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This question already has an answer here:

I am having two batteries of different specs (both, dry cell, UPS/inverter batteries)

Battery A: 12 V, 26 Ah

Battery B: 12 V, 7.2 Ah

I need a 24 V output so I have connected them in series.

My question is, will it damage my batteries?

If the configuration is okay, can I charge the 12 V 26 Ah battery alone? Wouldn't it push a higher current to battery A and keep the voltage at a steady state?

I don't have a 24 V charger, but I do have a 12 V charger. Yes I can charge them both parallely but I don't want to do that as I would have to open up my robot to access the batteries.

P.S. Sorry if it's a silly question. I am not much of an expert in this.

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marked as duplicate by winny, Andy aka, Voltage Spike, Bimpelrekkie, PeterJ Jun 11 '18 at 11:50

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • \$\begingroup\$ What are the loading current ratings for each battery? \$\endgroup\$ – Andy aka Jun 8 '18 at 9:41
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It will not damage your batteries on discharge if you stop the discharge before the smaller battery has discharged too far. Depending on the cycle life you want out of the batteries, this may be anywhere between 11v and 12v (hint, higher end-point voltage, higher cycle life).

Similarly, when charging, stop the charge based on the voltage on both batteries, which will usually mean the smaller battery. If float charging, limit each battery to a max of 13.8v, not the pair of batteries to 27.6v

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  • \$\begingroup\$ So you are advising them to be charged individually to 13.8v . Hmm i guess thats turning out to be the only option on the table . I dont have a balance charger . \$\endgroup\$ – Chris Thaliyath Jun 8 '18 at 7:57
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It is not recommended to put two different capacity batteries in series.

The lower capacity battery will be over-discharged and over-charged, since the large on will keep on going even though the small one is already depleted or full.

Damage is almost guaranteed.

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  • \$\begingroup\$ Alright , so i guess my only option is to use a 24v charger . \$\endgroup\$ – Chris Thaliyath Jun 8 '18 at 7:19
  • \$\begingroup\$ @ChrisThaliyath You should charge them with a balancer, or separate. \$\endgroup\$ – Jeroen3 Jun 8 '18 at 7:54

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