How many Ah have the batteries? You will need to do the math, because the whole capacity in A of the battery is not the same of the current the batt is able to provide. This is dependant of the 'technology' of the battery, but if is from a car, I guess they are lead-acid.
You need to divide 50A by the Ah rating of the batteries -I suppose they are equal- and that number will say you how many hours the battery will remain usable. For example a 50A batt with 10 Ah ratio will give you 10A for 5 h, and 50A batt with 5Ah ratio will give you 5A for 10h. Triying to demand more A is possible, but the time will decrease, *and* the battery life too. In a situation of heavy current demand, the battery will provide all A possible for them but will die (not discharge, die) shortly. For example, I have seen an accident where the terminals of a car battery were shorted with a dropped steel wrench, and in a pair of seconds the wrench acted as a fuse and *melted*, and the battery of course died without hope of ressurrection.
Then you need to know how many A your steppers need to energize the windings, and do the math again: number of windings energized at one time per A rating of the windings, per number of motors running at the same time. if the whole A you need is below the Ah of your supply, you can expect at least the time the Ah says. If it is above close, less time, if it is above far, go with it but you are overstressing the batts and decreasing their life.
All of this is nearly true, but take it with caution because in the real world the V of the batts is not the same all the time, drops when the batt is near depletion, so all the numbers are maximum optimal attainable.  |