The maximum charge rate variable imposes a limit on the rate at which the system can charge the battery bank. That limit is directly proportional to the amount of "unfilled capacity" in the battery, where the unfilled capacity is defined as the battery's maximum capacity minus its current absolute state of charge.
For example, consider a battery whose maximum capacity is 350 Ah and whose maximum charge rate is 0.4 A/Ah. If at some point in time the battery'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 battery 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 battery. If you set the maximum charge current variable to 25 A, then HOMER will ensure that the charge current never exceeds 25 A, no matter what the state of charge.
1. The kinetic battery model imposes a separate limit on the rate of charge.
2. This discussion relates to a single battery. To find the maximum battery charge power, HOMER calculates the product of the maximum charge current times the nominal voltage times the number of batteries in the battery bank.