12v charging, 24v starting

  • Thread starter quize
  • Start date
  • #1
hi all,

i have this qns which i been figuring, but cant get the right circuit.

i have 2 12v batteries. which i want to wire in series to give it a 24v starting to a motor.
however by doing that, i am unable to charge both batteries as my rectifier is only able to charge 12v. I need to make it to become "one" battery by making it parallel.

i think about it and i guess it need a DPDT switch to do the job.

but not sure about the circuit layout.

can any kind soul advise me. thanks


Answers and Replies

  • #2
This is fairly easy. Lets call one battery the first and one the second battery.

1) Connect two wires to the leads where the second battery is currently contacted.
Now Imagine, if you would rip out the second battery and connect it for charging, where would you put it (answer in parallel to the first battery - mind the polarity).

2) Connect two wires to the place where you would connect it for charging.

3) Put the for wires on the output of your switch.

4) Connect the second battery to the input of your switch. In one position the battery is connected as before ind the other it is connected for charging.

  • #3
Science Advisor
The batteries are used for starting a motor, so they can have loads of a hundred amps or so, even at 24 volts.

So, you really don't want to be putting switches in circuit unless you are prepared to get some really big switches and wire them up with starter motor cable.

You could charge up the batteries independently with the 12 volt charger. Just move the charger wires but leave the battery wiring alone.

You could try putting another 12 volt battery charger in series with the one you have got.

You could hunt around for a 12 volt to 24 volt DC to DC converter. I don't know if they exist.
You would have to put some current limiting on it to make it suitable for battery charging.

You could modify your battery charger with a voltage doubler circuit. This would give you some trickle charging.

The best and cheapest solution is to get a 24 volt battery charger.
  • #4
hmm i have draw out a circuit maybe some can advise if this is correct as what was mentioned

i have attached the original circuit, and the added on.


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  • #5
Science Advisor
I have attached a circuit which may be what you were describing.

This shows the batteries being in parallel most of the time and hopefully charging from the vehicle battery charging system. This normally involves putting rectifiers in series with each battery, but I haven't shown that.

When the starter switch is activated the relay contacts move across and connect the two batteries in series.

This is not a practical circuit, though, as the currents involved would be huge.


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  • #6
looks about right, switches for these purposes exist
  • #7
Truck manufacturers such as Kenworth,Mack,Freightliner&White used such a switch
(called a series-parall switch) on 1945-1965 trucks.

Find a dealer mechanic or starter alternator service to help locate one.
  • #8
12 volt to 24 volt DC/DCf converters do exist and can be purchased from Power Stream for a little over $100.