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.
Sorry, but that is not likely to work. Both the voltage and amperage provided by a charger for a laptop battery will not be correct for a car battery. It is likely that you will damage the charger.
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.
9 or so would give you the correct voltage but they would not give you the amperage that a car battery would give you and that you would need to start the car.
when cranking the engine the battery voltage should not fall below 9.6volts if so replace battery
Yes, but they will not be as bright as they would be with the correct voltage.
Yes, if your car battery is the same voltage as the ATV battery.
yes
Normally in off load, its about 12.6 volts. 12.6 is correct for a fully charged battery. 12.4 it is 75% 12.2 it is 50% 12.0 it is 25%
Larger.
12.6 volts
Yes
12-14v