# Car battery light

• posted on September 4, 2005, 9:37 pm

Can anyone tell me as a rough guide how long could I use a 12v light (hours) powered by a fully charged 12v car battery.
Thanks Keith
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• posted on September 4, 2005, 9:44 pm
Depends on the wattage of the bulb and the power of the battery i.e 21 watt bulb on a 75ah battery shoud last about 40hrs
Keith Hampson wrote:

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• posted on September 4, 2005, 10:16 pm

Apart from the top posting I agree with Chris.
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• posted on September 4, 2005, 10:29 pm
Not 'anyone' , It would probably have to be someone with enough technical knowledge as to be able to work out a rough guide !!! :-)

(hours)
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• posted on September 4, 2005, 11:07 pm
Keith Hampson wrote:

Its not the voltage that dictates this, its the Current the lamp draws..
So its impossible to say
-- ®ÖĞ Remove spamtrap to reply.
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• posted on September 5, 2005, 12:57 am
Rod said! Its not the voltage that dictates this, its the Current the lamp draws.. So its impossible to say
WRONG
The sums go something like this Watts = Amps x Volts, so amps = watts/volts Say it is a 100amp hr battery, that is 1 amp for 100 hours So a 25W bulb uses around 2 amps, 25/12, so around 50 hours.
Chris said, Depends on the wattage of the bulb and the power of the battery i.e 21 watt bulb on a 75ah battery should last about 40hrs
The sums go like this & prove him correct 75/(21/12)B.8 or 75[AH battery]/(21[Watts]/12[Volts])B.8 hours
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regards
dave batter
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• posted on September 5, 2005, 8:26 am

No, RIGHT! The OP did not quote any watts for the bulb and without that knowledge you can't work it out.
However, like you, one can make some assumptions, although they may not be correct.
Rob Graham
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• posted on September 5, 2005, 8:02 pm
kitchenman wrote:

With only the Voltage figure it is impossible to work out. you are guessing the other fiqures -- ®ÖĞ Remove spamtrap to reply.
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• posted on September 5, 2005, 8:25 pm
kitchenman wrote:

With only the Voltage figure it is impossible to work out. you are guessing the other fiqures
Kitchenman was not guessing. He gave a plausible answer and did not guess the figures (variables). He gave a worked out example that the OP could use to answer his own vague question. If the OP cannot work out the answer from Kitchenmans example then he should say so and give more info on his battery and lamp.