This PC screen is the Peukert's Compensation setting screen for the Victron BMV-602. Here it is set for 1.12 or the proper factory suggested setting for a Lifeline AGM battery.


I touched on the Peukert effect above but there is more to it.


Most cruising boaters never draw their house banks in excess of the 20 hour rate, as an average load. Take for example the boater with the 450Ah battery bank. The 20 hour rate on this bank would be 22.5A. At 77F and a 22.5A load this bank should deliver 450Ah’s once broken in. Most boaters with a 450Ah bank are drawing it down at well under 10A, on average. If you do not have an accurate Peukert’s constant programmed in you will be seeing an inaccurate SOC reading because it will not correctly calculate for Peukert.


***WARNING** Most Ah counters only correct for Peukert when discharge loads exceed the 20 hour rate (high discharge corrections) and fail to calculate for discharge rates below the 20 hour rate (Low discharge corrections).


Sadly most boats with Ah counters I set foot on have never touched the Peukert setting on the Ah counter. The fault of the owner? No, not when many Ah counter manuals suggest that leaving it at the factory setting should be okay...


Lead acid batteries used on boats range from a Peukert of about 1.11 to a high of 1.60. These are large variances in how your bank will discharge and how many ampere hours you can remove at XX rate of discharge.


If a battery monitor does not have a way to program for the Peukert’s constant then the manufacturer should have a very good explanation as for how they compensate for rate of discharge as related to SOC. If the unit has no Peukert correction then the manufacturer better be able to explain how they compensate for varying discharge rates. If they can't explain this, walk away.. Choose your battery monitor wisely.


Different batteries have widely varying Peukert’s constants and these too change with age. Having the correct factory Peukert’s constant programmed in is an important aspect in how these devices report & calculate SOC.


Even if you do program it correctly, in regards to the Peukert’s constant, this too changes with age. In the beginning, with new batteries, you might even see it lower than what the manufacturer says it is, but as the battery ages, it usually increases. A Peukert’s test can be done. It involves two discharge tests at varying discharge rates and then a calculation to figure it out. If you are willing to go to this length Victron has a good Peukert’s Calculator on their web site.


In short, if your Peukert’s constant is not set correctly, as well as Ah capacity, and Coulombic efficiency you stand little chance at getting the accuracy you may desire from your Ah counter.