How can I tell when the car battery is almost dead?

I don't know much about car battery, but I believe that it recharges automatically when you run the engine. But if I wanna charge my phone or laptop when the engine is off, how can I tell when I should stop using the battery so that it has enough charge to start the car? Do I use some kind of voltmeter or something? If so how many volts/amps/watts does the car need to have left to be able to start?How can I tell when the car battery is almost dead?
When the engine almost starts is the closest answer. This is an unknown factor, i dont know how good the charging system, or the condition of the cables that carry the current because old damaged and corroded cables draw more current than new clean cables, and or the age and condition of the car battery either. Most cars today need at least 12.5 volts to start the engine. You are better off recharging the laptop in the house or with the engine running to be safe.

good luck.....
Sluggish starting, dimming lights, malfunctioning/weak buzzers and such, etc. are all a sign of a weak battery. Everything requires at least 12 volts to even crank, though most batteries hold a bit more charge (12.5 - 13).How can I tell when the car battery is almost dead?
your engine will turn over slow or just go tick, tick, tick. if your going to charge your phone start it up once in awhile to keep the battery charged.
U CAN CHARGE UR PHONE 1 HOUR

BUT DON'T KEEP THE SWITCH ON

IF U SHOULD KEEP ON THE KEY

BETTER START AND KEEP FOR CHARGEHow can I tell when the car battery is almost dead?
When the car start is will take just alil bit longer then it would normally do..
battery wont charge unless engine is running
When the headlights are orange instead of bright white.. that's a good time to stop using up the battery.
at lease 12.5 and yes a volt meter is good to have.