Realm of Excursion - View Single Post - How to calculate (predict) voltage drop? Can somebody check my math?



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Old 04-08-2013   #5 (permalink)
recon440
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Default Re: How to calculate (predict) voltage drop? Can somebody check my math?

Thanks. There is definitely some oversimplifications and assumptions I'm making but the numbers I'm arriving at are quite reasonable.

ie.
-I'm assuming that the alternator is a constant voltage source which is obviously not the case when you are sucking more than it can provide. Net voltage is a result of equilibrium between two sources (which means at after the alternator runs out of juices there is a region where it tapers off and is actually supplying less than 14.4V) and plays a balancing act with the battery.
-The other assumption is in the battery voltage drop. I've seen 30s 500A load tests where the battery is getting pulled down to 10V but I would imagine that if I were to draw 350W for a second or two it it would probably sag to 12.5V or so (another assumption).
-I'm sure there would be a different way of figuring it out if we know the true Wire resistance, true internal resistance of the battery, real output curves of the alternator, etc...

I'm not sure about the sag on a battery (I don't put 500A constant loads on a battery (where essentially the deciding factor in these tests is the battery's internal resistance and capacity)). Would it be reasonable to assume that a quality AGM battery could maintain 12.5V for a second or two while pulling 100-150A?
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