# Why is only one out of 2 batteries is discharging when connected in series?

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.

• What is their actual capacity... and congratulations, you just destroyed one Apr 12, 2018 at 11:13
• This can happen if one battery has (significantly) lower actual capacity than the other one. Since both batteries source the same current in a series configuration, the weaker battery may be empty while the other is, say, still 90% charged. Apr 12, 2018 at 11:32
• A difference in capacity and/or ESR in conjunction with way too low undervoltage protection (why are you still operating when your supposed 24 V battery measures 0+12=12 V?). Apr 12, 2018 at 11:42

## 3 Answers

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.

• If you are experiencing this problem then there is something wrong with the battery connected to the positive terminal. Since the batteries are connected in series the current through them is exactly the same and they must both discharge at identical rates. Swap your batteries around and see if "The battery connected to the positive terminal will drain first every time". It certainly will not stand up to any technical analysis. May 22, 2021 at 16:22
• This answer is wrong. Batteries and/or individual cells should be balanced and drain evenly. Your own experience with a single set of batteries does not extrapolate to every set of serially connected batteries in existence. May 22, 2021 at 18:05