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Battery Amp Hours rating - Not linear with load? Answered

Important for people to know that battery amp hour ratings are done at a specific discharge time, often 20 hours for multi-cell batteries. If you discharge the battery at a higher amp rate than the rated time discharge calculates, you will get progressively lower amp hours as the amp load goes up. In the telephone industry our cells were rated at 8 hours and we figured 80% of that amp hour rating if we discharged over 4 hours, and 73% at 3 hour discharge. Just like a car gets lower MPG (Km/L) at higher speeds. No free lunch. So, 100 amp hour (20 hour rated) is loaded above 5 amps will give you less than 100 amp hours of discharge capacity. In this case, expect about 8 amps discharge to last about 10 hours, and 10 amps to around 7 hours.

Also the charge discharge cycle isn't 100%, you typically need to put 15-20% more amp hours into the battery than was discharged. Losses are a fact of life. Folks should also look at depth of discharge charts versus battery life (see manufacturer's website) when making battery choice decisions. If you work the battery hard, it won't last as many charge/discharge cycles.