Battery Selector 3-way Toggle Switch - Wiring Query

I have a 36v system with two batteries, one 20AH, and one 10AH. I know most folks here would put them in parallel and just disconnect them for charging, but for various reasons, I want to carry both batteries and switch between them at will.

I bought a 3-way toggle switch - A on, A&B off, B on. Turns out of course that I got an AC switch rather than DC but according to the AC-DC calculator I found, that switch should be able to handle 76 amps at 36v. Which is more than twice my peak load so I should be OK on that.

However, I'm not sure how to wire the darn thing. It has three screw terminals (one for A, one for B, and one for the power out to the controller). I understand that the positive wire from each battery should go to A and B and that the power out wire should be the positive wire to the controller.

But what do I do with the negative wires? I've seen 'ground them to the bike frame' and I've seen 'connect them to each other' neither of which I am comfortable doing without one of you guys with the enormous brains explaining how it's safe/cool/functional.

Thanks for any help.

docrocket said:

But what do I do with the negative wires? I've seen 'ground them to the bike frame' and I've seen 'connect them to each other' neither of which I am comfortable doing without one of you guys with the enormous brains explaining how it's safe/cool/functional.

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Just connect them together.

BTW I'd go with two toggle switches instead of one SPDT switch. That way when you do want to use them together (and you will at some point) you can.

billvon said:

Just connect them together.

BTW I'd go with two toggle switches instead of one SPDT switch. That way when you do want to use them together (and you will at some point) you can.

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OK, so all three negative wires all just glommed together? And that doesn't create a connection during the charge cycle? I really don't want to blow anything up.

Hmm, so two toggles would be basically the same thing as wiring them parallel, right? The output from each toggle would go into one joined wire going to the controller, so if they were both on, I'd have more amperage but not more voltage. And I could turn one off, or both? Sorry to belabor the point, just want to be sure I get it.

docrocket said:

OK, so all three negative wires all just glommed together? And that doesn't create a connection during the charge cycle? I really don't want to blow anything up.

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Any current flow needs a closed circuit. By opening the top of the circuit, no current can flow into the disconnected battery.

Hmm, so two toggles would be basically the same thing as wiring them parallel, right? The output from each toggle would go into one joined wire going to the controller, so if they were both on, I'd have more amperage but not more voltage. And I could turn one off, or both? Sorry to belabor the point, just want to be sure I get it.

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Yes. You would have four options:

Both off - bike off
A on, B off - battery A on
B on, A off - battery B on
Both on - both batteries on. Then they share both charge and discharge, at least as much as their differing ESR's allow.

Voltage is always the same. Caveat - you have to make sure they are similar (not identical) in voltage before connecting them both.