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Curious 101

if you leave your light on how long would it take for battery to go dead.

What light? How big is your battery? How dead? So dead it won’t start the car? Or so desd the battery can’t be recharged? It depends on those 3.

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If you had a full 50 amp-hour battery and used it to run 12 watts of dome lights inside your car, then the math says you’d get about 50 hours.

Please show the math. I am curious, too, and get those formulas all mixed up.

And that’s one good reason why new cars use LEDs in the dome light, and every other light in the car, if they can.

@lion9car only has part of the math correct. At 12 volts, a 12 watt light would use 1 amp of current. 12 watts/12volts = 1 amp. A 50 amp-hour battery can supply 1 amp for 50 hours… in theory.

In actual practice, the battery would draw down to about 40% state of charge in 30 hours {(50 x (1- 40%) = 30 hours} and would not be able to start the car.

But headlights alone draw 35 watts EACH or 70 watts plus the tail-lights so the battery would drain far faster.

That’s true, if you want to use that definition of dead, which is reasonable.

And now we can debate the definition of dead! Oh boy! Never did that before on Car Talk.


I do, I DO! :skull:

In a practical sense, it is “dead” since it will no longer do its job of starting the car.

I have run the battery dead enough to not crank the engine a couple of times, running a cooler overnight while camping. I am not sure its amp draw. I was careful to park on level or slightly downhill asphalt road, so it was easy for my buddy to push the car while I popped the clutch once it was rolling well. It started right up: proof that a battery that can’t run a starter motor might be able to power up the ignition and fuel pump.

These days, any draw that pulls the battery down below 9 volts would prevent a start even without the starter.The ECU won’t wake up to drive the injectors and fuel pump.

I the way-back carburetor days, a car with a stone dead battery could still be bump-started with a push. Fuel in the carb didn’t need the electric fuel pump and the points ignition would spark from the dead. My 1971 Datsun 4-speed went a long way with a junk battery since I parked it pointing down-hill at home and school.


My 99 Honda may be closer, electrically, to that 1971 Datsun than to today’s cars!

Can push start: one more reason to prefer a manual transmission.


But a battery below 9 vlt can be revived to life. So it is just sick.

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Yeah that would be my definition for practical purposes. If it starts the car it wasn’t quite dead but it it doesn’t and the lights don’t come on it’s dead enough to call a tow truck. To boil it all down to practical terms, I think if you left your headlights on all afternoon, it would no longer start the car.

2-3 days for the dome light as mentioned above, but it also depends on the car. A newer car might not last as long b/c if the dome light is on the computers may be awake too, and the powered-up computers will add significantly to the current drain on the battery. So a dome light left on just overnight could drain the battery enough to prevent starting in some newer cars, depending on the situation.

We may need the Dead Parrot sketch from Monty Python.

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Couldn’t help me self.:scream:


All of the newer cars with which I am familiar are programmed to turn the dome light off after 20 minutes or so.

My favorite sketch!!