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I have a BatteryMinder 12v trickle charger and two dead 6v "sealed" lead acid batteries. I assume that the 6v "sealed" batteries means that it is just an ordinary flooded battery that has been sealed shut.

Can I charge the two 6v batteries by connecting them in series and that putting the charger leads at both ends?

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  • \$\begingroup\$ Hi Tyler! It's been two days since you've gotten your answers. Both are nice, but you haven't accepted an answer so far. \$\endgroup\$
    – Marcus Müller
    Jul 28, 2020 at 6:56

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Short answer is no. The same current will pass through both batteries to charge them. What happens when one battery is fully charged and the other isn’t quite done? The charger will keep sending the current and overcharge one battery.

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  • \$\begingroup\$ By that logic, the separate cells in a 12v lead acid battery would get unequally charged. \$\endgroup\$
    – Tyler Durden
    Jul 26, 2020 at 1:22
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    \$\begingroup\$ @TylerDurden all the seperate cells in a 12V LA battery charge and discharge almost equally, because they start their life in the same package at the same time. This is not true for your two independent 6V batteries. There's no guarantee that these batteries have the same charge (remaining) before charging. Even if you fully charge them individually, connect them in series, and discharge them together, still one of them will discharge more than other. \$\endgroup\$
    – Rohat Kılıç
    Jul 26, 2020 at 1:38
  • \$\begingroup\$ @RohatKılıç Uh, well, my understanding is that the cells in a battery do not discharge equally, first of all. Second of all, when charging, if one cell charges faster than another, I would expect that more of the current would go to charge the lower cell, which would tend to balance them. \$\endgroup\$
    – Tyler Durden
    Jul 26, 2020 at 3:13
  • \$\begingroup\$ The idea that the current would go to the less charged cell and balance itself is referring to charging batteries in parallel, not series. You could use a 6V charger and connect both 6V batteries in parallel and charge them both at once using a single charger. Even with this you want the batteries balanced initially, but it is much simpler and easier to do with different batteries than trying to charge them in series. \$\endgroup\$
    – Joe Mac
    Jul 26, 2020 at 6:18
  • \$\begingroup\$ @TylerDurden your understanding is wrong: These cells are in series. Current can't go anywhere else? That's one electrical circuit and the current is the same at every point. \$\endgroup\$
    – Marcus Müller
    Jul 26, 2020 at 7:05
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If you have a constant current trickle charger, you can simply connect the 6V battery to the 12V charger and it will be fine. Do make sure to check the short circuit current of the charger so that it is below C/10 of the battery you are charging. You might not want to let the charging proceed to completion (i.e. do not top off).

For high current chargers and dead batteries, no, since it may cause reverse polarity in one of them. This is why you should not completely discharge multi cell batteries.

For high current chargers and batteries that are not completely dead, you can if you monitor voltage of each battery separately. You have to do this when they are not charging after a suitable rest time (which may be in minutes). Consider that, the end point of charging is determined not by how much current has passed, but by the final voltage of the battery. However, one battery will likely finish charging before the other one does. At that poin, it may be possible to top off the less charged one with low current.

IMHO, it may just be easier to charge each one separately, with a 7V output constant current buck converter charger for each battery, with both chargers operating from the same 12V supply.

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