Charging lead acid batteries in series

Question

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?

Answer 1

This is a problem when series-charging lead-acid batteries and it is generally not recommended. The battery's condition is dependant on the specific gravity of the sulphuric acid electrolyte. Of course the 6 individual 2V cells in each battery share the same electrolyte which is why they can be charged in series but separate batteries can't.

Answer 2

I'll have a crack at answering, but I'm really not a reliable source here.

My experience with LiPos in model aircraft says that series charging is fine, as they are usually a number of cells in series. However it is required to keep the cells 'balanced' ie. the same voltage.

Balancing is done by charging each cell individually. This is a complex task in a LiPo as each cell remains in series. It should be as simple as charging your batteries individually.

Ideally the cells should remain at the same voltage after discharging in parallel, but this really depends on the quality/technology/relative-age of the batteries. The further apart the voltages, the more you have to 'over-charge' one cell to get the other up to voltage. Obviously there is only so far this can go before it starts gassing.

It would also be a good idea to use a charger that adjusts voltage to maintain a constant current. Typical lead acid batteries can be charged at 0.1C (a 1Ah cell can be charged at 0.1A). A 'smart' charger will also make balancing the cells much easier.

Answer 3

My UPS uses 2 lead-acid sealed batteries in series. It charges them only to 27.4 Volts, and it does that rather slowly (IIRC ~8h charge time), but a charger of this type and voltage can stay connected to the batteries "forever" without damaging them.

Once you try to charge them with higher currents the slight difference between the cells will be a problem, since one battery will have higher voltage and thus start gassing out at some point. A really inelligent desgin would measure the middle voltage between the batteries, but I have seen this only on high-power LiPo/Li-Ion chargers.

So, the answer is that if you have the time for slow charging, you can leave the batteries in series, but if you want to charge as fast as you can - separate them.