12.6 volts
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
Larger.
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.
Yes, IF the operating voltage of the stereo is the same as the battery output voltage of the vehicle.
No, you cannot charge a 28 volt aircraft battery with a 12 volt car charger. The charger voltage needs to match the battery voltage for safe and effective charging. Using a charger with a lower voltage can damage the battery and pose safety risks.
dc - the voltage is a constant 12 volts nominally in a normal car battery.