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post #5 of Old 10-07-2012
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Re: Battery capacity - Peukert stuff

Originally Posted by Stu Jackson View Post
I don't think so. The P equation has only time and discharge amperage involved, Cp= I n t where n is a log function of I and t. It has nothing to do with battery bank capacity, it tells you the difference in time remaining when larger draws are made.

In fact, most battery monitors, if they're like the Links, only affect the time remaining function and NOT the amp hours consumed. Why? 'Cuz the only effect the Peukert function has is time remaining (which, of course, HAS to be based on any given battery bank capacity). That's simply linear -- bigger bank, bigger time remaining.

The Link manuals, especially the Link 2000, are pretty good at explaining this stuff. Xantrex has a discontinued models manual download section on their website.

The P equation essentially says: a higher load beyond the 20 hour rating takes more out of a bank, of ANY size, a lighter load takes less.

If you do the math on the exponential function, you'll see the differences.

In the real world, if you are imposing unusually larger loads on your house bank, it'll last less time because higher draws reduce the power availability, but only on time remaining, not on amp hours consumed.

That said, it really makes little difference.

Do the math.
Stu I don't think that's quite right. When you add another battery in paralel you're adding more capacity but you're also cutting down the load on each battery. This makes it non-linear.

A 100Ah (@20 hour rate) battery can supply 5 amps for 20 hours.

Two of these cells in paralel are rated for 20 hours at 10A. Running them at the 5A from above means they're now operating at half their reference rate and they'll go for more than 40 hours because of the equation. So for a given load, doubling the capacity more than doubles the real-life time.

I ran through the math and it bears this out (note the capacity is within the exponential). It's about 10% as bhcva said.

Last edited by asdf38; 10-07-2012 at 01:39 PM.
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