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They say to make your batteries last the longest, you never want to discharge them past 50%, which on a 12v should be around 12.06 volts. My question is, is this while the batteries are "idle" or while power is being drawn from them? I was reading around 12.3 volts on my battery earlier, turned my inverter off and the voltage shot back up to 12.5. Is it safe to allow my batteries to go below 12 volts while they're under load, as long as they'll go back above 12 when idle? Thanks.
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Only on a fully rested open circuit battery. But even then is only ball park. The only way to accurately measure the state of charge of a Pb battery is with a temperature compensated hydrometer. Voltage is meaningless on battery under load or charge.
As to your second question, the answer is maybe. It depends on how many amps you are taking from the battery with respect to the batteries AH capacity, and the Battery Internal Resistance. Let's try an example. Let's say your battery Open Circuit voltage is 12.2 volts, AH capacity is 100 AH, the battery Internal resistance is .03 Ohms, and you are taking 10 amps. The battery will go from 12.2 Volts Open Circuit to 11.9 volts. The math is simple and Ohms Law. Voltage = Current x Resistance, So we have 10 amps x .03 Ohms = .3 volts. 12.2 - .3 = 11.9 volts. Further lets say you only take that 10 amps for a few moments and disconnect it. The battery voltage will recover back to 12.2 volts or roughly 60% SOC
When you design a battery system you need to limit the voltage loss to 2% or less. On a 12 volt battery is not a lot at ,24 volts. So how do you do that. Real easy, use a rule of thumb. For Flooded lead acid batteries limit discharge current to no higher than C/8 where C = Battery Amp Hour Capacity ,and the number 8 is discharge time in hours. So C/8 on a 100 AH battery = 100 AH / 8 H = 12.5 amps.
So there is no cut and dry answer to your question other than Maybe. What I can tell you based on dropping .2 volts on your battery is about the maximum limit of load current your battery can supply without significant voltage drop. Realistically the battery can deliver 100 amps, but your inverter would trip off-line immediately from low voltage because at 100 amps the voltage would drop from 12.2 volts down to 9.2 volts. Even if the battery was fully charged at 12.6 volts would still trip off-line from under voltage at 9.6 volts.
On the other hand if you load current was say 1 amp, you only drop to 12.173 volts.
Thus as you can see battery voltage is about completely worthless on a working system. It is only useful on a battery disconnected and allowed to rest for a few hours.MSEE, PE -
Thanks for the very detailed response.Comment
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You are welcome.MSEE, PEComment
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