Originally Posted by LittleBigMan
The formula is basically this:
Watts = Volts X Amps or Amps = Watts / Volts
So for every hour ("Amp/hr") of battery power you need, divide the total watts you are using (40 watts) by the voltage (12.)
40 / 12 = 3.33 or 3.3 Amp/hr. So you would need at least a 3.3 AH battery for each hour of run-time you want.
BUT--
There is a problem. The smaller the battery, the less it can carry higher wattages. For example, a battery that supports 20W for an hour might not support 40W for half an hour; it might run for 20 - 25 minutes. So you have to use "overkill" when selecting an SLA battery for your needs.
40W / 12V = 3.3 AH
2 hours X 3.3 AH = 6.6 AH
"Overkill factor" -- minimum of 7 AH (or even more.)
Remember that SLA batteries, while cheap and reliable, don't like to be completely drained. It shortens their life. ALSO--always buy a fuse for safety reasons, even a 12V battery can start a fire. You can get details from your local battery store.
ok, with this in mind... i decided to add another 10W bulb. now i've got a bouble lighting system, 30W total and i can use them together or seperate. the best part is the fact that i can use both for 75% of the time and then just the 20W(which is plenty) for 25% of the time AND still have the over kill factor of .45( 4.55 needed from a 5AH battery). that's some good stuff....
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