How do I switch between 2 LIFEPO4 batteries

My wife uses a mobility scooter. I recently replaced the 2 x 12V 12A AGM batteries (in series) with 1 LIFEPO4 24V 20A battery (480 Wh), which turned out to be a great success. A smaller lighter battery with a significantly extended range. We now want to take it with us on holiday but the airline limits battery watt hours to a maximum of 300Wh, and only one battery. So I bought another battery 24V 12A, which is 288Wh. OK so far, we can go on holiday.

When we come back I would like to be able to mount both batteries on the scooter and have a switch to run the scooter from either battery. I know that the LIFEPO4 batteries cannot be run in parallel because of their individual BMS controls, so any switch has to isolate battery 1 and energise battery 2 and vice versa.

Does anyone know how to do this? Any help or ideas would be appreciated.


I think what you want is something like an RV Battery Switch and are used to switch between 2 different battery sources since RVs often have a “house” battery and a vehicle battery.

like this: just never put it on 1+2:


@dougl Is correct, there are quite a few solutions for RV’s, boats etc. available since this is quite a common problem.

The cheapest/easiest would be two of the very cheap battery isolators you can find in most car/boat/hardware shops:

Then just have a single key available with the scooter, so only one battery pack can be engaged at a time.

If you want easy switching from the control stick (which may be needed, depending on your wife’s dexterity etc…) then a big 24V relay (suitable for the current) and a remote toggle switch would work.

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Thank you for your suggestions re battery change over switches, but a pal of mine came up with a great solution, right off the bat, a DPDT toggle switch. Double Pole Double Throw 6 pin switch. Pins 1 & 2 are battery 1 and pins 5 & 6 are battery 2 with pins 3 & 4 being the supply to the scooter control board. An elegant inexpensive solution.

Here is the one I have purchased. Up to 48V 20A

Again, thanks to all and I hope this helps someone else.


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@DuncanC is there a fuse or breaker on the battery line which lead you to the 20A rating on the switch?

If your scooter doesn’t see 20A then that’s a very nice solution but watch out for the spade connectors if you soldered them because they’ll likely break at the solder joint. Far better to crimp.

Just don’t flip that switch while the scooter is under power! :smiling_face:

I don’t know whether you already know this, but better safe than sorry: Switching DC with meaningful current flowing through can weld the switch because there are no zero voltage crossings (as with AC) to quench the arc.

I can’t imagine why you would switch while running on purpose, but you might want to arrange the location to not be easily able to do it by accident.


Thank you Michael, I didn’t already know that. And with that knowledge I realise I should have bought an ON-OFF-ON switch like this.

I’ll make sure this is the one used, just to ensure safety if my wife or me accidentally switch while using power.

Thank you

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Good thinking. At least in theory if the switch is make-before-break (make contact with the new pole before breaking contact with the old pole), then you could have a moment of charging one battery from another at potentially high current (but I don’t know how much).

I didn’t mention that part because I think that break-before-make is the norm, both because break-before-make is easier to design and manufacture, and because most applications either prefer it or don’t care.

Having an off position is also a good idea for servicing it. :grin:

(Incidentally, I learned about this classification because on airplanes, the switches that change the supply electrical bus for components are explicitly break-before-make so that faults don’t propagate; it’s why the power may glitch when they switch from being powered from the gate to generating their own electricity before pulling back from the gate. I just thought it was cool when I learned that the power glitch was really a safety feature.)


I would recommend an RC or snubber circuit across the contacts due to the high current that you could possibly be switching. This will protect the life of the contacts.


Michael, I have checked and the new switch is position 1 - ON (battery 1 connected), position 2 - OFF (neither battery connected) and position 3 - ON (battery 2 connected). That makes a lot of safety sense to me. Thank you for the info.



Good! Just to be clear: Many ON-ON double-throw switches still have a momentary OFF in the middle (break-before-make) but all ON-OFF-ON switches, by definition, break-before-make. :smiling_face:

If you have any concern about accidentally engaging this switch while riding, @HalfNormal is right that the solution is a snubber. It would go on the load (common) side of the switch. Let us know if that’s a concern and if you need help. They can be made from individual components if you have them, or purchased as units.


Thank you for all your help. The battery changeover switch is in a safe place on the scooter which cannot be accidentally switched. I may even fit a switch cover for double security.

I thought you might like to see the dashboard panel I cut with my modified K40

For your info :-