DC-DC-converter-based active voltage-balancing system and method for parallel battery packs

1. A balancing circuit for actively balancing voltages of battery packs to be connected in parallel, the balancing circuit comprising:

IN-side switches;

OUT-side switches;

a direct current-direct current (DC-DC) converter having an IN terminal connected to the IN-side switches and an OUT terminal connected to the OUT-side switches; and

a controller configured to operate an IN-side switch to connect a V max battery pack having a highest voltage of the battery packs to the IN terminal, operate an OUT-side switch to connect a V min battery pack having a lowest voltage of the battery packs to the OUT terminal, and activate the DC-DC converter to transfer energy from the V max battery pack to the V min battery pack;

wherein the controller is further configured to

respond to an IN terminal voltage being sufficiently close to a voltage of a first battery pack by operating the IN-side switch to connect the first battery pack to the IN terminal, and

respond to an OUT terminal voltage being sufficiently close to a voltage of a second battery pack by operating the OUT-side switch to connect the second battery pack to the OUT terminal.

2. The balancing circuit of claim 1 , wherein, when all of the battery packs are connected to the DC-DC converter, the controller is configured to respond to a voltage difference among the battery packs being sufficiently small by stopping operation of the DC-DC converter.

3. The balancing circuit of claim 2 , wherein the controller is further configured to respond to a voltage difference among the battery packs being sufficiently small by opening the OUT-side switches and closing the IN-side switches to connect the battery packs to the IN terminal, closing one or more contractors of a system DC bus, operating the DC-DC converter to pre-charge the system DC bus, discontinue operating the DC-DC converter, and connecting the battery packs to the system DC bus.

4. The balancing circuit of claim 1 , wherein the DC-DC converter is one of a buck converter or a buck-boost converter.

5. The balancing circuit of claim 1 , wherein the controller is configured to rank voltages of the battery packs from a highest voltage to a lowest voltage or from a lowest voltage to a highest voltage.

6. A method of pre-charging a system direct current (DC) bus coupled to a plurality of battery packs to be connected in parallel, the method comprising:

ranking the plurality of battery packs in order of voltage;

connecting a first battery pack with a highest voltage to an IN terminal of a DC-DC converter;

connecting a second battery pack with a lowest voltage to an OUT terminal of the DC-DC converter;

converting an energy from the first battery pack to the second battery pack through the DC-DC converter,

connecting a third battery pack to the IN terminal in response to a voltage of the third battery pack being within a first threshold voltage difference of a voltage of the IN terminal;

connecting a fourth battery pack to the OUT terminal in response to a voltage of the fourth battery pack being within a second threshold voltage difference of a voltage of the OUT terminal; and

when the voltages of all of the battery packs are within a third threshold voltage difference of one another, connecting all of the battery packs to the IN terminal and connecting the OUT terminal to the system DC bus to pre-charge the system DC bus.

7. The method of claim 6 , wherein the connecting of the first battery pack with a highest voltage to the IN terminal of a DC-DC converter includes operating a controller to close an IN-side switch connected between the first battery pack and the IN terminal.

8. The method of claim 6 , wherein the connecting of the second battery pack with a lowest voltage to the OUT terminal of the DC-DC converter includes operating a controller to close an OUT-side switch connected between the second battery pack and the OUT terminal.

9. An energy storage system, comprising:

a plurality of battery packs to be connected in parallel;

a balancing circuit coupled to the plurality of battery packs; and

a pair of contactors coupled to the balancing circuit to connect the plurality of battery packs to a system direct current (DC) bus;

wherein the balancing circuit includes

a DC-DC converter having an IN terminal and an OUT terminal,

a plurality of IN-side switches connected between the plurality of battery packs and the IN terminal,

a plurality of OUT-side switches connected between the plurality of battery packs and the OUT terminal, and

a controller having a memory with instructions that, when executed, cause the controller to

control the IN-side switches and the OUT-side switches to connect a battery pack of the plurality of battery packs having a highest voltage to the IN terminal,

connect a battery pack of the plurality of battery packs having a lowest voltage to the OUT terminal,

connect any remaining battery pack of the plurality of battery packs to the IN terminal in response to a voltage of said remaining battery pack being sufficiently close to a voltage of the IN terminal, and

connect the any remaining battery pack to the OUT terminal in response to a voltage of said remaining battery pack being sufficiently close to a voltage of the OUT terminal.

10. The energy storage system of claim 9 , wherein the controller is further configured to respond to the voltages of the plurality of battery packs being balanced through operation of the DC-DC converter by connecting the plurality of battery packs to the IN terminal, closing system DC-bus contactors, and using the DC-DC converter to pre-charge a system DC bus.

11. The energy storage system of claim 9 , wherein the DC-DC converter is one of a buck converter or a buck-boost converter.

12. The energy storage system of claim 9 , wherein the controller is configured to rank voltages of the battery packs from a highest voltage to a lowest voltage or from a lowest voltage to a highest voltage.

13. A balancing circuit for actively balancing voltages of a plurality of battery packs to be connected in parallel, the balancing circuit comprising:

a plurality of direct current-direct current (DC-DC) converters corresponding to the plurality of battery packs, each DC-DC converter having an IN terminal and an OUT terminal;

a plurality of bypass switches, each bypass switch connected between an IN terminal and an OUT terminal of a respective one of the plurality of DC-DC converters; and

a controller configured to

connect a battery pack of the plurality of battery packs having a lowest voltage to the OUT terminal,

operate one or more DC-DC converters of the plurality of DC-DC converters corresponding to one or more higher-voltage battery packs of the plurality of battery packs having initial voltages greater than an average battery pack voltage (V ave ) to charge the OUT terminal,

connect one or more lower-voltage battery packs of the plurality of battery packs having initial voltages less than V ave to the OUT terminal when an initial voltage of a lower-voltage battery pack of one or more lower-voltage battery packs is sufficiently close to a current voltage of the OUT terminal, and

deactivate a DC-DC converter corresponding to a higher-voltage battery pack of one or more higher-voltage battery packs when a current voltage of the corresponding higher-voltage battery pack is sufficiently close to V ave .

14. The balancing circuit of claim 13 , wherein the controller is further configured to respond to all lower-voltage battery packs being connected to the OUT terminal and all higher-voltage battery packs being sufficiently close to V ave by disconnecting all battery packs from the OUT terminal, closing contactors of a system DC-bus, and pre-charging the system DC bus with each DC-DC converter of said one or more DC-DC converters.

15. The balancing circuit of claim 14 , wherein the controller is further configured to respond to completion of pre-charging the system DC bus by discontinuing operation of said each DC-DC converter, and connecting all of the battery packs to the system DC bus through the plurality of bypass switches.

16. A method of pre-charging a system direct current (DC) bus coupled through a DC bus contactor to a plurality of battery packs to be connected in parallel, the method comprising:

ranking the plurality of battery packs in order of voltage;

calculating an average battery pack voltage;

identifying higher-voltage battery packs as battery packs of the plurality of battery packs having initial voltages that are greater than the average battery pack voltage;

identifying lower-voltage battery packs as battery packs of the plurality of battery packs having initial voltages that are less than the average battery pack voltage;

connecting a first battery pack with a lowest voltage to the DC bus contactor;

activating DC-DC converters connected to the higher-voltage battery packs to provide energy to the DC bus contactor and increase the voltage of the first battery pack;

connecting at least one lower-voltage battery pack to the DC bus contactor when an initial voltage of the at least one lower-voltage battery pack is sufficiently close to a current voltage of the DC bus contactor; and

deactivating at least one DC-DC converter connected to a higher-voltage battery pack when a current voltage of the higher-voltage battery pack is sufficiently close to the current voltage of the DC bus contactor.

17. The method of claim 16 , further comprising disconnecting all of the plurality of battery packs from the DC bus contactor, connecting the DC bus contactor to a load and activating a DC-DC converter connected to one of the plurality of battery packs to pre-charge a system DC bus.

18. An energy storage system, comprising:

a plurality of battery packs to be connected in parallel; and

a balancing circuit coupled to the plurality of battery packs including a plurality of current-direct current (DC-DC) converters corresponding to the plurality of battery packs,

wherein each DC-DC converter has an IN terminal and an OUT terminal, a plurality of bypass switches, each bypass switch connected between an IN terminal and an OUT terminal of a DC-DC converter, and a controller having a memory with instructions that, when executed, cause the controller to control the plurality of bypass switches to

connect a battery pack having a lowest voltage to the OUT terminal of the DC-DC converter associated with said battery pack,

operate DC-DC converters associated with higher-voltage battery packs having initial voltages greater than an average battery pack voltage (V ave ) to charge the OUT terminal,

connect lower-voltage battery packs having initial voltages less than V ave to the OUT terminal when the initial voltage of a lower-voltage battery pack is sufficiently close to a current voltage of the OUT terminal, and

when a current voltage of a higher-voltage battery pack is sufficiently close to V ave , deactivate a DC-DC converter corresponding to the higher voltage battery pack.

19. The energy storage system of claim 18 , wherein the controller is further configured to respond to all lower-voltage battery packs being connected to the OUT terminal and the voltages of all higher-voltage battery packs being sufficiently close to V ave by disconnecting all battery packs from the OUT terminal, closing contactors of a system DC bus, and pre-charging the system DC bus with one of the DC-DC converters.

20. The energy storage system of claim 19 , wherein the controller is further configured to deactivate the one of the DC-DC converters, and connect all battery packs to the system DC bus through the bypass switches.