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Any electrical guys out there who might know how to calculate how long an automotive battery can supply 250 watts? Guessing there's some power loss to heat in the inverter as well.
Take the amp hour rating of the battery, multiply it by the voltage (12 for a 12 v battery) and thats the watt hours. IE the number of hours you can take 1 watt out, or the number of watts you can take for 1 hour.
So say you have a 70 amp hour (AH) battery, a large car battery that is, you say 70 * 12 = 840 watt hours. So say you are taking 200 watts, that means you will get 840 / 200 = 4.2 hours out of it.
The above is assuming 100% efficiency which you wont get. Inverters run from 80% to about 95% for the best, there will be some small losses in the battery too but probably not that much. Assuming a total efficiency of 80% that 4.2 hours becomes 4.2 * 0.8 = 3.36 hours (thats 3 hours 22 minutes, not 3 hours 36 minutes).
I think most inverters will survive being plugged into a slightly higher voltage from a battery under charge (a running car). The voltage off charge from a 12V battery is about 12V, but when its under charge (IE the engine is running) that will go up to about 14.4 volts. As I say though, although I havent tried it, most inverters should probably be able to take this without damage, as they are designed to survive inductive surges etc. So that means you should, if it comes to it, be able to plug your inverter (or converted UPS) into your car battery while the engine is running. Make sure the positive and negative are connected the right way round and improvise a fuse, like a short length of thin wire, if you don't have a fuse, because car batterys can deliver thousands of amps for a few seconds which can melt thick wires even.
While running at 2500 rpm or more the car alternator should be able to deliver 70 to 130 amps, so should recharge the 70 AH battery I discussed above in an hour or less. If the engine is at idle, it wont produce this. Its hard to say how much, but figure maybe 20 amps for safety, although its probably more. Bigger trucks have bigger alternators and supply more current.
Don't let the battery get deep discharged as it will damage it (IE dont run it flat). Keep recharging it by running the engine. Put the inverter close to the battery because you need thick wires from battery to inverter, thinner wires are ok from inverter to aquarium.
i wonder how i will be in a year when i move to denver!