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For those that understand that battery capacity is not constant and varies based on load:

Try changing the number of batteries you have in parallel and checking the runtime.

Example, a lead-acid battery with C/20 of 45AH and RC of 50 minutes (k=1.32) will run just about 2.8 hours at 10 amps, you would think if you double the number of batteries, you would double the runtime to 5.6 hours seeing as the amps per battery drops from 10 amps to 5 amps each. In reality, your runtime will be almost 7 hours (a whole 1.4 hours increase by lowering the per-battery load). Lower the per battery to 3.33 amps (3 batteries at 10 amps total), you get a little more than 11.9 hours..

Capacity at C/20 = 45AH (2.25 Amps X 20 Hours)

Runtime at 10.0 Amps is 2.8 Hours, Capacity at C/2.8 = 28AH (10.0 Amps x 2.8 Hours)

Runtime at 5.0 Amps is  6.97 Hours, Capacity at C/6.97 = 34.6AH (5.0 Amps x 6.97 Hours)

Runtime at 3.33 Amps is 11.92 Hours, Capacity at C/11.92 = 39.7AH (3.33 Amps x 11.92 Hours)

Runtime at 1.0 Amps is 58.3 Hours, Capacity at C/58.3 = 58.3AH (1.0 Amps x 58.3 Hours)

The industry standard is to report Capacity for a given amount of time at a given amount of current (the two are tied together), C/20 is a standard, C/10 not as common but generally used for smaller power sources and battery formats.