“But RC I have heard you say many times battery monitors generally lead folks to longer battery life, how can that be?”


That is correct and I still actually stand by that statement despite a lack of accurate programming. Let me share some reasons why I can say this...


Reason #1

Prior to having battery monitors many of my customers had simple analog volt meters. Most of these analog volt meters were quite inaccurate and many owners often discharged to well below 12V before recharging, in essence taking their battery to 70-80% DOD with each cycle.. Battery life for many was the typical 2-3 years we see with abused batteries.


Along comes the Ah counting battery monitor and all of a sudden the owner has a screen to watch, buttons to press and has some insight into the battery that never before existed. A good portion of these customers began to be really careful about battery depth of discharge (DOD) and the 50% they assumed previously for recharge now became “Wow I am at 65% SOC I think I will recharge.” Human attention was brought to batteries that never had such a fan base.


Reason #2

Previous to the Ah counting battery monitor most owners had not a clue as to what their alternator or charging systems were doing. They watched voltage and when it got to 14.4V or so they stopped charging. With the battery monitor they could at least hold off until a good portion of the Ah’s had been returned. Is this accurate? No, not at all, but it resulted in batteries getting to a higher SOC than the owners previously & regularly attained thus leading to overall healthier batteries.


Reason #3

Peukert rears his head once again. Remember when we talked about drawing the batteries at a slower rate of discharge and getting slightly more usable capacity from the bank due to the Peukert Effect? Keep in mind that many boat owners are discharging their banks at somewhere around 20-30% of the 20 hour rating. A bank of 450 Ah’s can support a 22.5A load for 20 hours at 77F. If we draw that bank at 6A at 82F it runs longer than 20 hours at that load. This can make a typical deep cycle battery bank, as installed on cruising boats, appear slightly larger based on the rate of discharge. Many owners look only at the –Ah’s screen and then unknowingly mentally assume a Peukert of 1.00, which does not exist, even in LiFePO4 batteries. With this type of low current discharge use, the actual DOD before recharge was slightly higher than was assumed by looking only at the -Ah's screen & thus a shallower DOD before recharge and a higher SOC after recharge.


With those three points it is easy to see how we take an average battery life of 2-3 years and extend it to 4-5 or more, even with a poorly programmed monitor. We can do better, if we program more accurately.


The Old School SOC test:

“ RC I want to try and keep my Ah counter more accurate, keep it on its toes so to speak, but I am just not inclined to do an actual 20 hour capacity test? What can I do?”


A marginally decent method for checking for the level of honesty in your battery monitor at tracking SOC is to:


-Check the SOC screen and write it down

-Check the –Ah’s screen and write it down (Hint: -Ah's and SOC should almost never agree)

-On a day when the batteries are between 75 & 80F disconnect them

-Allow them to sit for at least 24 hours (AGM & GEL may take longer) then test the specific gravity or the open circuit voltage (OCV) at the battery terminals.

-Compare this voltage or SG reading to your manufacturers stated SG to SOC or OCV to SOC scales. (Please don’t buy batteries from sellers who can’t provide you this information)

-How does this compare to your Ah counter?


It is important to understand that every 10% of battery capacity is represented by an approx 0.1V change in the rested open circuit voltage (OCV) reading. A short rest or an incorrect OCV reading is no better than an improperly calibrated battery monitor at finding your approximate SOC.


Different batteries from different manufacturers will have a slightly different rested OCV or SG for determining the approximate SOC. Generic scales for this, often found on the internet, may not apply well to your particular batteries. Always get your OCV>SOC or SG>SOC charts directly from the manufacturer of your batteries..


Understand also that this test only tells you how the monitor is tracking against SOC, not what your current Ah capacity is. Specific gravity (SG) or OCV readings tell you nothing about the actual Ah capacity your battery can deliver. For determining Ah capacity the only thing you can do is run a physical 20 hour Ah capacity test.


The battery bank above shown at 12.76V had rested for approx 3.5 hours completely disconnected at 80F. By 24 hours it was giving a reading of 12.54V, or approx 84% SOC for this battery bank. The Ah counter said the battery was 100% full a 16% difference in the relative SOC readings.


"But RC 16% is not that much is it?"


This was a 450Ah rated bank so a 16% counting error = a 72Ah variance. That is an entire day's energy use for this particular boat. To compound on that counting error, when these batteries were tested they were really only 383Ah's.... This is how the snowball rolls...............