WO2008151659A2

WO2008151659A2 - System and method for equalizing state of charge in a battery system

Info

Publication number: WO2008151659A2
Application number: PCT/EP2007/055712
Authority: WO
Inventors: Bertil Nygren; Jan R. Svensson; Willy Hermansson; Magnus Callavik
Original assignee: Abb Research Ltd
Priority date: 2007-06-11
Filing date: 2007-06-11
Publication date: 2008-12-18
Also published as: WO2008151659A3

Abstract

A high voltage battery system used in electrical transmission or distribution applications comprising a plurality of high temperature batteries (10) connected in series, and each battery (10) is adapted with; a battery heating means (14), a battery cooling means (13), auxiliary electronic equipment (11) powered by the battery (10) and controlled by a master control function (9). The auxiliary electronic equipment (11) comprises measuring means and communication means (17). In the system the battery heating means (14) of each battery is powered by the battery (10) and each battery heating means (14) is controlled by the auxiliary electronic equipment (11). Furthermore, the system is adapted with master control function (9) for equalizing state of charge across the plurality of high temperature batteries (10).

Description

System and method for equalizing state of charge in a battery system 2007-06-11

TECHNICAL AREA

The present invention relates to a system and method for equalizing the state of charge of individual batteries in a battery string. The battery string delivers a high DC voltage and the battery string is used in a transmission/distribution system.

TECHNICAL BACKGROUND

Large battery systems have a number of different uses in transmission/distribution systems. The batteries can be used for load leveling, peak shaving and power conditioning applications. They can provide backup power at a power failure e.g. at essential installations such as a hospital. The batteries can also be used to supply additional power to installations when the need for power peaks locally e.g. steel smelter or when a train starts.

Molten salt batteries is a class of high temperature electric batteries that use molten salts as electrolyte. The batteries offer high energy density and high power density but have to be operated at a high temperature, around 300 degrees C. A battery unit comprises a heat insulated box containing a plurality of series connected battery cells . The battery unit has two terminals comprising an electric circuit in the range of 1.5 kV. For example, connecting four such battery units in series will thus reach a voltage level of 6 kV and more battery units in series will create a battery string with even higher voltage. Two or more such battery strings can be connected in parallel to allow more power to be stored in the system.

A criterion for the function of the battery, e.g. to be able to store and release electric energy, is that the temperature inside the battery cell is kept approximately between 270⁰C and 340⁰C. At operation mode such as when the battery is being charged or discharged heat is generated within the battery. At idling mode or standby mode, however, no heat is generated inside the battery. The batteries are insulated but they have a continuous heat loss to ambient. Thus, in order to maintain the temperature in the working range, the batteries have to be heated in idling mode or standby mode.

One possible way to heat the batteries is to supply heat by an external source, for example, hot air from the outside to the battery compartment. However, this is complicated since in the battery string, several batteries are connected in series and the batteries are on a high potential, whereas the heat generator is not. Furthermore the heat piping system adds complexity and cost to large battery systems.

Another way to heat the batteries is by fitting all the batteries with a heating resistor e.g. a heating mat. This heating resistor could be powered from the outside but with many batteries in series in a high voltage application, it could be difficult to supply electric energy at a high potential. A solution to this problem would then be to power the heating resistor from the battery itself. This solution will generate more imbalances in state of charge of the different batteries in the string. Due to minor property differences between the individual batteries there will always be different heat losses. Normally, when one battery needs heating, all the batteries are heated with the same current to minimize the resulting difference in state of charge between the batteries in the string. The current drawn from batteries to heat are measured but due to measurement uncertainties and minor differences they are not exactly known. These uncertainties in current measurement will over time generate unbalances in the state of charge in the batteries.

A further source for unbalances in the state of charge in the series-connected batteries is the possible leak current between poles and the enclosure of the batteries which may vary between batteries and the size of this leak current is generally unknown.

When several batteries are connected in a string, it is essential to maintain the deviations of charge status as small as possible between the individual batteries in the battery string. The battery with the lowest state of charge will determine the total capacity of the battery string at discharging. During charging, the battery with the highest state of charge will be at risk of being over-charged, and over-charging a battery can damage it. Thus, charging the battery string have to stop when one battery reaches fully charged, therefore, unbalance in state of charge will limit the maximum energy stored in a battery string.

WO2006015083 (Moore) entitled "METHOD AND APPARATUS FOR BALANCING MULTI-CELL LITHIUM BATTERY SYSTEMS" describes a method and apparatus that equalize cell-to-cell imbalances in a multi-cell lithium battery system. Each cell is adapted with a switched balancing resistor that selectively shunts selected cells with selected value resistors to remove charge from the highest charged cells. Energy drawn by the balancing resistors is lost. The solution suggested by WO2006015083 is in the field of low voltage (cell voltage 3, 6V) , low temperature battery cells. The invention suggests that the method should be used when charging the batteries.

US2007046260 (ISHIKAWA) entitled "Apparatus for regulating state of charge in battery assembly" describes an apparatus for regulating state of charge in a battery assembly. They propose an apparatus including: discharge resistors which discharge a plurality of unit cells included in the battery assembly and connected in series. The suggested solution would be very difficult to implement in a high voltage battery system since all the connections and switches have to be able to isolate the full DC voltage of the battery string. The energy drawn by the balancing resistors is lost.

SUMMARY OF THE INVENTION

An embodiment of the present invention is to provide a system used in electrical transmission or distribution applications wherein the battery heating means, which draws power from the battery, are used to equalize the state of charge of the different batteries connected in series in a battery string.

An embodiment of the present invention is to draw energy from each battery with a higher state of charge than the battery with the lowest state of charge until all batteries have equal state of charge.

An embodiment of the present invention is to adapt each battery with an auxiliary electronic equipment powered by the battery and adapted with measuring means and communication means and the auxiliary electronic equipment is controlled by a master control function. The auxiliary electronic equipment controls the battery heating means which heats the battery. The master control function can be a centralized control system or a distributed control system and the master control function has information of the state of charge of all the batteries in the string. The master control function is adapted to equalize the state of charge across the plurality of high temperature batteries.

According to an embodiment of the invention, the master control function is adapted to equalize the state of charge of all batteries by letting said auxiliary electronic equipment draw an individual current from each battery with a higher state of charge than the battery with the lowest state of charge and said individual current powers the battery heating means .

According to an embodiment of the invention, the measuring means is adapted to measure a plurality of quantities that allows the state of charge to be determined.

According to an embodiment of the invention, the auxiliary electronic equipment communicates said measured quantities and/or the state of charge of the individual battery to the master control function over the communication means.

According to an embodiment of the invention, said master control function is adapted to determine and store the state of charge of all the batteries based on communicated measurements or store the communicated state of charge values from the auxiliary electronic equipment .

According to an embodiment of the invention, the master control function is adapted to determine the state of charge of the plurality of high temperature batteries based on measurements and the determined state of charge adjusted using stored operational data such as operating time, number of cycles, depths of discharge, cell failures, etc. According to an embodiment of the invention, if the temperature of a battery exceeds a threshold level during equalizing of state of charge said auxiliary electronic equipment connects the battery cooling means which can be an external cooling air flow or a cooling air fan on the battery.

According to an embodiment of the invention, the equalizing by drawing a current from the batteries is done when the battery system is idling or in standby i.e. when the battery string is not charging or discharging.

According to an embodiment of the invention, the equalizing by drawing a current from the batteries is done when the battery system is charging or discharging.

According to another embodiment of the invention, a method for equalizing the state of charge of a plurality of high temperature batteries connected in series and used in electrical transmission or distribution applications, where the state of charge of each battery can be determined using the steps

- determining how much charge has to be drawn from each battery to equalize the state of charge, and - connecting an individual battery heating load to each battery, and

- drawing from each battery the charge needed to equalize the state of charge of all batteries, and

- disconnecting the individual battery heating load of each battery.

According to an embodiment of the invention, a method comprises the step of controlling the charge drawn from each battery by measuring the current going through the individual battery heating load and controlling the time the heating load is connected to each battery.

According to an embodiment of the invention, a method comprises the step of cooling the battery if the battery temperature becomes too high from current going through the individual battery heating load during equalizing the state of charge .

Even when batteries are not delivering power, a small current is drawn from each battery to heat the battery and to drive measurement and communications equipment. The small current continuously drawn from the batteries will have measurement and control errors and this will, over time add to further unbalance of the state of charge status of the batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings constitute a part of this specification and include exemplary embodiments to the invention, which may be embodied in various forms.

Figure 1 shows schematically a battery string.

Figure 2 shows in more detail one battery in the battery string. Figure 3 illustrates schematically how the charge status of the different batteries changes with time during equalizing when charging the batteries.

Figure 4 illustrates schematically the current taken from the individual batteries during charging of the battery. Figure 5 illustrates schematically how the charge status of the different batteries changes with time during equalizing at standby .

DETAILED DESCRIPTION OF THE DRAWINGS Detailed descriptions of the preferred embodiment are provided herein. It is to be understood, however, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure or manner.

Figure 1 shows schematically a battery string. The battery string 5 comprises a number of batteries 1-4 that are connected in series to provide high voltage DC power. Each individual battery 1-4 is built up of several battery cells all enclosed in a common enclosure. An industrial battery system can comprise several battery strings with batteries connected in series and several strings that are connected in parallel .

Figure 2 shows in more detail one battery in the battery string. A battery 10 comprises an enclosure 15 which holds a number of battery cells 16. The enclosure 15 acts as a thermal isolation that keeps the cells 16 at a working temperature. The battery 10 is connected 12 in series with other batteries in the string. Each battery has an individual auxiliary electronic equipment 11 connected in parallel with each battery getting energy supply from the individual battery. The auxiliary electronic equipment 11 comprises measurement equipment for e.g. battery voltage, battery temperature, battery isolation status and temperature control, communication means which could be wireless transmission equipment 17 for the measured battery data to a master control function 9. The master control function 9 could be a centralized control system or a distributed control system. The main function of the master control function 9 is to collect measurements from the measurement means on all batteries, calculate the state of charge of all batteries, and communicate the difference in state of charge or how much charge each battery has to discharge to equalize the battery string. The master control function 9 also comprises storage means for process parameters, measured values and system data, e.g. operating times, number of cycles, depth of discharge of cycles, battery temperatures, cell failures, loading currents, discharging currents, heating currents, voltages etc. With this information the master control function 9 is adapted to estimate how different batteries will perform and possibly adapt the target state of charge for a battery in view of these historical data.

The auxiliary electronic equipment 11 further comprises a power supply for the DC-heating of elements 14 to keep the battery temperature at the operating temperature level in e.g. stand-by mode. The auxiliary electronic equipment 11 also controls cooling means 13 of the battery which can lower the battery temperature if it becomes too high. The cooling means 13 can be a connection to an external cooling air source or a fan powered by the battery and controlled by the auxiliary electronic equipment 11.

Figure 3 illustrates schematically how the state of charge status of the different batteries changes with time during charging. The state of charge of the different batteries has during charging and discharging of the batteries over time begun to differ and the state of charge are now calibrated or leveled according to an embodiment of the present invention. At the time ti the state of charge status of the batteries are 60%, 55%, 65% and 62% respectively, and the battery is starting to be charged. The charging continues and at time t₂ the third battery "3" has reached a state of charge of 100%. The charging of the battery string continues and energy is drawn from the third battery to prevent it from over-charging. At the time t₃, the forth battery "4" has reached a state of charge of 100% and energy is now also drawn from this battery to prevent it from over-charging. At the time t₄, the first battery "1" has reached a state of charge of 100% and energy is now also drawn from this battery to prevent it from overcharging. The charging continues until all batteries have reached 100% and the individual batteries in the battery string are now fully balanced.

Figure 4 illustrates schematically the current taken from the individual batteries during equalizing of the individual batteries in the battery string 5. All batteries have similar construction. Each battery unit is connected to an individual auxiliary electronic equipment 11 comprising a plurality of sensing transducers for measurement of battery status and communication electronics. The auxiliary electronic equipment 11 draws a current from the battery and feeds the battery heating means 14 e.g. a heating mat. The communication module on the auxiliary electronic equipment 11 is galvanically isolated and thus at the same potential as the battery unit. The module may communicate within a wireless local area network, such as a WLAN or a Bluetooth network. The sensed battery values, such as voltages, currents and temperatures are preferably transmitted in digital form. To save power consumption the communication can be arranged in short part of a time period. Thus the communication means need only be electrified during a small percentage of time. The communication may preferable take place within the 2 GHz band. The power supply comprises the auxiliary electronic equipment 11 can be arranged with a back up battery or alternative electric energy providing means. The equipment 11 also powers the heating element 14 in the battery which is used to heat the battery.

If the battery temperature exceeds an upper operating temperature during charging, discharging or equalizing, the battery temperature will be cooled by the battery cooling means .

One embodiment of the present invention is to cool the batteries with a flow of external cooling air, as in figure 4. The flow of this external air is controlled 24 by the individual auxiliary electronic equipment 11. The individual pipes 22 leads air from the source of external cooling air 23 to each battery. The pipes are adapted with a valve means 21 that is controlled 24 by the individual auxiliary electronic equipment 11.

In another embodiment of the present invention, the individual battery is cooled by a fan which is powered by the battery. The operation of the fan is controlled by the individual auxiliary electronic equipment 11.

The currents drawn from the individual batteries during equalizing at a charging situation described in figure 3 will be explained in the following.

Before and at t₂ the currents I₁-I₄ drawn from the individual battery to heat it are zero.

Between the time t₂ and t₃, the current I₃ is non-zero and the currents I₁, I₂ and I₄ are zero.

Between the time t₃ and t₄, the currents I₃ and I₄ are non-zero and the currents I₁, I₂ are zero.

Between the time t₄ and until all the individual batteries have been fully charged the currents I₁, I₃, and I₄ are non-zero and the current I₂ is zero. It should be noted that if the battery heating load is connected first when a battery is fully charged, the charging current of the battery string can not be larger than any of the currents I₁-I₄ drawn from the individual battery otherwise the batteries could be damaged by overcharging.

Another way of equalizing the batteries during charging would be to connect the battery heating directly when the charging starts or a short time after the charging starts. For example, if the difference between two batteries in state of charge is 0,5 Ah and the battery heating draws 0.5 A, connecting the battery heating to draw a current of the battery with the highest state of charge for 60 minutes would equalize the two batteries and the charging current of the battery string could exceed the 0.5 A significantly without risking damages from overcharging.

Yet another way of equalizing the state of charge, during stand-by of the battery string, would be to draw a current, to heat the battery, from all the batteries in the string that have higher state of charge than the one with the lowest state of charge. One will draw so much charge from each battery that the charge of all batteries will equal the one with the lowest state of charge.

Figure 5 illustrates schematically how the state of charge status of the different batteries changes with time when equalizing the battery string at standby. The top diagram, at ti, shows the state of charge of four different batteries in series and the state of charge status of the batteries are 60%, 55%, 65% and 62% respectively. The batteries are now being equalized according to an embodiment of the present invention . The second battery 2 is identified as the one having the lowest state of charge and the aim of the equalizing method according to an embodiment of the present invention is to reduce the state of charge of all the batteries in the string to the same state of charge as the one with the lowest state of charge. At the time ti the batteries are starting to be equalized by drawing a current from batteries 1, 3 and 4. At time t₂ the first battery 1 has reached a state of charge equal to battery 2 which is the one with the lowest state of charge. After time t₂ the heating of battery 1 is disconnected but the heating of battery 3 and 4 continues. At the time t₃, the forth battery 4 has reached a state of charge equal to battery with the lowest state of charge. After time t₃ the heating of battery 4 is disconnected but the heating of battery 3 continues. At time t₄ the third battery 3 has reached a state of charge equal to battery with the lowest state of charge and all the batteries now have the same state of charge. The battery string is now equalized.

During operation of the battery string all batteries in the battery string will be both charged and discharged by the same current . No imbalance of state of charge status between individual batteries will be created. However, at the stand-by mode when the current is zero, the thermal losses in an individual battery have to be compensated for by heating the battery by a current drawn from the battery.

Accurate measurements of the heating currents Ii to I₄ can be used to monitor the discrepancies that are created due to the individual differences in heat losses. If one battery in the string has lost too much charge (identified by measuring Ii to I₄) , the other batteries has to be discharged by heating the heating element and at the same time be cooled in order to remain at the chosen temperature.

Simultaneously the whole battery string will be charged by current to compensate for average charge losses in the battery string during heating.

The current measurements of Ii to I₄ are not error free and over time there will slowly develop differences in state of charge of the batteries that can be detected by accurate measurements of the voltage and other battery parameters across the individual batteries during stand-by, during discharge or during charge cycles.

When the difference in voltage is greater than a certain value a new calibration to 100% state of charge is made with the method presented in the present invention.

It should be noted that while the above describes exemplifying embodiments of the invention, there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention as defined in the appended claims.

Claims

1. A high voltage battery system used in electrical transmission or distribution applications comprising a plurality of high temperature batteries arranged in at least one string of batteries connected in series, and each battery is adapted with; a battery heating means, and a battery cooling means, and an auxiliary electronic equipment powered by the battery and controlled by a master control function, the auxiliary electronic equipment comprising measuring means and communication means, characterized in that the battery heating means of each battery is powered by the battery and each battery heating means is controlled by the auxiliary electronic equipment and said master control function, which has information on the state of charge of all batteries, is adapted to equalize the state of charge across the plurality of high temperature batteries .

2. A battery system according to claim 1, characterized in that said master control function is adapted to equalize the state of charge of all batteries by letting said auxiliary electronic equipment draw an individual current from each battery with a higher state of charge than the battery with the lowest state of charge and said individual current powers the battery heating means .

3. A battery system according to any of the claims 1-2, characterized in that said measuring means is adapted to measure a plurality of quantities that allows the state of charge to be determined.

4. A battery system according to any of the claims 1-3, characterized in that said auxiliary electronic equipment communicates said measured quantities and/or the state of charge of the individual battery to the master control function over the communication means.

5. A battery system according any of the claims 1-4, characterized in that said master control function is adapted to determine and store the state of charge of all the batteries based on communicated measurements or store the communicated state of charge values from the auxiliary electronic equipment .

6. A battery system according to claim 5, characterized in that said master control function is adapted to determine the state of charge of the plurality of high temperature batteries based on measurements and the determined state of charge is adjusted using stored operational data such as operating time, number of cycles, depths of discharge, cell failures, etc.

7. A battery system according to any of the claims 1-6, characterized in that if the temperature of a battery exceeds a threshold level during equalizing of state of charge said auxiliary electronic equipment connects the battery cooling means .

8. A battery system according to claim 7, characterized in that said battery cooling means comprises a controlled external cooling air flow.

9. A battery system according to claim 7, characterized in that said battery cooling means comprises a cooling air fan on the battery controlled and powered by the auxiliary electronic equipment .

10. A battery system according to any of the claims 1-9, characterized in that the equalizing by drawing a current from the batteries is done when the battery system is idling or in standby.

11. A battery system according to any of the claims 1-9, characterized in that the equalizing by drawing a current from the batteries is done when the battery system is charging or discharging.

12. A method for equalizing the state of charge of a plurality of high temperature batteries connected in series and used in electrical transmission or distribution applications, where the state of charge of each battery can be determined, comprising the steps of

- determining how much charge has to be drawn from each battery to equalize the state of charge, and

- connecting an individual battery heating load to each battery, and

- disconnecting the individual battery heating load of each battery .

13. A method according to claim 12, further comprising the step of controlling the charge drawn from each battery by measuring the current going through the individual battery heating load and controlling the time the heating load is connected to each battery.

14. A method according to any of the claims 12-13, further comprising the step of cooling the battery if the battery temperature becomes too high from current going through the individual battery heating load during equalizing the state of charge .

15. A method according to claim 14, further comprising the step of cooling the battery by opening a valve means to an external cooling air source.

16. A method according to claim 14, further comprising the step of cooling the individual battery by operating an individual cooling fan powered by the individual battery.

17. A method according to any of the claims 12-16, further comprising the step of equalizing the state of charge of the batteries during charging of the battery string.

18. A method according to any of the claims 12-16, further comprising the step of equalizing the state of charge of the batteries during stand-by or idling of the battery string.

19. A method according to any of the claims 12-16, further comprising the step of equalizing the state of charge of the batteries during discharging of the battery string.

20. A method according to claim 17, further comprising the step of equalizing the state of charge of the batteries during charging to full charge state.

21. A method according to claim 17, further comprising the step of equalizing the state of charge of the batteries during charging to a predefined state of charge.

22. A method according to claim 17, further comprising starting to draw the current from each battery at different times so that all the batteries reach the same state of charge level at the same time.