Quote:
Originally Posted by orange4boy
If you have an ammeter in your car you can estimate your battery life by the capacity rating. Example is a Trojan 12v 27tmx with a 175 minute capacity. This battery can provide 25 amps of power for 175 minutes in ideal conditions and down to 100% discharge. You should really only discharge to 50% (12.06V) max for deep cycle batteries. So half that figure is about an hour and a half.
Watts / volts = amps. Amps x volts = watts.
Low beams are typically 55 W each X2 = 105 / 12 = 8.75 amps. The heater fan depends on the setting and the vehicle. Many lights can be replaced with
LEDs which only use a fraction of the power.
From another forum...Don't know the car model but would be good enough to estimate.
The more current you draw the less capacity your battery can provide.
Your homework 
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Thanks for the info! I'm probably going to need a new battery this winter, so I may look into using a deep cell.