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#### Plebmaster

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Odd query really... Is there such thing as a method (or calculation) to work out how long a battery will remain charged for?

So if I know the total charged AMPs and Voltage, and I know the constant pull on the battery (AMPs) whilst it's not charging, can I work out the time before the battery is completely discharged?

Thanks,
Luke

#### straightliner

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yes

The formula for converting Amps to Watts is : Amps = Watts / Volts

To work out the total number of amp hours you divide by the voltage (12v) total watts /12 volts = amp hours.

#### AndyF_RSX

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You need the A-h rating for the battery and then it's a *relatively* simple task of just taking the amp draw from the battery and dividing the A-h by the draw to give you an approximate life.

However, if you're discharging the battery quickly then the A-h rating becomes less than the stated rating so you need to put in a fiddle factor to the power draw.

Then... you note that it's not a good idea to discharge the battery fully and you should still leave around 20% of it's charge in there to be able to recharge it properly and you get the following:

Time to discharge = rated capacity (A-h) / (current draw ^ 1.1) * 0.8

As an example.... if you have a Halfords HBO54 battery on your car then it's rated capacity is 45 A-h

If you have a current draw of 0.5 A then it will last

45 / (0.5^1.1) * 0.8 = 77 hours or just over 3 days.

Hope that helps

Andy

#### Plebmaster

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Cheers gents :smthumbup

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