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.
12-14v
12.6 volts
Yes
Sure, if the alternator voltage regulator is defective it can overcharge the battery.
The number of cells in a battery determine the battery's voltage. Different types of batteries have different cell voltages. The cell voltage is dependant on the reactive metals used. A typical car battery will have six cells. Because a typical cell voltage has a source voltage of approximately 2.0 volts, this makes a car battery produce a voltage of 12 volts.