hi,

i'm trying to teach my self, but getting confused.

i'm checking out this big battery and i want to calculate how long i

will be able to run different combinations of lights with it.

the battery has a 60 amp hour rating.

how long can i run 200 watts worth of lights from that?

how about 1000 watts worth of lights?

2000 watts?

teach me please. thanks for your help.

You need to turn "Watts" into "Amps" and "Time".

Ohm's Law will help you do that. Take a look at

<

http://www.the12volt.com/ohm/ohmslaw.asp> for more info.

Let's assume you're working with a 12 volt battery - You don't say what

voltage you've actually got, so I need a starting place. Change "12" to

whatever voltage rating your battery actually is and recalculate

accordingly.

You now need to know what the resistance of the light bulb is. Start

with your first one - 200 watts - You know the wattage of the load, and

you know the voltage of the supply. You want the resistance of the load.

Ohm's law says that the bulb's resistance can be calculated from:

Watts = volts^2/ohms.

So we solve for ohms:

200 = 12^2/ohms

200 = 144/ohms

144 / 200 = 0.72 - lets check that - 144/0.72 = 200, which is correct.

So, assuming your battery is 12 volts, your bulb must have a resistance

of 0.72 ohms to be able to dissipate 200 watts.

Now you can figure out the amperage it burns, using another facet of

Ohm's law:

Amps = volts/ohms

So Amps = 12/0.72, or approximately 16.6 amps.

Now you've got the critical piece of the puzzle that you need to answer

your question.

You've got a 60 amp-hour battery - which means it can supply 60 amps for

one hour, or 10 amps for 6 hours, or 1 amp for 60 hours, or 0.1 amps for

600 hours, or... (If you haven't caught the pattern, the equation is

Amp-hours = load(in amps) * time loaded (in hours))

So to get your "run-time" for a 200 watt bulb on this 12 volt, 60

amp-hour battery, divide battery capacity (60) by amps (16.6) to get

hours of run-time. In this case, that would be a bit more than 3.5 hours.

Don't expect to get that exact amount of run-time, though - resistance

from less-than-ideal real-world wiring is going to burn up some of the

battery's capacity.

The other examples are left for you to play with. The exact same math

applies to all of them. Don't forget to adjust for your actual battery

voltage - if you've got a 6 volt battery, replace all uses of 12 in my

examples with 6. If you've got a 9 volt battery, use 9 instead of 12,

and so on.