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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?

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  • \$\begingroup\$ This is why you replace both batteries at the same time and using the same model. All large banks have series charging. Charging them separately is just dodging the fact that one battery is in worse condition than the other. \$\endgroup\$
    – user39962
    Mar 7, 2015 at 14:36
  • \$\begingroup\$ You are doing something right here .Seperate charging is better . \$\endgroup\$
    – Autistic
    Feb 11, 2016 at 0:10

5 Answers 5

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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.

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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.

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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.

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  • \$\begingroup\$ I think these two statements contradict each other: "LiPos in model aircraft says that series charging is fine," and "it is required to keep the cells 'balanced' Balancing is done by charging each cell individually". The first says charging in series is okay, the second says the cells are charged individually, which seems to imply they are not in series. Yes? \$\endgroup\$
    – gbulmer
    Aug 12, 2014 at 22:46
  • \$\begingroup\$ Typically one will balance by connecting a balancer to the battery with all cells still in series. The balancer will usually apply a small load across any cells that are too high. Generally RC folk seem to like balance chargers that balance automatically while charging. But balancing is really only done with lithium rechargables. \$\endgroup\$
    – EternityForest
    Aug 13, 2014 at 10:32
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    \$\begingroup\$ LiPos are way more sensitive than lead acid - hence the balancing act. \$\endgroup\$
    – user39962
    Mar 7, 2015 at 14:42
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    \$\begingroup\$ Lead acid cells are balanced by overcharging, a process called "equalisation". \$\endgroup\$
    – david
    Apr 22, 2015 at 5:09
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Have you considered charging them in parallel at your "normal" 14.1V? It still will be best if the batteries are balanced or nearly so, but the worst case scenario is that each will behave as if you were charging them individually and leaving them connected for an extended period of time. Of course, during initial charging your load will be drawing twice the current if your system is capable of delivering it.

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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.

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  • \$\begingroup\$ What do you mean "share the electrolyte"? The cells will generally have been filled at the same time, and any maintenance performed on one will generally be performed on all of them, but each cell is a separate compartment. Any continuous path of acid from one cell to another would represent a short circuit. \$\endgroup\$
    – supercat
    Aug 12, 2014 at 23:05
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    \$\begingroup\$ Generally not recommended? I guess every telecom center in the world is doing it wrong. \$\endgroup\$
    – user39962
    Mar 7, 2015 at 14:40
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    \$\begingroup\$ Cells do not normally share electrolyte, and series charging of Lead-Acid cells, in a battery, is the normal recommended procedure. \$\endgroup\$
    – david
    Apr 22, 2015 at 5:08
  • \$\begingroup\$ I agree with @david, nothing wrong with charging lead acid batteries in series as long as they are of the same type, age and state of charge. \$\endgroup\$
    – jms
    Oct 9, 2016 at 6:54
  • \$\begingroup\$ isn't a lead acid battery actually six 2.1 V cells is series? so even with just one battery you are charging the (internal) cells in series. \$\endgroup\$
    – dominic
    Jul 12, 2017 at 2:50

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