12-13 volts DC
For a standard "12 volt" battery, the voltage should be 12.6 to 12.8 while car is off, 13-14 while running. Some newer cars (e.g. recent model Hondas) have higher voltage batteries.
get a multi meter that reads dc voltage & touch the black (negative -) wire to the negative battery post & the red (positive +) wire to the positive battery post with car running. car off is battery voltage, car on is alternator voltage.
when cranking the engine the battery voltage should not fall below 9.6volts if so replace battery
Yes, if your car battery is the same voltage as the ATV battery.
yes
4.5v
Larger.
12.6 volts
Yes
12-14v
Sure, if the alternator voltage regulator is defective it can overcharge the battery.
Voltage is the same but amp hour capacity of the car battery is much higher.