This will depend on the size of the 12 v battery.
Now adays about 36 - 48 months is all you get from one.
Charge it with a 12 volt battery charger.
Yes, as long as it will fit in the space provided.
No
12 volt
As long as the ATV is wired 12 volt, not 6.
No, if they are wired in series you have 36 volts assuming each battery is a 12 volt battery. They will have to be wired in parallel to output 12 volts. In that case you can charge them with a 12 volt charger but it will take 3 times as long as charging just one 12 volt battery.
It takes between 8 and 12 hours to charge a 12 volt battery at 2 amps. The actual time will depending on how much charge is in the battery initially.
About 2 hours if the battery is 100% discharged.
No, a 12 volt charger will overcharge a 8 volt battery and destroy it.
The battery is a 12 volt.