 901 similar to that in the previous example, to control a DC-DC converter 903. The microcontroller 901 would trim the output of the DC-DC converter to control the current provided to the battery to maintain the chosen charge profile. The microcontroller 901 may be controlled by control signals 902 from the BMS.

The DC-DC converter may be a single unit or may be formed from an array of variableoutput DC-DC converters. Some proprietary DC-DC converters can output hundreds of watts and permit voltage trimming above and below nominal, and/or output voltages higher than the input. Some of these converters can be used in parallel for higher power.

The arrangements above make use of separate charge and discharge current paths in combination with a charging module and BMS. This allows full control of the charging process so that charging can be positively terminated without taking the battery off line. Also by terminating charging at a low current, the risk of damaging voltage spikes associated with interrupting a large current are avoided.

Also, as the charging can be controlled, charging can be prevented if the battery/cell temperatures are outside the permitted temperature range for charging. As noted above, whilst the temperature may be outside of an acceptable range for charging purposes, the temperature may be still within an acceptable range for discharge, allowing the battery to continue to supply power if needed and also provide the voltage regulation effect.

The effective peak voltage at the battery terminals can be controlled in circumstances where the alternator rated output voltage exceeds allowable/desirable limits for the battery. In this way, the battery is effectively protected in the event of unexpected high voltages such as those caused by alternator runaway.

The charge current can be limited to suit the BMS requirements regardless of alternator output. This allows CC/CV charge management with the required profile to optimise safety and battery life.

Once the battery has reached a specified discharge voltage, it can be safely isolated. Similarly, if cell temperatures exceed (or fall below) the limits for discharge the battery can be isolated. The system can be connected to a conventional battery charger of the type typically used for PbAc batteries. The charger would be connected to the bus to provide the source of power. However, the charging controller would then ensure that the correct charging profile was used to manage charging of the battery. This avoids the need for special Li-ion chargers where external charging of the battery is required. Again a conventional trickle charger designed for maintaining a PbAc battery can be used since the BMS would allow charging to take place until the Li-ion battery had reached the desired level of charge and then simply isolate the battery from the bus, although the battery would still be connected to the bus via the unidirectional connection 503, should it be required.

The battery could also be recharged by connecting to other power sources including solar or wind generators provided their voltage and current outputs were adequate. Again, the regulation of the current and voltage provided by these sources is less critical as the charging is controlled by the charging module, as long as the sources were able to provide sufficient power.

In the arrangements described above, the BMS and charging modules are shown separately. However, the elements such as the cells, BMS, charging module, by-pass capacitors can be packaged in the same case, providing drop-in replacement in existing PbAc systems with no modifications required to the rest of the electrical system. Equally, the BMS and charging module may be provided separately so that they do not need to be replaced when the battery replaced.

The invention may also be embodied in a separate unit connected between the battery and the bus to allow a basic Li-ion battery to be used with one terminal connected to earth or the positive bus (as shown in figures 5 and 8 respectively) and a separate unit incorporating the two unidirectional connections provided between the other battery terminal and the positive bus and earth respectively.

Whilst in most applications, multiple cells are required to form a battery, the invention is still applicable to single cells and references to battery should be considered to be applicable to a cell and vice versa in this document.

Claims

1. A battery connection system for connection to a rechargeable battery, the system including:

a first connection for connection to said battery;

a second connection;

a first unidirectional current path arranged to allow current flow in only a first direction between the first connection and the second connection to provide a discharge path;

a second unidirectional current path arranged to allow current flow in only a second direction, opposite to the first direction, between the first connection and the second connection to provide a charging path; and a current controller for controlling the current flow through said second unidirectional current path.

2. A battery connection system according to claim 1 wherein said first unidirectional current path includes at least one of a power diode and a power transistor, for preventing current flowing in a direction opposite to said first direction.

3. A battery connection system according to claim 1 or 2 wherein said second unidirectional current path includes at least one of a power diode and a power transistor, for preventing current flowing in a direction opposite to said second direction.

4. A battery connection system according to any preceding claim wherein said second unidirectional current path includes a power transistor controlled by said current controller, for controlling the current flow through said second unidirectional connection.

5. A battery connection system according to any preceding claim wherein said second unidirectional current path includes one or more variable output DC to DC converters, controlled by said current controller, for controlling the current flow through said second unidirectional current path.

6. A rechargeable power source including a battery connection system according to any one of the preceding claims and further comprising a battery.

7. A rechargeable power source according to claim 6 wherein said battery is a Lithium ion battery.

8. A rechargeable power source according to claim 6 or 7, further comprising:

a positive output connected to said second connection of the battery connection system, wherein said first connection of the battery connection system is connected to a positive terminal of said battery; and a negative output connected to a negative terminal of said battery.

9. A rechargeable power source according to claim 6 or 7, further comprising:

a negative output connected to said second connection of the battery connection system, wherein said first connection of the battery connection system is connected to a negative terminal of said battery; and a positive output connected to a positive terminal of said battery.

10. A rechargeable power source according to any one of claims 6 to 9, further comprising a capacitor and capacitor control means for selectively connecting the capacitor between said positive power connection and said negative power connection of said rechargeable power source.

11. A rechargeable power source according to any one of claims 6 to 10 further comprising an alternative power source connected to a common power bus in parallel with said rechargeable power source.

12. A rechargeable power source according to claim 11 wherein said alternative power source is a mechanically powered alternator.

13. A rechargeable power source according to claim 11 wherein said alternative power source produces an unregulated voltage.