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Super simple question. I am new to electronics and have to connect 4 12V lead-acid batteries in series. Once I connect the negative terminals of each battery to the positive terminals of the others, can I treat the whole battery system as a 'single' battery? That is, when connecting this system to the load should I follow the standard convention to connect the positive terminal first, then connect the negative terminal?

The main reason I am asking is that I saw a more experienced person first connect the negative terminal of the battery to the negative bus, then continue to just arbitrarily make connections until finally, they connected the positive terminal of the series system to the load. It seemed like they were not following any sort of safety convention, but were quite confident about their approach.

I know this is fairly harmless, but working with batteries has always freaked me out a little bit. Any advice on this would be helpful!

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  • \$\begingroup\$ I think you need to be careful when you say this: Once I connect the negative terminals of each battery to the positive terminals of the others. \$\endgroup\$
    – Andy aka
    Jun 19, 2020 at 13:43
  • \$\begingroup\$ ^^ Yeah he didn't word it well..... \$\endgroup\$
    – Kyle B
    Jun 19, 2020 at 13:45
  • \$\begingroup\$ What exactly is the load? Is this like an automotive system with a chassis ground? \$\endgroup\$
    – Elliot Alderson
    Jun 19, 2020 at 13:48
  • \$\begingroup\$ There is a 'convention' when connecting car batteries to charge them, because of a concern that the batteries may be releasing flammable/explosive hydrogen gas. If you get the connection order wrong, it's possible to make a spark and ignite the gas. (If I understand your question, that's what you're getting at). In your case, connect your batteries w/o concern, and finally connect the LOAD LAST. Then any spark that occurs will not be in the area of your batteries. \$\endgroup\$
    – Kyle B
    Jun 19, 2020 at 13:50
  • \$\begingroup\$ The other reason for connecting negative last when wiring cars (e.g.for jump starting) is this : if you connect negative first, then drop the battery's positive lead onto the chassis, you have just shorted out the battery. (Most cars are negative earth : positive earth cars would be a different matter) \$\endgroup\$
    – user16324
    Jun 19, 2020 at 15:03

1 Answer 1

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'Connect positive first' is a convention taught when jump-starting cars, because it's safer than the alternative. If you first connect the two negatives, then have made one positive connection, then accidentally brush the final clip against almost anything, it's likely to be chassis, and will short one of the two batteries. If you first connect the two positives, then any accidental connection with the negative lead will be to chassis, which is where it was going to be connected anyway, so no problem.

If the four 12 V batteries are identical, you can treat them as a single 48 V battery, once they are connected in series. Lead acid can be safely overcharged under defined circumstances (current below a threshold, topping up wet cells as required), which balances the batteries. There's no need for an explicit BMS as used with lithium cells, which must not be overcharged under any circumstances.

There is no need to connect a particular terminal first or last, unless the configuration of your conductors, like in the car example, gives you a specific reason.

Always include a suitable fuse in the circuit when using lead acid batteries. They are capable of very high currents. Thin connecting wires could start a fire, or burn you (I've burnt myself, I've burnt other people, I'm finally learning). Thick connecting wires could sustain high power arcs which could start a fire, or damage the batteries. A suitable fuse means one with a current rating not greater than the current rating of the smallest wire in the circuit.

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