hi i have just got a dvd player in ma car and if i watch a dvd with the engine off obv its gunna run ma batt down so i need to no what meter i need in the car 2 show how much power i ave left in the batt so it dont die out on me!!!

Volt meter. You'll have to experiment with your car, but most batteries will show 14.4v while running and somewhere between 12.3-13v while off. If your volt meter drops to somewhere around 11.5v you might have trouble starting it, but that's where the experimentation comes in.

From my experience, voltage isn't a good measurement of how much juice you have left. A battery with 12 volts is said to have only 25% charge, while 12.75 volts should have above 75% charge. Generally, people do a resistance voltage test. You hook up a high ohm resistor and test for voltage. That's called a load test. I dont really remember what resistance value it was.