AH calculations

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AH calculations

Postby ShreddinPB » Tue Aug 21, 2012 12:50 pm

Hey guys.. I am a little short on knowledge here on how to calculate how long batteries will last under draw..
I have an LED strip that says it takes 2A. If I want to run 4 of them, I assume then they use 8A.
How long will that take to drain a 17Ah batter?
Is it as simple as 17/8 = 2.125 hours?
Also, SLA and car deep cycle batteries should not be drained completely right?
If they should only be drained to 40%, then that means I would get only about an hour of light running all 4 at once?
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Re: AH calculations

Postby some seeing eye » Tue Aug 21, 2012 2:06 pm

You are correct assuming the voltage of the battery and the load is the same. I would shoot for 50% capacity, depends on the battery type. You can buy more batteries and buy a cheap electrical multimeter that can read the actual draw in Amps. Usually the label on the load is the max, which many not equal the typical.

You can buy more deep cycle lead acid, sealed lead acid, AGM, or gel cells. Then you can take them to Snow Koan Solar for charging. If you have a lot of heavy batteries, it's useful to bring a little red wagon, which will also tow behind a bicycle. They can explain how to use the meter to tell when the battery is charged or discharged.

Sure there are battery stores, and solar or marine supply stores nearby you. Car batteries are not deep cycle, they are max cranking batteries. But a deep cycle marine battery can physically look the same.
Last edited by some seeing eye on Tue Aug 21, 2012 2:26 pm, edited 2 times in total.
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Re: AH calculations

Postby crstophr » Tue Aug 21, 2012 2:06 pm

Yes you've got it!
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Re: AH calculations

Postby maladroit » Tue Aug 21, 2012 2:16 pm

If the LED strips do anything cool, like color cycle, the current rating might be maximum. However, if it was doing an RGB cycle, the average current might be 1/3 maximum so you'd get a lot more runtime. This also applies to blinking and other patterns.
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Re: AH calculations

Postby ShreddinPB » Tue Aug 21, 2012 3:32 pm

Awesome thanks guys!
I already have a multimeter, lots of them actually. I am just a tinkerer tho so have never had to calculate AH before.
I am planning on getting the deep cycle battery from Costco that I have seen posted about in many threads.

I have one white led strip and one RGB that does cycles, I thought the RGB would suck a lot more battery but I guess during the down time on them, they arnt using as much energy
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Re: AH calculations

Postby gyre » Wed Aug 22, 2012 2:47 am

Batteries actually have an amp-hour curve, typically decreasing in capacity as draw goes up.

There is usually a graph for different temperatures too.
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