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I recently got my hands on eight 12V 9AH lead acid batteries. Could I simply place them in parallel and charge them with a battery tender without any special measures? Are there any risks involved? Could it damage the batteries?

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  • \$\begingroup\$ Used or new batteries? \$\endgroup\$ – Jeroen3 Jul 29 at 5:37
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    \$\begingroup\$ Used batteries. \$\endgroup\$ – ElectronicsNoob Jul 29 at 6:12
  • \$\begingroup\$ Charge them separately with a good (3 or more stage) battery charger and see if they hold their charge for a day (setlling at about 12.6 or 12.7 V), or if they charge at all. If they do, you can probably safely charge them together. There are always risks involved when charging lead acid batteries. Keep them well ventilated and fused. The more batteries you put in parallel, the greater the chance of one of them failing and taking the others with it. \$\endgroup\$ – StarCat Jul 29 at 6:53
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Most lead-acid batteries charge at a constant 14 4 volts, so charging several in parallel is really just a charge-current issue. If the charger cannot supply enough current it will likely lower the charge voltage to protect itself. As the batteries charge up the voltage will rise, but should NOT go over 14.4 volts, or you could "cook" the batteries, releasing deadly vapors and ruin the batteries ability to hold a charge.

In a nutshell, you can do this but stay away from "wimpy" chargers. Ideally it should have charge current equal to the sum of the AH values of all connected batteries. This means their is a limit to how many you can charge in parallel, as the charge time gets to be untenable. A good 20 amp charger should be fine.

Note that many lead-acid batteries may have a 13.8 volt idle charge rating, so be sure before you invest in a charger.

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  • \$\begingroup\$ Lead acid batteries will not be properly charged at just 13.8 V. All (not some) lead acid batteries I know need a “bulk” charge voltage over 14 Volts (look up the datasheet of any lead acid battery to confirm this). 13.8 V is just to maintain the charge (“float voltage”). You will never completely charge a lead acid battery by just applying 13.8 V. \$\endgroup\$ – StarCat Jul 29 at 6:58
  • \$\begingroup\$ @StarCat I already have the 14.4 volt version in my answer \$\endgroup\$ – user105652 Jul 29 at 7:01
  • \$\begingroup\$ It comes down to following the manufacturer’s advice when charging batteries. That almost exclusively involves a bulk stage at >14 V. I do agree that you’re less likely to damage an unknown battery at 13.8 V, you will just not charge it properly. \$\endgroup\$ – StarCat Jul 29 at 7:19
  • \$\begingroup\$ @StarCat Duly noted and will edit answer. My Toyota truck charged at 14.4 volts, yet it cooked several batteries \$\endgroup\$ – user105652 Jul 29 at 7:21
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Batteries don't often fail low voltage or short circuit, but if they do, then a 'very parallel' arrangement could be bad as all the good batteries gang up on the bad one to force a high current through it. Protect each battery with a fuse in series.

Before connecting them in parallel, make sure one or more of them aren't duds, check each individually into a load. You may have to charge them individually to do this.

Before connecting them directly in parallel, roughly equalise the voltages. This is more important with lithium than with lead or nickel chemistries, but it's still worth making sure that you don't have large imbalances in the voltage, even with lead batteries.

Once you've connected them in parallel, you can treat them like a high capacity battery.

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Parallel connection of lead-acid batteries is done routinely in a lot of cases - including almost all UPS devices, small boats, offroad cars, etc...

The more identical batteries are, the better.

They ABSOLUTELY must be the same voltage. They MUST be of the same type (flooded/gel/AGM, starter/traction/standby), it is good if they are the same brand and even better if they are all brand new.

It is also good for them to be in the same state of charge (i.e. fully charged) when you connect them first.

You get a battery pack having the same voltage as each of them, the sum of capacities and if the connections are symmetric - the sum of their CCA.

In most cases, if one of the batteries fails and you don't notice, the rest of them follow quickly.

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