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This maybe is not a Corvette question but I know you geniuses know the answer !! My AGM battery has a 70AH rating. My cooler draws 40W DC. If my calks are correct I can only run the cooler 20 hours before the battery will be depleted? I=P/E. (40/12-3.4A) 70/20 = 3.4A Am I correct? If this is true, I'll be taking the cooler into the motel rather than leaving it plugged into the Vette overnight.
 

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70 AH / (40W / 12V) = 21 H

You are rounding, and also you have a minus sign where there should be an equals sign. The last equation needs to be set apart from the previous one, and multiplied through by the time divided by the current.

However, you did arrive at roughly the correct answer.

By the way, it will probably last longer than that: the voltage in your battery will be higher than 12 Volts, and the cooler probably doesn't truly draw 40 W even if it is rated for that. Also, it isn't likely to draw 40 W continuously. It will probably turn on when it gets warm, and turn off when it goes below a pre-set temperature. So, I would think you will end up with much more than 21 hours of cooling time.

Of course, I could be wrong.
 

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That's how I did test the AC current draw
That is great! So how much was the current drawn? Did you measure it when the cooler was running as well as when it had kicked off when it was cold enough?

If you know how much current it draws both active as well as dormant, and if you know about what percentage of time it kicks on, then you can reasonably accurately calculate how long your battery could power it.
 

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Why don't you just drink the beer? Then you don't have to worry how long the cooler needs to run and you will have more than enough battery power to start the car.... Not that you care after drinking all that beer...
 
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