Electronic – Charging lead acid batteries in series


I recently bought 2 12V lead acid batteries (AGM type) for my mobile music needs where I need 24V, so I discharge them in series.

At the moment I charge both batteries separately, which is a bit annoying. So I would like to charge them in series, but I am not yet sure if this is a good idea.

When charging them separately I use a bench power supply with current and voltage control set to 14.1V. So, when charging them in series I would set my power supply to 28.2V, but then one of the batteries starts gassing and when measuring the individual batteries I see a voltage difference of about 0.3V.

So, am I doing something wrong here, is it just a bad measurement and nothing to worry about or is it generally a bad idea to charge batteries in series?

Best Answer

It is normal to charge lead-acid batteries in series. As they are used, the cell voltages will change, which is why they are not charged in parallel. If they were charged in parallel, the one with the high voltage wouldn't get much current, and the one with the low voltage would get too much current.

With the cells in series, they all get the same amount of current, and all get approximately the same amount of charge. Since they will not charge and discharge exactly the same, the battery voltage and level of charge will gradually drift apart.

To handle this, it is common to periodically do an "equalization" charge, where you overcharge the string slightly to bring up the charge of the undercharged cells. You do this because lead-acid batteries handle overcharge better than they handle undercharge.

You have done that, and at least one of the cells has gassed. Check the fluid level, and next time charge to a slightly lower voltage. Only do equalization every couple of months. If some of the cells fail, it will not be possible to charge the battery fully. When that happens, it is time to throw out the battery.