That is Impossible. You can however make two 6 volt batteries into a 12 volt system by wiring the two 6 volts batteries in Series. You will then have an output voltage of 12 volts but the amperes will only be what one of the batteries have, In other words the volts will double but the amps will stay the same as if you only had 1 battery connected.
Install a 3 volt battery and forget any modifications. It will not operate on 12 volts.
One is 9 volts and the other is 12 volts. They do not interchange!
12 volts battery.
Modern automobile batteries are 12 volts.
A fully charged 12 volt battery will output 12.6 volts are read with a digital volt meter.
12 volt system A fully charged 12 volt battery will have 12 to 12.7 volts. With you engine running you should have 13 to 13.5 volts
It will be a 12 volt battery.
12 volts
You cannot make a 6 volt battery into a 12 volt batter, but you can make a 12 volt battery out of two 6 volt batteries. Connect two 6 volts batteries together in series and you have 12 volts. Please define "in series".
The 12 volt battery has double the electromotive force of the 6 volt. Electromotive force is measured in volts.
To get 24 volts from two 12 volt batteries, hook the positive post from one battery to the negative post of the other.
No, an automobile battery charger outputs 12 Volts. This is far too many volts to charge a 5 Volt battery. You would destroy the 5 Volt battery with this charger.