In order to determine the if the SOC of the Smart Gauge was correct I first had to accurately determine the Ah capacity of the batteries I was testing. I tested both new and used batteries to see how the SGBMU would adapt to being inserted into a system with used batteries.. One FLA battery was seven years old.


The 20 Hour Capacity Test:


Deep cycle batteries for marine use, in the US, are rated at what is called the 20 hour rate and this rate is labeled as Ah's or ampere hours.. The 20 hour Ah rate is the load the battery can deliver at 77-80F for 20 hours before the terminal voltage hits 10.5V. For a 100Ah battery this load would be exactly 5A. 100Ah / 20 hours = 5 amps.


When conducting these tests I had to keep the load as constant as I could over the duration of each discharge. This is difficult because as the voltage decays or falls, as SOC drops, the current changes due to Ohm's law. As a result of this testing I now have a custom designed and built constant-load that will hold current precisely where you set it and then disconnect the bank when it hits 10.5V this makes my life MUCH easier!


During this testing I kept my down stairs office at 75F. Luckily that room is its own heating zone. While this is 2 degrees lower than "ideal" test conditions the impact in terms of accuracy is very small.


Determining Ah Capacity On Used Batteries:


In order to compare the Smart Gauge, to the two Ah counters, I had to first determine the batteries physical, at this point in time, 20 hour rating. This sometimes involved three complete discharge capacity tests all the way to 10.5V.


*First Capacity Test:

Load applied at labeled 20 hour rate and Ah's counted. because the battery does not have this capacity I noted the Ah's it supplied before hitting 10.5V and transferred that Ah capacity to the next test.


*Second Capacity Test:

For simplicity's sake lets assume a 100Ah battery that only delivered 77Ah's. The first test was at 5A/75F until the bank hit 10.5V. But, if the bank hit 10.5V at only 77Ah's delivered, I then recalculated the test based on 80Ah's. This is a small fudge factor I learned while performing these tests. What I wanted to do was come up with a new 20 hour rate for the battery in its current condition. So now I have an 80Ah bank and 80Ah's / 20 = 4A load. The second test was then run at 4A and the Ah's delivered were recorded to see if it hit 20 hours. If it matched, and we hit 20 hours, and 80Ah's were delivered, then no third test was needed.


*Third Capacity Test:

If Ah capacity did not match on the second test I then recalculated and performed a third test. I never had to go beyond a third test and was usually within .5 - 1 Ah. Close enough for this testing and far more accurate than any boater would ever hit on-board with a traditional Ah counter, unless they got lucky.


After each capacity test the battery was immediately, and slowly, recharged at the new 20 hour rate. This recharging was done at constant current until voltage was at 16.0V (not for the GEL). This was constant-current (CC) only charging with no voltage limit (well technically 17V). It required my attention near the top end of charge and added many days to these tests because to recharge after the capacity test took over 20 hours!


This type of discharge/recharge is often referred to as a reforming charge. It can tend to put some capacity back into the bank and can help minimize the abuse of taking the battery to 10.5V to find actual capacity. Tedious and time consuming though!!


BULLET POINT: It should be noted that not one single lead acid battery I tested produced the rated Ah capacity. Not a single one.. (Head-Bang) New batteries take many cycles to fully break in and deliver rated capacity and used batteries lose capacity over time. During testing some of the new batteries slightly increased capacity, and some used batteries lost a 1% or so. Ah capacity on lead acid is an ever moving target. If you think your Ah counter is giving you accurate SOC data consider this bullet point and what it took me to find the actual, at this point in time, Ah capacity of these batteries. The ONLY battery tested, that delivered its rating, was LiFePO4..




PHOTO: This photo just represents the screen I have on the PentaMetric battery monitor. I can change the screen display to add up to three shunts and three voltage sources. I normally use this tool to track solar performance and do A/B comparisons but it is great for data like this too... Ah's were tracked simultaneously with both the PentaMetric and the Victron BMV-602.