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I have just installed two brand new 12v lead-acid batteries in series to for my 24v “house system” on my boat. I connected them to my 24v charger to ensure they were charged to 100%, to synchronise my battery monitor. However, the voltages of the two batteries whilst on charge was markedly different: batt 1 15.1v, batt 2 13.8v. This immediately triggered an imbalance alarm from the battery monitor (mid-point voltage difference). After about half an hour the charger entered its float stage, and the voltages were then: batt 1 14.1v, batt 2 13.4v. Do I have a problem, or is this behaviour perfectly normal? And if this is normal, how much of an imbalance should I accept?

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  • \$\begingroup\$ Normally you want them balanced << 1% because the 0 to 100% is only 1V and overcharge will boil the acid out of the cell if sustained. Do you have an active balancer? I would expect 14.5V max 14.2V nom. \$\endgroup\$ Commented Jan 29, 2021 at 18:38
  • \$\begingroup\$ Assuming these are not AGM \$\endgroup\$ Commented Jan 29, 2021 at 18:44
  • \$\begingroup\$ I would contact whomever sold you these batteries. THere may be some special pre-conditioning charge that you have to do. I could see if possibly, these two batteries didn't START at the same charge level (i.e. one was more drained than the other). Charging in series would not fix that. Maybe you need to fully charge each individually with a 12V charger first so they have a common starting point??? Assume these are "identical" batteries to begin with, right??? \$\endgroup\$
    – Kyle B
    Commented Jan 29, 2021 at 18:47
  • \$\begingroup\$ Balance them and try again. \$\endgroup\$
    – user16324
    Commented Jan 29, 2021 at 19:20
  • \$\begingroup\$ I second BrianDrummond and KyleB. Fully charge each of them separately, then connect them together again and see how they behave. \$\endgroup\$
    – bitsmack
    Commented Jan 29, 2021 at 21:45

2 Answers 2

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Yes, you do have a problem.

Series-connected batteries should be balanced, otherwise you always overcharge one of them and over-discharge the other. Neither is good for the battery life and you also get less cycle capacity.

Ways to solve it:

  1. A balancing device. A search for "24v battery equalizer" will get you some ideas.
  2. Charge the batteries in parallel (once, before connecting them in series). The batteries may gradually disbalance over time, but the process is slow and generally negligible for sane batteries.
  3. Use "Balancing charge". In essence, it is a mild overcharge that gets every cell charged to the max while overcharging the already charged ones. Rather harsh and makes batteries to lose some water, but does the job. It also equalizes the cells within the battery. Some controllers do this once in a while, some batteries don't like it very much.

The normal imbalance for a 12v lead batteries is less than 0.5v when charged and way less (less than 0.1v) in intermediate state of charge

p.s. I expect brand-new lead batteries to be of equal (near-100%) state of charge.

Getting two unbalanced batteries means something is not absolutely OK. Either one of the batteries has higher self-discharge, or the batteries are from different batches and were stored for different amount of time.

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  • \$\begingroup\$ Note that equalising and balancing are not the same thing, although they are related! "Equalising" has a specific meaning that applies even to the cells within a single battery, that of a deliberate overcharge to remove sulphate crystals, which shorten the life of the battery. See batteryuniversity.com/article/bu-404-what-is-equalizing-charge \$\endgroup\$ Commented May 8, 2023 at 8:10
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This is normal, and is a result of state of charge imbalance, when the battery reaches above 13.8V it starts splitting the water into hydrogen and oxygen which bleeds off power allowing the other battery to charge. How much imbalance should you accept is a difficult question because no-one really knows about imbalance in lead acid batteries, but it should be okay as long as it doesn't consistently have large imbalances where one battery is over 15V for very long periods.

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