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I am using 2 Lead-acid batteries to run a DC motor. (Say battery A and /battery B). Both had potential of 12V before starting the motor. After two hours of running the motor, battery A still had potential of 12V, but the battery B showed 0V. Does this mean only one of the batteries is discharging? If yes, why?
Battery spec is 24V, 350 watt.
Battery A still had potential of 12V, but the battery B showed 0V.
When you connect batteries in series, you have to make sure they are of equal capacity (Ampere hours), equal Voltage and equal wear and age.
Basically you may only create a series pack with two brand new batteries.
This is important to prevent one battery limiting the entire pack. When discharging with unequal batteries your drain one of the batteries to damaging levels, and with charging you overcharge the other.
You can buy lead acid battery equalizers that make sure they are both the same voltage during charging and discharging so they wear out at the same rate.
However, this is not strictly required. But it can be helpful when you have such a heavy load that the differences in internal resistance at high currents have an effect on the balance.
For other battery chemistries, such as lithium batteries, balance is more strictly managed since these batteries are a fire hazard when you (dis)charge them too much.
Voltage is not a reliable indicator of charge state.
Both batteries can have a terminal voltage of 12V, but one be half charged and the other nearly dead. Running them in series will work until the weakest one is completely discharged.
At that point, the battery that still has charge begins pushing current through the discharged one. This "charge current" is running in the opposite direction from the current when the battery is being normally charged. Sort of like if you connected a 12V battery charger to a battery backwards.
This "backwards charging" will usually destroy the battery. This has happened to your battery. A discharged battery will usually show some voltage. Yours is showing 0V, so it is shot.
The battery connected to the positive terminal will drain first every time. The idea that they drain evenly is incorrect. I check my trolling motor batteries for voltage every time I bring my boat back from the lake. The one connected to the positive lead is low and the one connected to the negative lead is 12+V.
