can 6 ,12 volt battery be wire to make in to one 24 volt battery for solar use in home
To get 24 volts from two 12 volt batteries, hook the positive post from one battery to the negative post of the other.
If you have two 6 volt batteries you can take a jumper wire and connect the negative post on one battery to the positive post on the other battery then connect the remaining pos and neg posts to the 12 volt battery charger.
Install a 3 volt battery and forget any modifications. It will not operate on 12 volts.
You can wire two 12 volt batteries in series... Connect negative of the first battery to positive of the second battery. That common connection becomes ground. The positive of the first battery becomes +12, and the negative of the second battery becomes -12. However, do not expect to be able to charge the second battery in a system designed for only one battery, such as in an automobile.
To use two 12 volt batteries in a 24 volt system they must be wired in series. For this explanation I will refer to them as battery #1 and battery #2. This answer describes the wire connections for a vehicle with a negative ground electrical system. The cable from battery#1 Negative terminal connects to ground (Frame or engine block). The cable from battery#1 Positive terminal connects to Battery #2 Negative terminal. The cable from battery#2 Positive terminal connects to Starter solenoid.
Charge it with a 12 volt battery charger.
wire it in series
You would wire them in "Parallel". That means positive post to positive post and negative to negative. Two 12 volt batteries wired this way doubles the capacity and amperage capability of the system. It does not cause any harm to the electrical system as the system sees them as one big battery.
No
12 volt
24 volts (although you'll ruin the 12v battery this way). If you're trying to get 36 volts, you want to wire them in series.
No, a 12 volt charger will overcharge a 8 volt battery and destroy it.