More on the effects of Peukert's Equation.

Make sure you have read (and understood) the section on Peukert's Equation before reading further. If you haven't, go back to the technical page and read it (unless, of course, you already know all about it).

This will simply make no sense unless you understand Peukert's Equation and it's effect on battery capacity.

A common problem on any sort of "off grid" power system is insufficient battery size. No matter how good your charging system, if the battery bank only has 200 amp hours available, and you need 300 amp hours, then you have a problem.

When large battery banks are involved people often live with an insufficiently sized bank because of the expense of buying more batteries. However, it isn't quite as bad as a cursory glance would first lead one to believe. The reason is Peukert's effect......

Whatever discharge rule you use (be it the 50% rule or any other discharge floor) will make no difference to this section.

Let's take an example of a 400 amp hour battery bank. This (by the 50% rule) gives you 200 amp hours of available power.

Let's assume you want this battery bank to supply your power needs for 24 hours. But in actual fact it is only lasting you 12 hours. First thought is "I need to double the size of the battery bank"

This is incorrect, and the reason, again, is Peukert's effect.

You are discharging 200 amp hours in 12 hours. So your average current drain is about 16.7 amps. 16.7 amps for 12 hours = 200 amp hours.

This is actually not the exact figure, but it's close enough.

Go to your new Peukert Calculator. Enter 400Ahrs for the battery bank. 1.3 for Peukert's Exponent and 16.7 amps for the current draw. You'll see it calculate the total amp hours available as being 422 amp hours. But remember this is until totally flat. The 50% rule says we use half of this. So far so good.

Anyway, look at the available run time. 25.28 hours. 50% rule gives us 12.64 hours.

Now put in 800 Ahrs for the battery bank size (double the size). You'll see the total available amp hours has increased to 1040. Which is substantially more than double what we started with. Available run time has increased to 62.26 hours. 50% rule gives us half of this, or 31.13 hours. That is far more than you needed.

Re-adjust the battery bank size until you get the run time to show 48 hours. 50% rule again giving you the required 24 hours. You'll see that a 700 Ahr battery bank will do it. Not the 800 you thought you needed.

So instead of buying 4 new 100 Ahr batteries you only actually need 3.

Have a play with the calculator trying different battery bank sizes and see how much difference it makes to the run time. Have you spotted a pattern?

Well if not, I'll tell you. The run time is increased according to Peukert's Exponent according to the equation Tc = Ccn where Tc is the change in run time expressed as a ratio, Cc is the change in battery capacity expressed as a ratio and n = Peukert's exponent. So with Peukert's exponent at 1.3, a change in battery capacity from 400Ahrs to 800 Ahrs is 2 times. 21.3 is 2.46 so the run time will be increased by 2.46 times for a 2 times increase in battery capacity.

By this equation we can state that increasing a battery bank from 400 Ahrs to 800 Ahrs (increase of 2 times) will increase the run time by 2.46 times which just so happens to be exactly what we calculated in the example above.

The higher Peukert's exponent, the more effect this will have.


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Page last updated 02/04/2008.
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