Charging batteries in series

Electrical and electronic topics for small boats
janellekia
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Joined: Fri Jul 17, 2020 4:39 am

Charging batteries in series

Postby janellekia » Tue Jul 21, 2020 1:23 am

I have two 12-Volt deep-cycle marine batteries in series and a 24-Volt charger that is supposed to taper down and stop when the batteries are fully charged.

Q1: What happens if one battery is weak and takes a long time to charge or never gets to full-charge?

Q2: Will the charger keep going and possibly burn out the good battery?

Q3: Or, will the charger stop when the good battery is full-charge and leave than other one undercharged?

jimh
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Location: Michigan, Lower Peninsula
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Re: Charging batteries need advice

Postby jimh » Tue Jul 21, 2020 8:09 am

On the basis in the description of the charger as being designed to “taper” the charging current, and then assuming the charger accomplishes the “taper” based on sensing the voltage across the 24-Volt battery string, one can infer that if the total voltage of two series-connected 12-Volt batteries does not rise to exceed the threshold voltage for the “taper” of the charging current to occur, then the “taper” will not occur.

All that can be said with certainty in charging two batteries in series is that the charging current for each will be the same.

The best answer can be obtained from the company that made the charger. There could be a more sophisticated algorithm in control of the charging current in a particular 24-Volt charger device.

Generally for charging cells in series, all cells should be closely matched. When you charge a 12-Volt battery you are charging six cells in series. If one of the six cells is shorted or has a very high resistance, it affects the charging of the other five cells. The best results occur when all cells are identical.

ASIDE: charging many batteries in series is a common method. A large uninterruptible power supply ( UPS) might have 40 12-volt batteries in series being charged at about 520-Volts DC. For that to work properly all the batteries must be in almost identical condition.

For any battery with multiple cells in series, the total battery is only as good as the weakest cell. For that reason in the particular case of two 12-Volt batteries connected in series, the two batteries should be identical in brand, model, age, and condition. Since all your questions impose a condition when the batteries are not identical, the solution is not to be found in the charger but in getting new batteries.