WO2012143422A1

WO2012143422A1 - Measurement system and method for a series connection of energy storage devices

Info

Publication number: WO2012143422A1
Application number: PCT/EP2012/057128
Authority: WO
Inventors: Rudolf Vidael; Eric Verhaeven
Original assignee: 4Esys
Priority date: 2011-04-19
Filing date: 2012-04-19
Publication date: 2012-10-26
Also published as: EP2699914A1

Abstract

The present invention is directed to a measurement system for measuring a series connection of energy storage devices comprising: a. a number of energy storage devices (C(1)…C(N)) in a series connection, b. a differential amplifier having a first input A and a second input B for coupling it over an individual storage device or over a section of the series connection comprising a plurality of said storage devices, said individual storage device or said section of the series connection having a more negative terminal and a more positive terminal, c. a network of switching means for connecting said more negative terminal to said differential amplifier's input A, and said more positive terminal to said differential amplifier's input B, or vice versa. Further, the present invention is directed to an assembly comprising a series connection of energy storage devices and a measurement system as described above. Additionally, the present invention is directed to method for measuring a series connection of energy storage devices comprising the steps of: a. providing a number of energy storage devices (C(1)…C(N)) in a series connection, b. providing a differential amplifier having a first input A and a second input B, c. selecting one individual energy storage device or a section of the series connection comprising a plurality of energy storage devices, said individual storage device or said section of the series connection having a more negative terminal and a more positive terminal, d. connecting said more negative terminal to said differential amplifier's input A, and said more positive terminal to said differential amplifier's input B, or vice versa. Additionally, the present invention is directed to the use of such system and method for measuring a series connection of electric double-layer capacitors, lead acid or NiMH batteries, lithium energy storage devices or Li-ion energy storage devices.

Description

MEASUREMENT SYSTEM AND METHOD FOR A SERIES CONNECTION OF ENERGY STORAGE DEVICES.

FIELD OF THE INVENTION

The present invention relates to a measurement system for measuring energy storage device voltages in a series connection of energy storage devices. Further, it relates to an assembly comprising a series connection of energy storage devices and such measurement system. Additionally, the invention relates to the use of such system or method for measuring a series connection of electric double-layer capacitors, lithium battery devices or Li-ion battery devices.

BACKGROUND OF THE INVENTION

Recently, a lot of effort has been put in enhancing the use of energy storage media, in particular large numbers of energy storage devices in series connection. Such series connection is for example used in hybrid drive trains for buses, waste collection vehicles, fork lift trucks and electric cars.

Usually, a series connection of energy storage devices for medium and high voltage applications comprises a large number of energy storage devices. For example, since the maximum voltage of an energy storage device is limited to for example about 2.5V to 3.0V in case of a double layer capacitor, a number in the range of 20 to 25 energy storage devices need to be serially connected to form an energy storage device stack delivering a voltage of for example 60V.

A general problem of series connections of energy storing devices is that varying characteristics of each individual energy storage device due to, for example, differences in self-discharge, capacitance, internal resistance, and temperature, generates an inequality between the individual energy storage devices resulting into a so-called unbalanced stack wherein voltage differences between the individual energy storage devices exist. In order to measure the voltage of the individual energy storage devices in the series connection, systems for measuring a series connection of energy storage devices are developed.

In the state-of-the-art two approaches have been used for this energy storage device measurement. One approach is based on resistor voltage dividers (for example WO2008/120163). Each measurement point is connected to an input of a respective voltage divider and the divider outputs are connected to a multiplexer that is connected to a differential amplifier. The voltage dividers divide down the voltage of each measurement point to bring it down to the input range of the differential amplifier. The output of this amplifier eventually represents the energy storage device voltage.

Another approach is based on intermediate flying capacitors (US2008/0272791 ). The capacitor sequentially stores the charge voltage of the energy storage devices such that an analog-to-digital converter (ADC) connected to the capacitor may process a representation of the voltage of the energy storage devices.

These approaches are limited in accuracy, measurement speed and have the major drawback that they extract a substantial and variable amount of charge (current) from the energy storage devices.

Considering the above drawbacks, it is an object of the present invention to provide a measurement system and method for a series connection of energy storage devices with improved accuracy, improved speed, and decreased current consumption, i.e. decreased charge extraction, from the energy storage devices.

In particular, it is an object of the present invention to provide a measurement system and method for a series connection of energy storage devices with sufficient accuracy, sufficient speed, and sufficiently low current consumption to be used for controlling a continuous active energy storage device balancing system based on charge transfer. The measurement needs to be precise as the voltage differences relate to the unbalance of the series connection. A precision in the order of millivolts is required. The measurements of all energy storage devices need to be finished in a very short and limited time in the order of milliseconds. Most important, the measurement principle should not have an influence on the charge transfer of the balancing system between the energy storage devices.

It is a further object of the present invention to provide a system fulfilling high common-mode, and safety and isolation requirements.

SUMMARY OF THE INVENTION

The present invention is directed to a measurement system for measuring a series connection of energy storage devices comprising:

a. a number of energy storage devices (C(1 )...C(N)) in a series connection, b. a differential amplifier having a first input A and a second input B for coupling it over an individual storage device or over a section of the series connection comprising a plurality of said storage devices, said individual storage device or said section of the series connection having a more negative terminal and a more positive terminal,

c. a network of switching means for connecting said more negative terminal to said differential amplifier's input A, and said more positive terminal to said differential amplifier's input B, or vice versa.

Further, the present invention is directed to an assembly comprising a series connection of energy storage devices and a measurement system as described above.

Additionally, the present invention is directed to method for measuring a series connection of energy storage devices comprising the steps of:

a. providing a number of energy storage devices (C(1 )...C(N)) in a series connection,

b. providing a differential amplifier having a first input A and a second input B, c. selecting one individual energy storage device or a section of the series connection comprising a plurality of energy storage devices, said individual storage device or said section of the series connection having a more negative terminal and a more positive terminal, d. connecting said more negative terminal to said differential amplifier's input A, and said more positive terminal to said differential amplifier's input B, or vice versa.

Additionally, the present invention is directed to the use of such system and method for measuring a series connection of electric double-layer capacitors, lead acid or NiMH batteries, lithium energy storage devices or Li-ion energy storage devices.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG 1 to 14 illustrate several embodiments of a system or method in accordance with the present invention.

DESCRIPTION OF THE INVENTION

In the context of the present invention, an energy storage device may be any device adapted to store electrical charge, for example electrochemical batteries, capacitors, electric double-layer capacitors (EDLCs or so-called supercaps), lithium battery devices or lithium-ion battery devices.

According to a first embodiment of the present invention and as illustrated in FIG 1 , a measurement system is provided for measuring a series connection of energy storage devices comprising:

a. a number of energy storage devices (C(1 )...C(N)) in a series connection, each energy storage device,

b. a differential amplifier having a first input A and a second input B for coupling it over an individual storage device or over a section of the series connection comprising a plurality of said storage devices, said individual storage device or said section of the series connection having a more negative terminal and a more positive terminal,

c. a network of switching means for connecting said more negative terminal to said differential amplifier's input A, and said more positive terminal to said differential amplifier's input B, or vice versa. By providing for each energy storage device its own two switching means it is possible to connect every energy storage device or series connection of energy storage devices in two ways to the inputs of the differential amplifier: with the more positive terminal of the storage device to the positive input of the differential amplifier and the more negative terminal to the negative input, or in the opposite way with the more negative terminal of the storage device to the negative input of the differential amplifier and the more positive terminal to the positive input.

Additionally and as illustrated in FIG 2 for even N and FIG 3 for odd N, in another embodiment in accordance with the present invention, it is possible to limit the number of switching means by eliminating some of the switching means. The remaining switching means can be split in two groups, wherein for even N switching means A(1 ) to A(L) are adapted for connecting the more negative terminal of individual storage device C(N), C(N-2), C(2) to said amplifier's input A, and wherein switching means B(1 ) to B(M) are adapted for connecting the positive terminal of C(N) and the more negative terminal of individual storage device C(N-1 ), C(N-3), C(1 ) to said amplifier's input B, or vice versa for odd N.

The values of L and M depend on the number of energy storage devices which are subject of this measurement N. When N is odd, then L=(N+1 )/2 and M=(N+1 )/2. When N is even, then L=N/2 and M=N/2+1 .

Additionally, in another embodiment in accordance with the present invention, a method is provided for measuring a series connection of energy storage devices comprising the steps of:

In an embodiment according to the present invention, the energy storage devices may be measured one by one, or over a section of the series connection, wherein the differential amplifier is connected by an active network comprising two groups A and B of switching means (A1 ...A (L)) and (B1 ...B(M)). The step of connecting the more positive and more negative terminal of the selected energy storage device or of the section of the series connection occurs then for even N by connecting the more negative terminal of individual storage device C(N), C(N-2), or C(2) to said amplifier's input A by switching means A(1 ) to A(L), and connecting the positive terminal of C(N) or the more negative terminal of individual storage device C(N-1 ), C(N-3), or C(1 ) to said amplifier's input B by switching means B(1 ) to B(M), or vice versa for odd N.

In order to measure energy storage device C(N-1 ), the more negative terminal of individual storage device C(N) and the more negative terminal of individual storage device C(N-1 ) are simultaneously connected respectively to the amplifier's input A and input B. In order to subsequently measure energy storage device C(N-2), the more negative terminal of individual storage device C(N-1 ) and the more negative terminal of individual storage device C(N-2) are simultaneously connected respectively to the amplifier's input B and input A. Consequently, measuring consecutive energy storage devices results in an alternating sign of the voltage appearing at the differential amplifier's input (supposed that the voltages of the energy storage devices have an equal sign). This can be achieved using a differential amplifier that is able to cope with positive and negative voltages.

Alternatively measuring voltages of consecutive devices or sections in a series connection of energy storage devices as described above can be achieved by introducing a voltage polarity correction. This polarity change can be obtained at the input of said differential amplifier using an additional switching means as illustrated in FIG 4 or said voltage polarity change can be obtained by using an additional amplifier in series with said differential amplifier as illustrated in FIG 5 or said voltage polarity change is obtained by using an additional amplifier in parallel with said differential amplifier as illustrated in FIG 6.

By introducing this polarity change the full dynamic range of the ADC input can be used for measuring both positive and negative voltage inputs and a smaller effect of the differential amplifier input offset voltage is achieved.

An energy storage device is connected to the differential amplifier(s) by closing two switching means, one of both groups A or B. While these switching means are closed, the differential amplifier presents the measurement to the ADC which converts it to a digital value. To connect the adjacent energy storage device to the differential amplifier, one switching means (the one not common with both energy storage devices) has to be opened and the switching means of the same group A or B connected to that adjacent energy storage device has to be closed. So only one switching means has to be opened and one has to be closed to be ready for the next measurement.

The advantage of using two groups of switching means may be that a limited number of switching means is necessary on average, and except for the calibration, only one switching means per energy storage device is required.

Another advantage may be that the speed of the measurement can be increased as only one switching means needs to be opened. On the same group of switching means another (well chosen) switching means has to be closed to be ready for the next measurement. The absence of an intermediate capacitor needs to be charged and no extra switching means need to be opened or closed. Consequently, most of the measurement time may be spent on the analog-to-digital conversion.

Another advantage is that, because of the high common mode rejection of the differential amplifier, no capacitor needs to be charged, disconnected and connected to the ground of the measuring system to do the actual measuring. This results in a further advantage that only very low currents are drawn from the energy storage devices such that only very low voltage drop due to contact resistances occurs., thereby increasing the accuracy of the measurement system.

Optionally, each group of switching means may be extended with extra switching means to implement extra functions like calibration.

As each group of switching means has a common connection towards the amplifier and different nodes towards the energy storage devices, an unintentional connection between two nodes in the energy storage device group can occur when two switching means of the same group would be closed at the same time. Therefore, a safety component in a series connection with the switching means may be used to limit the current in such case.

In an embodiment of the present invention, an analog-to-digital converter (ADC) is used to digitize the output voltage of the difference amplifier. A controller controls the active network selecting the energy storage device to be measured and receives the digital value from the ADC. The controller processes the ADC value. The measurement result is eventually sent by the controller and/or stored in a memory. Additionally at least two additional amplifiers or diodes can be added at the input of the ADC for limiting the voltage swing at the ADC input terminals as illustrated in FIG 7. In an embodiment of the present invention, for safety reasons the active network may comprise resistors R(1 ...N+1 ) to limit the current to the differential amplifier's inputs in specific situations as illustrated in FIG 8. Such situation occurs for example when, by malfunction of the switching means control, in the same group of switching means the two switching means farthest away from each other, for example A1 and A(L), would be closed resulting in a short circuit at high current. Preferably, these resistors are sufficiently below the input resistance of the differential amplifier to reduce common mode influences in the difference amplifier.

In another embodiment of the present invention, likewise for safety reasons the active network may comprise fuses F(1 ..N+1 ) with appropriate voltage and current rating, which may be open permanently or temporarily as illustrated in FIG 9. For example, a resettable positive temperature coefficient (PTC) fuse can be used. In another embodiment of the present invention, likewise for safety reasons the active network may comprise resistors R(1 ..N+1 ) and fuses F(1 ..N+1 ) with appropriate voltage and current rating as illustrated in FIG 10.

Alternatively, the resistors and fuses may be replaced by any appropriate component which disconnects the energy storage devices from the groups of switching means if the current is larger than a certain threshold. For example, fusible resistors with appropriate power and voltage blocking rating and sufficiently low resistance may be used.

Additionally, special precautions can be taken in the control circuit to minimize the possibility of a short circuit in one of the groups of switching means. One possibility to achieve this is to have two decoders, one for each group of switching means as illustrated in FIG 1 1 . Such a decoder can only close one switching means within one group at a time.

In accordance with the present invention, the switching means may comprise for example a FET, or a combination of diode and FET, or optically isolated switches such as an optical relay or optoMOS, or an electromagnetic relay or transistors

The differential amplifier has two inputs A and B, the voltage difference between those inputs determining the amplifier's output voltage. Preferably, this amplifier may be sufficiently precise allowing the use of a large common mode component connected to the inputs. The amplifier needs to be able to measure positive and negative voltages in the required range or a polarity correction can be introduced.

In an embodiment in accordance with the present invention, high precision (trimmed) resistors may be placed at the input of the amplifier (e.g. component LT1990) in order to achieve high common mode rejection. These resistors should preferably be matched carefully, since connecting a voltage to the amplifier's inputs through unmatched external resistors with a non-negligible resistance compared to the input resistors of the amplifier will deteriorate the precision of the measurement significantly. Additionally, preferably the resistance of possible fuses should be matched to the resistance of the inputs of the amplifier. In case an amplifier is used that is able to handle positive and negative voltages without voltage correction, a correctly chosen amplifier output reference voltage is necessary. The difference between the amplifier output voltage and the output reference voltage is positive for a positive input voltage and negative for a negative input voltage. Preferably, the reference output voltage is set halfway of the supply voltage of the differential amplifier. In that case the full range of the energy storage device voltage should be covered by half of the voltage when only positive (or only negative) voltages are to be measured.

A controller and control circuit for controlling each of the switching means may be incorporated in the measurement system. Such controller may also receive the data from the analog-digital converters (ADC's), may process these measurements (e.g. sign inversion or calibration), and/or may send the information to another controller or store it in a memory.

Such control circuit having a different common mode voltage than the common mode voltage (the average voltage over an energy storage device) over the switching means itself. This voltage difference may be bridged in the following ways:

A first possibility is to use a series of components (e.g. component DG444 (Vishay Siliconix, Intersil) or ADG444 (Analog Devices)) to translate the 'close' command from the control circuit to the gate of a switching means at a different common mode voltage. A drawback thereof is that charge injection occurs in the line to the amplifier's input, thereby influencing the measurement and allowing merely limited common mode voltage.

Another possibility is to bridge the common mode voltage using optical coupling, for example by using OptoMOS switches (e.g. component TLP172). These provide an optical connection between the control circuit and the switching means. This way the control circuit is isolated from the measurement circuit. These switching means are small and fast and have a negligible influence on the high precision difference amplifier as they don't inject charge in the line to the amplifier's input.

Another possibility may be using transistors or electromagnetic relays. The controller usually comprised a processor and may directly control the switching means by applying an appropriate sequence. Such processor may preferably use one output for each switching means.

Preferably, the controller may be used in combination with decoders (e.g. 74HC238). This results in a limited number of outputs of the controller to communicate with the encoders. If one encoder is used per group of switching means, then the specific decoder chosen can only close one switching means at a time. This way it is impossible to actively order two switching means to close at the same time. The specific switching means used can have a non-zero switching means-off time, so a small overlap is still possible when one switching means is opened and the next is closed immediately. Even with the decoder a small time of no closed switching means is necessary to be sure no switching means of the same group are closed at the same time. To change from one closed switching means to the next closed switching means in the group, the enable signals are first used to mask the output of the decoder resulting in opening of all switching means connected to that decoder (all switching means of that group). The address for the converter is changed by the controller and after that output is unmasked by applying the correct enabling signals to the decoder.

As already explained above, in order to minimize the possibility of a short circuit in one of the groups of switching means, a single decoder per group of switching means may be used. By doing so while applying the proposed masking method before changing the address of the decoder, no two switching means of the same group can be closed at any time.

When the state change of a decoder would be faster than the turn-off time of the switching means, a small overlap may exist of two closed switching means at the same time. Therefore, a correctly chosen current limiting component in a series connection with the switching means can be used to sufficiently limit the current through the switching means and make this situation not destructive for the system in short term. Alternatively to decoders, a ripple register may be used. This combination of flip-flops will send on every clock pulse the 'close' signal to the next switching means. This way only the start signal needs to come from the processor and with every clock pulse the next energy storage device can be measured. The processor needs to keep track of the energy storage device being measured at a certain time.

Additionally, the controller may be responsible for the calibration of the measurements. Calibration is necessary to acquire measurements with high accuracy. Calibration needs to be repeated from time to time, to be able to take aging and environmental influences into account.

Two types of calibration may be possible. In the first one, the measurement system is calibrated using reference values. Several measurements of zero volts or high precision reference voltage at different common mode voltages make it possible to estimate the offset and gain values depending on the environmental factors and the common mode voltages of the cells. One possible embodiment of this is illustrated by scheme in FIG 7 for even N and FIG 8 for odd N. For this illustration, the calibration method is:

• receive the ADC result for an amplifier input of zero volts difference with common mode of the more negative side of energy storage device C(1 ) (in FIG 7 assert signals A(cali1 ) and B(1 ), in FIG 8 assert signals A(1 ) and B(cali)), the result is Vzerolow;

• receive the ADC result for an amplifier input of a high precision voltage Vref difference voltage with minus side equal to the more negative side of energy storage device C(1 ) (in FIG 7 and FIG 8 assert signals A(cali1 ) and B(cali)), the result is Vgainref;

• measure zero volts with common mode of the more positive side of a top energy storage device of the group (in FIG 12 and FIG 13 assert signals A(cali2) and B(M)), the result is Vzerohigh;

These measurements of a high precision voltage reference or common mode can be implemented on every cell, not only as illustrated in FIG 12 and 13. The difference between Vzerolow and Vzerohigh can be used to know the influence of the common mode variation. If the difference is low, a good measurement is obtained while only taking into account Vzerolow (Vzero=Vzerolow). When the difference between Vzerolow and Vzerohigh is significant, a zero voltage reference can be obtained by applying the following equation for the measurement of energy storage device n of the total of N energy storage devices:

Vzero, n = Vzerol ow + (Vzerohigh— Vzerol ow)——— , . .

i¥ ( 1 )

This calibration can be performed regularly. The calibration can be performed before every measurement of all energy storage device voltages. This allows to take into account the current common mode voltages, temperature and other environmental influences.

The second type of calibration is based on every energy storage device individually: A special reference group can be attached to the system where all energy storage devices can be put on a high-precision reference voltage and a certain common mode voltage. This way a table can be created with a zero value (Vzero, n) and a gain value (Vgainref,n) for every energy storage device. This off-line calibration doesn't allow taking into account aging and drift phenomena as well as temperature and other influences. When the data obtained from the ADC for the measurement of an energy storage device is Vadc, then the actual energy storage device voltage is (using the appropriate Vgainref and Vzero):

Vadc— Vzero

Vref

(2)

When the measured energy storage device was inversely connected to the differential amplifier (due to the specific switching means connection), than the actual energy storage device voltage is -Vmeas.

In an embodiment of the present invention, the controller may filter the measurements, for example by taking the average of a number of measurements (for example of 15 samples) and/or by applying a finite impulse response (FIR) filter to obtain a weighted sum of 15 samples with an equal factor for all samples.

In an embodiment in accordance with the present invention, an assembly comprising a series connection of energy storage devices and a measurement system as described above is provided.

A typical measurement cycle is illustrated in FIG 14.

In a particular embodiment in accordance with the present invention, an assembly comprising a series connection of energy storage devices, an energy balancing system, and a measurement system as described above is provided.

When a balancing system is connected to the energy storage devices and the same connections are used for the measurement system as for the balancing system, one system can influence the other. The low currents required for the voltage measurement system don't influence the balancing system.

The balancing system can however influence the measurement system when a significant unbalance is present in the system. This unbalance will introduce large currents through the connections. These currents introduce voltage drops over connection and contact resistances. This influences the apparent energy storage device voltage at the differential amplifier. To eliminate this influence, the balancing mechanism can be adaptively controlled during measurement of the energy storage device voltages.

A method and system as described above may be used for measuring a series connection of electric double-layer capacitors, lithium energy storage devices or Li- ion energy storage devices.

Claims

CLAIMS:

1 . A measurement system for measuring a series connection of energy storage devices comprising:

2. A measurement system according to claim 1 , further comprising an analog-to- digital converter (ADC).

3. A measurement system according to claims 1 or 2, comprising an additional switching means for obtaining an output voltage polarity change of one or more individual storage devices or one or more sections of the series connection at the input of said differential amplifier.

4. A measurement system according to claims 1 or 2, comprising an additional amplifier for obtaining an output voltage polarity change of one or more individual storage devices or one or more sections of the series connection, said additional amplifier being in series or in parallel with said differential amplifier.

5. A measurement system according to claim 4, comprising at least two additional amplifiers or diodes at the input of the ADC for limiting voltage swing at the ADC input terminals.

6. A measurement system according to claims 1 to 5, wherein said network of switching means is split into two groups, wherein for even N switching means A(1 ) to A(L) are adapted for connecting the more negative terminal of individual storage device C(N), C(N-2), C(2) to said amplifier's input A, and wherein switching means B(1 ) to B(M) are adapted for connecting the positive terminal of C(N) and the more negative terminal of individual storage device C(N-1 ), C(N-3), C(1 ) to said amplifier's input B, or vice versa for odd N.

7. A measurement system according to any of the above claims, further comprising a controller in combination with separate decoders for groups of switching means.

8. An assembly comprising a series connection of energy storage devices and a measurement system according to any of the above claims.

9. A method for measuring a series connection of energy storage devices comprising the steps of:

b. providing a differential amplifier having a first input A and a second input B, c. selecting one individual energy storage device or a section of the series connection comprising a plurality of energy storage devices, said individual storage device or said section of the series connection having a more negative terminal and a more positive terminal,

d. connecting said more negative terminal to said differential amplifier's input A, and said more positive terminal to said differential amplifier's input B, or vice versa.

10. A method for measuring a series connection of energy storage devices according to claim 9, wherein the output voltage polarity of one or more individual storage devices or one or more sections of the series connection is changed

1 1 . A method for measuring a series connection of energy storage devices according to claims 9 or 10, comprising limiting voltage swing at the ADC input terminals by adding at least two additional amplifiers or diodes.

12. A method for measuring a series connection of energy storage devices according claims 9 to 1 1 , comprising splitting said network of switching means in one or more groups that are adapted for connecting the more negative terminal of individual storage device C(N), C(N-2), C(2) and one or more groups that are adapted for connecting the positive terminal of C(N) and the more negative terminal of individual storage device C(N-1 ), C(N-3), C(1 ) to said amplifier's input B, or vice versa.

13. A method for measuring a series connection of energy storage devices according claims 9 to 12 comprising controlling the network by using a controller in combination with separate decoders for groups of switching means.

14. Use of a measurement system according to claims 1 to 8 for measuring a series connection of electric double-layer capacitors, lead acid or NiMH batteries, lithium energy storage devices or Li-ion energy storage devices.

15. Use of a method according to claims 9 to 13 for measuring a series connection of electric double-layer capacitors, lead acid or NiMH batteries, lithium energy storage devices or Li-ion energy storage devices.