Car batteries are rated by "Reserve Capacity". To convert from 
reserve capacity to amp hours one must first understand the 
definition of "Reserve Capacity" which is the time in minutes that a 
new fully charged battery will deliver 25A at 80 degrees Fahrenheit 
and until it discharges to 1.75V per cell. So you take the reserve 
capacity and divide it by 60 to get the hours. Then take the hours 
and multiply it by 25 to get the amp hour capacity.

For example a Sears Marine Deep Cycle Battery (model 27494) is rated 
at 135 minutes of reserve capacity. 135 minutes divided by 60 minutes 
= 2.25 hours, 2.25 hours times 25 amps = 56.25 amp hours.

I have found in real life the calculation comes out higher than an 
actual battery can deliver due to temperature, physical condition of 
the plates and electrolyte, state of existing charge, etc. It does 
get you in the ball park though.

Hope that helps.

David

At 11:10 AM 6/14/2009, you wrote:
>Let's say you have a device that you leave permanently plugged in to the
>vehicle, and that draws about 100 mA. How do you calculate (or estimate) the
>time it would take for the device to drain the battery to the point where it
>won't have enough power left to start the engine?
>
>To clarify, I am more interested in determining the number of hours it is
>safe to leave the device plugged in, rather than the number of hours it
>takes to kill the battery for sure (although both values are useful). :)
>
>Vitaliy
>
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