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Batteries 4 Dummies. The Peukert Lesson

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  • Batteries 4 Dummies. The Peukert Lesson

    Batteries 4 Dummies. The Peukert Lesson
    In today's lesson we are going to discuss Peukert's Law. Specifically as it applies to Flooded Lead Acid batteries (FLA). Peukert's Law effects all battery chemistry types, but FLA is severe. Yes another stupid physics lesson named after some fossil that died a hundred years ago. Who cares? Well you should if you are reading this and do not know. So here we go in 1897 a German scientist W Peukert discovered a distinct significant characteristic in batteries he defines as:

    The capacity of a battery in terms of the rate at which it is discharged. As the rate increases, the battery's available capacity decreases.

    When you buy a battery it has a Amp Hour capacity rating at some rate. Typically the rate is 20 hours, and you will see some at 8, 2 or even 100 hours. Example a plain ole golf cart 6 volt battery with a 225 Amp Hour capacity if discharged at the 20 hour rate. Understanding Amp Hours and charge/discharge rates (aka C rates) is important so now some math. Relax easy math you learned in 5th grade Jethro.

    Amp Hours is the product of current flowing (amps) over a period of specified time, so Amp Hours = Amps x Hours or AH = A x H.

    From the above formula tells you::
    Hours = Amp Hours / Amps or H = AH / A
    Amps = Amp Hours / Hours
    or A = AH / H

    Told you that was easy, all that from just two numbers. So when you go buy that 6 volt - 225 ah battery what they are saying to you is; if you discharged the battery at 11.25 amps for 20 hours. (solve for 20 hour discharge. 225 ah / 20 hours = 11.25 amps.) But what if you actually going to discharge the battery at say 60 amps in a golf cart or coffee pot on your inverter? Well greedy Mr Peukert is going to rob you blind of capacity. 60 amps on a 225 ah battery is what hour rate? 225 ah / 60 a = 3.75 Hours right? How much does that effect the 225 AH capacity of the battery? A lot is the short answer, and for many of you will not know exactly, but I can get you close with correction factors based on averages. Any battery worth having, hint hint, will publish what we call C Rates where C = the hour discharge rate like 3.75 hours, or in the form of Discharge Table and charts. Here is a good example that will make your draw drop. it is a golf cart battery a Rolls S-290 a 6 volt 220 AH battery. One of the best out there. That same battery discharged at the 100 hour rate (2.25 amps) is a 293 AH battery and if discharged at the 1 hour rate of 79 amps is a 79 AH battery. So in a 3.75 hour rate of 60 amps that same battery is now really a 2 hour rate yielding only 112 AH. Ouch!

    So that concludes the lesson. Do you care about Peukert Law now? Yes if you are a lead acid battery user. Not so much for lithium and others. OK what follows is something you can apply, a Correction Factor Table or a number greater than 0 for you to apply to your 20 hour rated capacity to find the real capacity at your discharge rate. Let's start with the easy one the 20 hour rate = 1 The formula is: Corrected AH Capacity = F x AH@20 hour spec. Where F= the following correction factor, and the battery AH rating. So 20 hour discharge is easy to do in your head because when F = 1 then 1 x 225 ah = 225 ah.

    50 Hour + = 1.2
    20 Hour = 1
    16 Hour = .95
    10 Hour = .85
    8 Hour = ..80
    5 Hour = .70
    3.5 Hour = .60
    2 Hour = .50

    Take note of what happens when you discharge faster than 8 hours, you fall off a cliff. That ought to be a clue of the C/8 max rating I always refer too. In the Telecom sector this exact same battery would be sold as a 175 AH battery or the 8 Hour specified rate. So when you buy a battery, you damn well better know what Hour Discharge Rate they are specifying at in a Flooded Lead Acid battery. As a point of reference the Peukert Effect on AGM is not as severe because AGM batteries have much lower Internal Resistance then FLA batteries, thus about half the Peukert Effect allowing them to be used as fast as C/4 where C = the 20 hour AH rating of the battery.

    Hope that helps some of you.

    Dereck

    EDIT NOTE:

    About 12 years ago when I was laid off from the Telecom sector I taught DC and AC principles to EE students. What is above is just about what every student in EE receives on batteries in a 1-day 2-hour lecture. You gotta 10 minute Readers Digest glimpse of the lecture
    Last edited by Sunking; 08-17-2016, 03:05 PM.
    MSEE, PE
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