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Need new batteries? Going from 12V to 18V or 6V to 12V? Wiring Questions?
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By Turbo1002
I have been searching the forum looking for a drawing with no official results. How to wire three 6 volts batteries in series for run and parallel for 6 volt charging. Would like to make up two posts to connect the charger to and flip a switch. All the drawings I have seen show anywhere from one dpdt relay to six relays with diodes. None of them actually work. Can someone please point me in the right direction. Thank you.

Yeah, pretty complicated plus the charger can't really balance charge more than one battery when wired in parallel, it only sees average voltage of the batteries, not individual charge levels. It's also possible that if one battery dies, it would discharge the others.

Best route is individual charge ports for each battery and charge them one at a time (or 3 chargers to charge them at the same time). You could also have an 18v charger (hard to find but do exist) to charge the batteries in series and then do an individual balance charge from time to time
I'll take a shot at this. This uses a manual toggle switch, instead of relays.
4PDT Toggle switch
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This is an animated gif, doesn't seem to play in the preview though. For clarity, fuses have been omitted. If you plan to wire this up, make sure you have fuses on each battery.

The tricky part is finding a suitable 4PDT switch that can handle the current when running at 18V.

I found this switch on Amazon which handles up to 20A @ 125V, which should be good enough for this situation. (The typical SPDT bosch-type relays are usually rated for about 30A or so). I'm sure you can find similar switches on ebay as well. ... B001TJ6O9M

BTW, bosch automotive style 6V SPDT relays do exist, they're just harder to find and more expensive than the 12V versions: ... tive+relay
I've wrestled with this problem myself and have come to the conclusion that you don't need all the relays at all. It's over-thinking the problem (don't feel bad- I did the same thing)

Just wire a charger port to each of the batteries individually, and the batteries themselves can remain wired in series to the car for more voltage. No switches, no relays.

Look here, and scroll down to #7: ... tery-packs

"Figure 7 shows two 12 Volt batteries connected in series. The resulting battery pack voltage is 24 volts. As you can see, each battery is connected to a single 12-volt charger. This is probably the best way to ensure that each battery is completely recharged to its full capacity after each time that the battery pack is discharged. This eliminates most of the problems associated with cycling batteries connected in series strings."

I've also asked an electrical engineer friend and he confirmed that this is indeed not only correct, it's better than one charger of higher voltage powering several batteries.

I haven't done it yet, but I am about to re-rig my daughter's dune racer for 24v (but with two 12v chargers) in exactly this way.

I think one large breaker/fuse for the series battery power, plus small fuses to each charger, is the correct way in this case.
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Yes, that is absolutely the best way to do it. That's how I do it with 12+6 setups as well.

The way I understood the OP was he wanted to charge all 3 6v batteries in parallel simultaneously with a single charger so that's kind of the direction this thread went.
By Rob222
Am I understanding this correct also :
To charge all 3 6v batteries simultaneously with a single 6v charger they need to be wired in parallel.
To run them at 18v they need to be wired in series.
You can't have them wired in series and parallel at the same time.
The original OP wanted an "automatic" way that would switch over from series to parallel when the 6v charger was plugged in (the charger voltage would control the relay position). This would make charging from a single 6v charger fool-proof. It ended being relatively complicated to do.
Okay here is how this works.

You are running the three 6v batteries in parallel to get 18 volts to power the car.

You can also wire one charger in series to all three batteries AS LONG AS the series circuit is removed before operating the car via the parallel circuit. Else you would probably fry the charger.

So the easiest way that I figure to make this foolproof:
You rig three charger ports in the car itself, one each wired to the +\- terminals on each of the car's 6v batteries.
Then you make a wiring harness directly to the 6v charger with three charge plugs (for the three ports) all wired in series.

As long as you don't operate the 18v car with the series-wired charger plugged in, you are fine.

My preference would be to use three separate 6v chargers however, as when one battery does die then it will proceed to kill the other two batteries when you go to charge them.
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