HOMER Energy Support

Battery maximum charge rate and maximum charge current

Last Updated: Aug 12, 2016 12:52PM MDT

HOMER imposes three independent limits on the rate at which you can charge the battery.  The kinetic battery model imposes one limit, but I’m not going to cover it here.  I’m going to cover the other two:  the maximum charge rate and maximum charge current.

The maximum charge rate current is simple:  it’s the largest charge current the battery can accept.  The maximum charge rate is a little more complicated.  Its units are A/Ah, where the Ah refer to the unfilled capacity in the battery – the headroom.  As the battery fills up, the headroom decreases, so the maximum charge rate starts to become the limiting factor.  When the battery is near empty, the maximum charge current is more likely to be the limiting factor.

The graph below shows the effect of the maximum charge rate.  In all three cases the maximum charge current is 30 A, so you can never charge faster than 30 A.  As the battery approaches 100% state of charge, the maximum charge rate kicks in.  the higher its value, the later it kicks in:


Varying the maximum charge current just moves the horizontal line up or down, as in the following graph.  In this graph, the maximum charge rate is 0.4 A/Ah in all three cases:


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