How long will a 6volt 4AH battery,run a 3w Led light? thanks.

Theoretical runtime should be a maxiumum of 8 hours.

To allow for battery age and temperature variations, I'd bet on 6 hours just to be safe.

Teaching you how to fish:

P = I * E
watts = amps times volts
so amps = watts divided by volts

3 watts at 6 volts = 3/6 = 1/2 amp

an amp hour means it'll deliver an amp for an hour, or half an amp for two hours etc
1/2 amp from a 4 amp-hour battery = 8 hours.


Note that you have to derate batteries if you're trying to draw a ton of current from them because they have an internal impedance that will cause a lot of their energy to be converted to heat, but you're nowhere near that.

Also, never trust ratings. 8 amp hour batteries will give you that, maybe, after they've been cycled a few times, until they start to die.

Not enough info. Need to know the V/I characterstics of the LED and any associated circuitry. Based on LED voltage drop of 3.5 volts , I'd estimate worst case as something under 4.5 hours, and best case as much as 8 hr to battery exhaustion. It's never good to exhaust a battery so cut those down by at least 20%.

Thanks everyone,I can now use my raleigh led dynamo light as a replacement for my old fishing headlight.I always take a spare,but its good to have an idea of the run time.