Thinking about a Truck Fridge.

2oldman wrote:

~DJ~ wrote:
The specs say 24 watts per hour. That is 2 amps per hour?

Nobody's said it yet, so I will.

There's no such thing as 'watts per hour' or 'amps per hour.' It's watts, or amps. If your spec is 24watts, that means it will draw 24/12=2 amps from a battery. In one hour, that's 2 amp-hours.

Batteries are rated in AH (amp-hours), not amps/hour. Yes, it's a fine distinction, but some of us are picky that way.

If you want to get really picky, they are actually based on a 20 hour drawdown, meaning a 100AH battery should supply 5 amps for 20 hours. If you draw 50 amps from it, you probably won't get 2 hours like you may calculate, but somewhat less. One of the reasons for this has to do with the square of the current times the internal resistance, which is the power lost in the battery. As you can see, 10 amps squared is 100, but 50 amps squared is 2500. If you have 20 milliohms of internal resistance (pulled that number out of me bum, but it is probably in the ballpark), then you are losing only 2 watts inside the battery when providing 10 amps of current, but you are losing 50 watts inside the battery when providing 50 amps of current. There will be additional I^2*R losses external to the battery in things like the wiring which also are made a lot worse by larger I values.

Everyone asleep now? I am good at bedtime electronics stories.