Battery state-of-charge aggregation method

1. A method of managing a state of charge of a battery having a plurality of battery cells in a hybrid electric power system comprising a fuel cell, the method comprising:

calculating an individual state of charge for each battery cell;

calculating an average state of charge for the battery cells;

defining a minimum average state of charge threshold;

defining a maximum average state of charge threshold;

calculating an overall state of charge for the battery such that when the average state of charge is below the minimum threshold, the overall state of charge is calculated based on the state of charge of a subset of battery cells having the lowest state of charge relative to the other battery cells, and when the average state of charge is above the maximum threshold, the overall state of charge is calculated based on the state of charge of a subset of battery cells having the highest state of charge relative to the other battery cells;

preventing battery cell discharge if the average state of charge is below the minimum threshold and the overall state of charge is at or below a level at which the subset of battery cells having the lowest state of charge would overdischarge if discharged; and

preventing battery cell charge if the average state of charge is above the maximum threshold and the overall state of charge is at or above a level at which the subset of battery cells having the highest state of charge would overcharge if charged.

2. The method according to claim 1 , wherein the subset of battery cells having the lowest state of charge includes only the battery cell with the lowest state of charge and wherein the subset of battery cells having the highest state of charge includes only the battery cell with the highest state of charge.

3. The method according to claim 1 , wherein the number of battery cells used to calculate the overall state of charge changes over time as a function of the average state of charge of all of the battery cells at that time.

4. The method according to claim 3 , wherein the number of battery cells used to calculate the overall state of charge is substantially equal to 1 when the average state of charge is below the minimum threshold or above the maximum threshold, and the number of cells used to calculate the overall state of charge is substantially equal to the total number of battery cells when the average state of charge equals approximately 50%.

5. The method according to claim 3 , wherein the number of battery cells used to calculate the overall state of charge changes as a non-linear function when the average state of charge is between the maximum threshold and the minimum threshold.

6. The method according to claim 4 , wherein the number of battery cells used to calculate the overall state of charge changes as a linear function when the average state of charge is between approximately 50% and the maximum threshold and when the average state of charge is between approximately 50% and the minimum threshold.

7. The method according to claim 1 , wherein calculating the overall state of charge includes assigning a weight to the individual state of charge of each battery cell as a function of the average state of charge at that time and multiplying the individual state of charge of each battery cell by the assigned weight and wherein the overall state of charge is equal to the sum of the weighted individual state of charges.

8. The method according to claim 1 , wherein the minimum threshold is an average state of charge of approximately 20% and the maximum threshold is an average state of charge of approximately 80%.

9. The method according to claim 1 further comprising:

defining at least one intermediate threshold.

10. A method of managing a state of charge of a battery having a plurality of battery cells in a hybrid electric power system comprising a fuel cell, the method comprising:

calculating a state of charge value for each battery cell;

sorting the state of charge values for each battery cell in numerical order;

calculating an average state of charge of all of the battery cells;

defining a minimum average state of charge threshold;

defining a maximum average state of charge threshold;

calculating an overall state of charge for the battery such that when the average state of charge falls below the minimum threshold, the overall state of charge equals the state of charge of the battery cell having the lowest state of charge relative to the other battery cells, and when the average state of charge is above the maximum threshold, the overall state of charge equals the state of charge of the battery cell having the highest state of charge relative to the other battery cells;

preventing battery cell discharge if the average state of charge is below the minimum threshold and the overall state of charge is at a level at which the battery cell having the lowest state of charge relative to other battery cells would overdischarge if discharged; and

preventing battery cell charge if the average state of charge is above the maximum threshold and the overall state of charge is at a level at which the battery cell having the highest state of charge relative to other battery cells would overcharge if charged.

11. The method according to claim 10 , wherein the overall state of charge is calculated according to the following equation:

SOC

overall

=

∑

i

=

1

N

⁢

⁢

SOCs

⁡

(

i

)

*

w

⁡

(

i

)

.

wherein N is the total number of battery cells in the battery and w is a weight applied to the state of charge of an individual battery cell, which is a function of the average state of charge at a given time.

12. The method according to claim 11 , wherein the state of charge values are sorted from highest to lowest.

13. The method according to claim 12 , wherein the weight applied to the individual battery cell state of charges are calculated according to the following conditions:

if the average state of charge is above or equal to 50%, then a weight 1/x is applied to a first subset [x] battery cells, a weight

1

-

[

x

]

x

is applied to a second subset [x]+1 battery cells, and a weight of 0 is applied to all remaining battery cells; and

if the average state of charge is below 50%, then a weight 1/x is applied to a third subset [x] battery cells, a weight

1

-

[

x

]

x

is applied to a battery cell defined as N−[x], and a weight of 0 is applied to the other cells,

wherein x equals the number of battery cells used to calculate the overall state of charge and wherein [x] denotes a floor function, which gives the largest integer less than or equal to x.

14. The method according to claim 13 , wherein x is calculated so that:

⁢

if

⁢

⁢

SOC

avg

>

SOC

max

⁢

⁢

or

⁢

⁢

SOC

avg

<

SOC

min

,

then

⁢

⁢

x

=

1

;

if

⁢

⁢

50

⁢

%

≤

SOC

avg

≤

SOC

max

,

then

⁢

⁢

x

=

1

-

N

SOC

max

-

50

⁢

(

SOC

avg

-

50

)

+

N

;

⁢

and

if

⁢

⁢

SOC

min

≤

SOC

avg

,

<

50

⁢

%

,

⁢

then

⁢

⁢

x

=

1

-

N

SOC

min

-

50

⁢

(

SOC

avg

-

50

)

+

N

.

15. The method according to claim 10 , wherein the minimum threshold is an average state of charge of approximately 20% and the maximum threshold is an average state of charge of approximately 80%.

16. A method of aggregating state of charge values for all cells of a battery to calculate an overall state of charge value for managing the state of charge of the battery, the method comprising:

calculating the state of charge value for each cell in the battery;

sorting the state of charge values for each cell in numerical order;

calculating an average state of charge of all the cells in the battery;

defining a minimum average state of charge threshold;

defining a maximum average state of charge threshold;

calculating the overall state of charge for the battery such that when the average state of charge is below the minimum threshold, the overall state of charge equals the state of charge of the battery cell having the lowest state of charge and when the average state of charge is above the maximum threshold, the overall state of charge equals the state of charge of the battery cell having the highest state of charge;

preventing battery cell discharge if the average state of charge is below the minimum threshold and the overall state of charge is at a level at which the battery cell having the lowest state of charge would overdischarge if discharged; and

preventing battery cell charge if the average state of charge is above the maximum threshold and the overall state of charge is at a level at which the battery cell having the highest state of charge would overcharge if charged.

17. The method according to claim 16 , wherein the average state of charge and the overall state of charge are approximately equal to each other when the average state of charge is approximately 50%.

18. The method according to claim 16 , wherein the minimum threshold is an average state of charge of approximately 20% and the maximum threshold is an average state of charge of approximately 80%.

19. The method according to claim 16 , wherein calculating the overall state of charge of the battery substantially prevents the battery cell having the highest state of charge from being charged above a predetermined state and substantially prevents the battery cell having the lowest state of charge from being discharged below a predetermined state.

20. The method according to claim 16 , wherein the battery is part of a hybrid electric power system including a fuel cell.