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I am currently using two 12V car batteries connected in series connected to a 24V power supply (SMPS), so they act as a filter.

I need to do the same with a 12V power supply and I am not sure if to:

  1. Connect the power supply directly to ONE of the batteries (the same batteries that are connected in series to the 24V power supply.)

  2. If just connecting the load to one of the batteries will behave as a voltage divider and the 24V supply will act as a 12V supply (without having to connect the 12V power supply.)

  3. If I can't mix things and I need to use a separate battery.

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  • \$\begingroup\$ You need a separate battery for the 12V. \$\endgroup\$
    – user57037
    Dec 15, 2020 at 8:46
  • \$\begingroup\$ what would happen if I connect the other power supply to the same batteries? (both power supplies will be ON at the same time) \$\endgroup\$
    – F. A
    Dec 15, 2020 at 8:49
  • \$\begingroup\$ If these are just regular power supplies, it might work, actually. In some situations there could be some conflict between the supplies, though. I think I would find another way to do it if it were me. Like a 24V to 12V buck converter. Or maybe add a 12V supply to each battery (I think this would be safer than having one 24V supply and one 12V supply). \$\endgroup\$
    – user57037
    Dec 15, 2020 at 10:25
  • \$\begingroup\$ The thing is that both loads (12v and 24v) are for the same unit to be tested so the grounds must be connected together and that is why I thought using the same batteries. \$\endgroup\$
    – F. A
    Dec 15, 2020 at 11:05
  • \$\begingroup\$ It may work OK. It is just that the batteries will not divide their voltage evenly due to the fact that the lower battery has a load and a charger. But if the supplies are simple constant float voltage supplies it should actually be OK. Just keep in mind that the upper battery voltage will be V24-V12. It will not necessarily be V24/2 if you see what I mean. If they were both smart chargers I think it could be problematic. But if they are float chargers or constant voltage supplies it may be OK. \$\endgroup\$
    – user57037
    Dec 16, 2020 at 10:14

2 Answers 2

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#3 is the sensible choice.

#1 can be done with some care — make sure that both circuits have their ground at the same point (i.e. use the "bottom" battery for your 12V circuit, not the "top" one), or else you will potentially short out a battery from innocuous things like letting a piece of 12V equipment touch a piece of 24V equipment, or connecting any kind of cable with a ground lead from one to the other. Or just when you plug the power supplies in, if they're not isolated. Very bad news.

#2 creates a balance problem — since the batteries are charged in series, they both see the same charge current, and they both see the same load current from the 24V load, but one of them sees an additional current from the 12V load. This eventually leads to one battery full and the other battery dead.

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The simplest answer is a blanket no, do not supply a 12 V load and a 24 V load from two 12 V batteries in series.

However, for appropriate loads, and/or if you are careful to manage the batteries, it can be done.

The problem is battery balance. The 12 V load will drain one battery more than the other. This one will run out first. If you are monitoring the total 24 V to detect end of charge, then the first battery will be over-discharged.

12 V lead acid batteries in series tend to self balance when charged, because they can accept an indefinite period of over-charge, if the overcharge meets the battery specifications. For a wet cell, you can fast overcharge, but have to top up the electrolyte. For a gel cell, this means meeting a voltage limit, while the overcharge current drops to a very low value. With series connected gel cells, if you need to even out a severe imbalance, it could take weeks for the low value of current in the charged battery to fully charge the low one.

a) If you are going to charge the two batteries separately, and monitor both batteries for individual end of charge, then go ahead, and use whatever loads you like.

b) If the 24 V load is big, and the 12 V load is very small, then you can probably get away with monitoring the whole battery on both charge and discharge, as the imbalance will be very small.

c) If the situation is somewhere between those two, then you'll need to think about whether you run the risk of damaging the battery also supplying the 12 V load by over-discharge every time you use or charge the batteries. Is that something you want on your mind the whole time?

A good solution for a modest 12 V load is to run it from a 24 V to 12 V DC-DC converter.

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