What does c/20 capacity rate mean for batteries?

The capacity rating is in ampere hours (Ah) at a discharge rate of C/20, meaning a 20 hour rate. Thus 55Ah/20h = 2.75A means that it can deliver 2.75A for 20 hours, or some equivalent of that. The cranking amps is another rating, for how much current can be delivered under certain conditions like starting the car engine in cold weather with a time limit of probably 10 seconds. With a load of 500W and a nominal 12V the current is ~42A. This is far more than the current at rated capacity, so the capacity is less than expected. Taking the capacity as 55Ah then 55Ah/42A = 1.32h approx. In practice it will be less, because the capacity is less at such high currents, but the battery could overheat after some time. We can get a rough idea of the internal heating as: Power = I*I*R = 42 * 42 * 0.0028 = 5W, so it appears as if the heating is low. I suspect that figure for R will change for this high load current and as the battery discharges though. The best you can hope for 50% discharge is 1.32h/2 = approx 40 minutes. It is possible it could be half this due to changed ratings with that higher load current, or that overheating might occur. There is no specification for the battery under these conditions, so more moderate conditions that are specified are used to get this result..

55Ah with discharge time of 20 hours

The capacity rating is in ampere hours (Ah) at a discharge rate of C/20, meaning a 20 hour rate. Thus 55Ah/20h = 2.75A means that it can deliver 2.75A for 20 hours, or some equivalent of that. The cranking amps is another rating, for how much current can be delivered under certain conditions like starting the car engine in cold weather with a time limit of probably 10 seconds. With a load of 500W and a nominal 12V the current is ~42A. This is far more than the current at rated capacity, so the capacity is less than expected. Taking the capacity as 55Ah then 55Ah/42A = 1.32h approx. In practice it will be less, because the capacity is less at such high currents, but the battery could overheat after some time. We can get a rough idea of the internal heating as: Power = I*I*R = 42 * 42 * 0.0028 = 5W, so it appears as if the heating is low. I suspect that figure for R will change for this high load current and as the battery discharges though. The best you can hope for 50% discharge is 1.32h/2 = approx 40 minutes. It is possible it could be half this due to changed ratings with that higher load current, or that overheating might occur. There is no specification for the battery under these conditions, so more moderate conditions that are specified are used to get this result..

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maybe 20 percent of cranking amps

Any Battery will last longer in a freezer. A dead camera or cell phone battery can be brought back by freezing. It is a good ideal to seal them in a bag first!

maybe 20 percent of cranking amps

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