Voltage sharing of series connected battery modules in a plug-and-play DC microgrid

1. An apparatus comprising:

a battery module controller; and

a battery management system,

wherein the battery module controller:

generates, for an m th converter of N converters connected together to a DC microbus, a droop current ĩ d,m comprising a converter voltage error signal {tilde over (v)} err,m multiplied by a non-linear droop multiplier g d (i), wherein each converter of the N converters is a DC/DC converter connected between a battery module and the DC microbus, the battery module comprising one or more battery cells,

wherein the m th converter uses the droop current ĩ d,m , a common current reference ĩ all of a battery pack comprising the battery modules connected to the N converters and an input current ĩ m to the m th converter to control switching of the m th converter, the common current reference ĩ all provided by the battery management system, and

wherein the voltage error signal {tilde over (v)} err,m is based on an output voltage {tilde over (v)} o,m of the m th converter and an average converter output voltage {tilde over (v)} avg .

2. The apparatus of claim 1 , wherein the non-linear droop multiplier g d (i) varies between a minimum droop multiplier G d,min and a maximum droop multiplier G d,max , wherein the non-linear droop multiplier g d (i) is the minimum droop multiplier G d,min during battery discharging and varies linearly between the maximum droop multiplier G d,max at a maximum battery charging rate and the minimum droop multiplier G d,min at a zero charging rate.

3. The apparatus of claim 2 , wherein the non-linear droop multiplier g d (i) is

g

d

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i

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=

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G

d

,

min

⁢

:

i

all

≥

0

G

d

,

min

+

G

d

,

min

-

G

d

,

max

I

all

,

max

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:

i

all

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0

where:

i all is the common current reference of a battery pack comprising the battery modules connected to the N converters, and

I all,max is a maximum value for the common current reference which correlates to the maximum battery charging rate.

4. The apparatus of claim 1 , wherein the m th converter using the droop current ĩ d,m , the common current reference i ref and the input current ĩ m to the m th converter to control switching of the m th converter comprises the battery module controller:

generating, for the m th converter, a converter current reference ĩ ref,m comprising a sum of the droop current ĩ d,m and the common current reference i ref , and

generating, for the m th converter, a local current command ĩ err,m comprising a difference between the converter current reference ĩ ref,m and an input current ĩ m to the m th converter,

a duty cycle {tilde over (d)} m for the m th converter comprising the local current command ĩ err,m multiplied by a current compensator G ci (s), wherein the duty cycle controls switching of the m th converter.

5. The apparatus of claim 1 , wherein the voltage error signal {tilde over (v)} err,m being based on the output voltage {tilde over (v)} o,m of the m th converter and the average converter output voltage {tilde over (v)} avg comprises the battery module controller generating, for the m th converter, the voltage error signal {tilde over (v)} err,m for the m th converter comprising a difference between the average converter output voltage {tilde over (v)} avg and the output voltage {tilde over (v)} o,m of the m th converter.

6. The apparatus of claim 1 , wherein the battery management system:

generates the average converter output voltage {tilde over (v)} avg by dividing a DC microbus voltage v bus by the number of converters N; and

generates the common current reference ĩ all of the battery pack as a function of an average state-of-charge (“SOC”) of the battery modules of the battery pack and a bus voltage v bus of the DC microbus.

7. The apparatus of claim 6 , wherein the battery management system calculates an average SOC for each of the N converters of the battery pack by calculating a difference between an initial average SOC of the battery pack and an integral over a period of time (0 to t) of a common current i all divided by a total charge capacity of the battery pack.

8. The apparatus of claim 7 , wherein the battery management system generates a current offset for each of the N converters, wherein the current offset for an m th converter of the N converters is based on a current SOC of the m th converter compared to the average state of charge for the N converters and wherein the battery module controller for the m th converter adds the current offset for the m th converter to the common current reference ĩ all .

9. The apparatus of claim 8 , wherein the battery management system adjusts the current offset for each of the N converters until the SOC of each of the N converters reaches a desired SOC.

10. The apparatus of claim 8 , wherein the battery management system adjusts the current offset for each of the N converters until the SOC of each of the N converters reaches a same SOC.

11. The apparatus of claim 7 , wherein the current offset for a converter of the N converters varies between a maximum positive current offset that corresponds to a positive maximum common current reference I max of a positive value of the common current reference ĩ all and zero current offset at zero common current reference ĩ all and varies between a maximum negative current offset that corresponds to a negative maximum common current reference I max of a negative value of the common current reference ĩ all and zero current offset at zero common current reference ĩ all .

12. The apparatus of claim 6 , wherein for a voltage range of the bus voltage v bus of the DC microbus between a minimum bus voltage v bus,min corresponding to a minimum state-of-charge SOC min of the battery pack and a maximum bus voltage v bus,max corresponding to a maximum state-of-charge SOC max of the battery pack, the battery management system generates a negative common current reference ĩ all for a lower voltage range and a positive common current reference ĩ all for an upper voltage range above the lower voltage range.

13. An apparatus comprising:

a battery module controller; and

a battery management system,

wherein the battery management system:

generates, for N converters connected together to a DC microbus, an average converter output voltage {tilde over (v)} avg by dividing a DC microbus voltage v bus by the number of converters N, wherein each converter of the N converters is a DC/DC converter connected between a battery module and the DC microbus, the battery modules of the N converters forming a battery pack, the battery module comprising one or more battery cells; and

generates a common current reference ĩ all of the battery pack as a function of an average state-of-charge (“SOC”) of the battery modules of the battery pack and a bus voltage v bus of the DC microbus,

wherein for a voltage range of the bus voltage v bus of the DC microbus between a minimum bus voltage v bus,min corresponding to a minimum state-of-charge SOC min of the battery pack and a maximum bus voltage v bus,max corresponding to a maximum state-of-charge SOC max of the battery pack, the battery management system generates a negative common current reference ĩ all for a lower voltage range and a positive common current reference ĩ all for an upper voltage range above the lower voltage range,

wherein for an m th converter of the N converters, the battery module controller for the m th converter uses the average converter output voltage {tilde over (v)} avg and the common current reference ĩ all along with an input current input current ĩ m to the m th converter and an output voltage output voltage {tilde over (v)} o,m of the m th converter to control switching of the m th converter to control the output voltage output voltage {tilde over (v)} o,m of the m th converter and charging and discharging of the battery module connected to the of the m th converter.

14. The apparatus of claim 13 , wherein the battery module controller for an m th converter of N converters:

generates, for the m th converter, a droop current ĩ d,m comprising a converter voltage error signal {tilde over (v)} err,m multiplied by a non-linear droop multiplier g d (i),

wherein the m th converter uses the droop current ĩ d,m , the common current reference ĩ all and the input current ĩ m to the m th converter to control switching of the m th converter, and

wherein the voltage error signal {tilde over (v)} err,m is based on an output voltage {tilde over (v)} o,m of the m th converter and an average converter output voltage {tilde over (v)} avg .

15. The apparatus of claim 13 , wherein the battery management system calculates an average SOC for each of the N converters of the battery pack by calculating a difference between an initial average SOC of the battery pack and an integral over a period of time (0 to t) of a common current i all divided by a total charge capacity of the battery pack.

16. The apparatus of claim 15 , wherein the battery management system calculates an average SOC for each of the N converters of the battery pack by calculating a difference between an initial average SOC of the battery pack and an integral over a period of time (0 to t) of a common current i all divided by a total charge capacity of the battery pack.

17. The apparatus of claim 16 , wherein the battery management system generates a current offset for each of the N converters, wherein the current offset for an m th converter of the N converters is based on a current SOC of the m th converter compared to the average state of charge for the N converters and wherein the battery module controller for the m th converter adds the current offset for the m th converter to the common current reference ĩ all .

18. A method comprising:

generating, in a battery module controller for an m th converter of N converters connected together to a DC microbus, a droop current ĩ d,m comprising a converter voltage error signal {tilde over (v)} err,m multiplied by a non-linear droop multiplier g d (i), wherein each converter of the N converters is a DC/DC converter connected between a battery module and the DC microbus, the battery module comprising one or more battery cells; and

using the droop current ĩ d,m , a common current reference ĩ all of a battery pack comprising the battery modules connected to the N converters and an input current ĩ m to the m th converter to control switching of the m th converter, the common current reference ĩ all provided by a battery management system,

wherein the voltage error signal {tilde over (v)} err,m is based on an output voltage {tilde over (v)} o,m of the m th converter and an average converter output voltage {tilde over (v)} avg .

19. The method of claim 18 , wherein the non-linear droop multiplier g d (i) varies between a minimum droop multiplier G d,min and a maximum droop multiplier G d,max , wherein the non-linear droop multiplier g d (i) is the minimum droop multiplier G d,min during battery discharging and varies linearly between the maximum droop multiplier G d,max at a maximum battery charging rate and the minimum droop multiplier G d,min at a zero charging rate, wherein the non-linear droop multiplier g d (i) is

g

d

⁡

(

i

)

=

{

G

d

,

min

⁢

:

i

all

≥

0

G

d

,

min

+

G

d

,

min

-

G

d

,

max

I

all

,

max

⁢

:

i

all

<

0

where:

i all is the common current reference of a battery pack comprising the battery modules connected to the N converters, and

I all,max is a maximum value for the common current reference which correlates to the maximum battery charging rate.

20. The method of claim 18 , wherein the m th converter using the droop current ĩ d,m , the common current reference i ref and the input current ĩ m to the m th converter to control switching of the m th converter comprises:

generating, with the battery module controller for the m th converter, a converter current reference ĩ ref,m comprising a sum of the droop current ĩ d,m and the common current reference i ref ; and

generating, with the battery module controller for the m th converter, a local current command ĩ err,m comprising a difference between the converter current reference ĩ ref,m and an input current ĩ m to the m th converter,

a duty cycle {tilde over (d)} m in the battery module controller for the m th converter comprising the local current command ĩ err,m multiplied by a current compensator G ci (s), wherein the duty cycle controls switching of the m th converter.