HOMER Grid 1.9

The maximum charge rate variable imposes a limit on the rate at which the system can charge the storage bank. That limit is directly proportional to the amount of "unfilled capacity" in the storage component, where the unfilled capacity is defined as the storage component's maximum capacity minus its current absolute state of charge.

For example, consider a storage component with a maximum capacity of 350 Ah and a maximum charge rate of 0.4 A/Ah. If, at some point in time, the storage component's absolute state of charge is 310 Ah, then it has 40 Ah of unfilled capacity, so the highest charge current it could accept would be 40 Ah * 0.4 A/Ah = 16 A. If, at some other point in time, its state of charge was 335 Ah, then the highest charge current it could accept would be only 6 A. So the allowable charge current decreases with increasing state of charge.

Another variable, the maximum charge current, imposes an upper limit on the allowable charge current, regardless of the state of charge. If our example storage were empty, the maximum charge rate variable would imply that it could accept a charge current of as high as 350 Ah * 0.4 A/Ah = 140 A. But a current that high might be very damaging to the storage. If you set the maximum charge current variable to 25 A, then HOMER ensures that the charge current never exceeds 25 A, no matter what the state of charge.

Notes:

1.The kinetic storage model imposes a separate limit on the rate of charge.

2.This discussion relates to a single storage component. To find the maximum storage charge power, HOMER calculates the product of the maximum charge current times the nominal voltage times the number of batteries in the storage bank.