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Join Date: Aug 2007
Location: Manor, TX
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There are a lot of variables in that that you didn't mention. What you need to know is the ah capacity of your batteries, multiply by 2 because you have 2 batteries. Using a Diehard platinum as a example, you have 75 ah x2 at 20 hours which means you can draw 150 amps for 20 hours before they are dead. Now this assumes that the batteries are fully charged, new and the temperature is fairly normal. Now you need to figure out the required minimum power to get the truck to crank and replicate the exact same conditions, ie. no vent running, no lights on, same amount of time to crank, etc. Subtract this amount of power from the 150 ah c/20 rate and you will be in the ballpark.
Basically .96 ah would take 156.25 hours to kill the batteries by itself. Typically you don't want to go below 50% to avoid a premature battery failure, less if it's a starting battery. Let's say 80% soc to be safe so 150 ah x .2 / .96 would be 31.25 hours. But don't forget the truck's normal draw when off, typically >.05a but not sure on the 6.7 so 150 ah x .2 / (.96+.05) is 29.7 hours. To play it safe, you could move for a 90% soc which would be 150 ah x .1 /(.96+.05) which would be 14.85 hours. I would use a 15% correction factor for unknown condition of batteries and draw so 14.85 x .85 is 12.6 hours.
On the other hand you could just disconnect one battery so you know you have one good one to help start the truck. It's not a good long term solution but wouldn't hurt if you just did it a few times.
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