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Compass Marine How To | all galleries >> Welcome To >> Keeping Your Battery Monitor More Accurate > Peukert Effect on Capacity At Varying Loads
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Peukert Effect on Capacity At Varying Loads

There is no better way to explain this than to show you an actual hourly capacity chart from a battery maker who publishes good reliable data. Don't expect good data like this from Wal*Mart / Johnson Controls products or no-name batteries, it usually does not exist. This battery happens to be a very heavy duty L-16 deep cycle battery with a Peukert of 1.50.

So what can we glean from this chart?

100Ah Run Rate - If we go all the way to the top we can see that if we apply a small load of 4.86A to this 375Ah battery it will run for 100 hours before hitting 10.5V and deliver 486 ampere hours of usable capacity! Yes 486Ah's from a 375Ah battery when drawn at a low rate. This is the net positive side of the Peukert effect. By drawing this battery at 4.86A vs. 18.75A we've netted a usable capacity gain of almost 30%. If your battery monitor is not programmed to calculate for this, how accurate do you think your SOC will be....?

WARNING: Most Ah counters only compensate Peukert for HIGH DISCHARGE RATES! They completely leave out the low discharge side of the Peukert effect which can theoretically increase the banks usable capacity at a LOW DISCHARGE RATE. Most all Ah counters have no way to compensate for a 30, 40, 50 or 100 hour discharge rate....

Now look at the rate of discharge compared to our typical house-bank discharge rates on cruising boats. A good many cruising boaters discharge their bank at about 25-30% of the 20 hour rate? Well 4.86A is 26% of the 20 hour rate for this particular battery.

Let's contrast that 1.50 Peukert L-16 flooded battery to a Lifeline AGM battery, which has a Peukert of 1.12. Using a 120 hour discharge rate on the Lifeline AGM battery will yield nearly 114% of its 20 hour rating (Lifeline does not publish a 100 hour rate). This is a large difference in the actual SOC of these two batteries especially if you can't physically properly program the Peukert in your Ah counter for low discharge rates..

20 Hour Run Rate - This is a 375Ah rated battery and no surprise that at an 18.75A load it will run for 20 hours and deliver its rated capacity. 375Ah 20 hours = 18.75A discharge rate. This is Peukert net neutral.

1 Hour Run Rate - Conversely if we apply a 135A load to this battery we can only get 1 hour of run time from it before it hits 10.5V. This is the net negative side of Peukert.

As can be seen from this battery manufacturers hourly rate chart the Peukert effect is a setting that simply should not be ignored. even if it only compensates one sided. You really need to program your battery monitor for the correct Peukert in order to get the best information from it.

Peukert Effect Means:

-Loads above the 20 hour rate = Less usable capacity

-Loads at the 20 hour rate = Rated battery capacity

*-Loads below the 20 hour rate = Slightly more usable capacity


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