[uthe54]

heres how you figure out how long the battery should last.

a 55 watt light bulb on 12 volts is about 4.6 amps (watts divided by volts, eg55/12 = 4.58333). In theory, if you had a battery rated at 4.6 Amp Hour (Ah) you could mathmatically run the light for 1 hour. If it were a 9.2 Ah battery, it should run for 2 hours, etc.

now I say should for three reasons.

Reason 1 - just like in a flashlight, the light starts getting dark way before the battery is low. thats because the voltage is decreasing as the battey is begin drained, thus the amount of watts supplied to the light is decreasing. the Ah rating is (my understanding at least) when the bulb is completely out.

Reason 2 - it's extremely hard on a lead battery to discharge all the way. I think I remember reading somewhere that a car battery can only completely charge and discharge a small number of times - like 10 or something. A deep cycle/marine battery is supposedly better at withstanding full discharges, but it's still not good for them. I think once a lead battery has discharged to about 70% capacity, it starts to damage the battery.

Reason 3 - This is the rating of the battery brand new. obviousally as the battery is used and recharged, it'll lose its ability to hold a full charge.

With all that in mind, if you get something like say a 100Ah battery, you should last 21 hours in theory. multiply it by 30% (.3) and you get 6.3 hours to put the battery is at 70% - which is when it'll need to be charged without damaging the battery.

also note, Ah is not the same as cranking amps. For example, the batteries in all my buggies are 90CA, 9Ah - obviousally built for starting the buggy, and providing a buffer for the headlights/whatever other electronics are onboard - not for running headlights with no generator.

sorry for the long post. I hope you learned something.