Are Battery Conductance Testers Worth It?

01 Impedance Testers

20 Hour Capacity Test Results

PUBLIC APOLOGY: First let me begin this by eating some public CROW. Over the years I have recommended these testers for boat owners or techs to use, as had the ABYC and other reputable organizations. While I have always maintained a get a baseline > moving forward manner for use, it seems even this may not be as accurate as I had initially thought it was. While this type of use, baseline > forward, may give an indication of changes in the battery they do not easily correlate as well as I initially thought, or was lead to believe by the manufacturers of these products.

It was not until I got heavily involved in actual physical capacity testing (20 hour capacity tests) & reserve capacity or reserve minute testing that I realized the extent of the errors in these devices as related to usable Ah CAPACITY at the loads a boater would normally use.. The manufacturers make them sound like the second coming of God, and they may be for cranking amps, but they perform rather miserably for translating to the kind of data we as boaters need for our low rate discharge house battery banks.

I am not the kind of guy who likes to mislead folks so I have to be honest here. I am also not the kind of guy who, just because he owns something, it has to be the best, is not at all my style. I will be honest when things don’t work as I had hoped, and this is one of those cases.

Conductance Testers Vs. Ah Capacity Testing:

Over the past 5 months I had to opportunity to run hundreds and hundreds of hours of battery testing for one of the sailing publications (Practical Sailor May 2015 & August 2015). I can’t discuss the testing outcomes for that article but that PSOC testing spun off some other interesting tid-bits. What I can show you is how actual 20 hour Ah capacity tests compare to iconductance battery testers. The results are more shocking than you’d probably ever imagine.

For the PSOC (partial state of charge) testing all the batteries were cycled up to rated capacity and a baseline Ah capacity was established using the average of two controlled 20 hour capacity tests to BCI testing standards. This particular battery, a Deka/East Penn AGM tested new, once “cycled up” to capacity, at 104.5 Ah’s or just shy of its 105Ah rating.

It should be noted that I conduct 20 hour tests vs. shorter tests, such as a 25A fixed load reserve caapcity test because as a battery ages a high rate discharge test and a 20 hour test begin to diverge.

When new, cycled and broken in a Lifeline GLP-31 will deliver 105 Ah’s over 20 hours at a 5.25A load and also run for 195 minutes at a 25A load. As this battery ages the 25A load becomes less representative of the actual low rate usable capacity and a 20 hour test and fixed rate test will deliver differing results. For example at a 20 hour discharge rate the used battery may deliver 83% of its rating and under the 25A load it may only deliver 71% of its rating. How the battery is aging determines how much variation. It can vary widely so I stick with a 20 hour test. On boats we discharge house banks at less than the 20 hour rate so the 20 hour test is the most useful and most relative test we have. A 25A test may fail your battery for your typical use before it is really dead for your use. Capacitance testing often over exaggerates your actual state of health, for house bank use, which can lead to dangerous premature failures.

After the rather brutal PSOC testing was completed the now diminished tested Ah capacity of this battery was approx 73 Ah’s or a loss of 30% of its usable Ah capacity. So how did the conductance testers compare to an actual 20 hour capacity test? You’re about to find out…..

Screen Shot of 20 Hour Results

For capacity testing I was using two lab grade DC electronic loads to apply a constant current at a fixed amperage. This tester is extremely accurate and holds current extremely stable.

Here we are looking at the ending Ah capacity for the battery pictured previously. Yes, it lost nearly 30% of its Ah capacity during this particularly difficult testing..

02 Impedance Testers

03 Impedance Testers

Midtronics EXP-1000HD Screen Shot

It should be noted that I own and spent my own hard earned money on all the impedance testers used in this article. I first noted these discrepancies years ago and tried to learn to translate the results from actual Ah capacity to what these devices spit out. I also spent aa great deal of time on the phone with the folks at Midtronics, & Argus while they were still in business, trying to come up with a translation “factor” from CCA to Ah capacity, for different battery types. Suffice it to say there is not one that is repeatable and reliable.

The Midtronics line of impedance testers are considered the gold standard for cranking capacity testing. Many battery manufacturers, such as Deka supply these testers. at great cost, to their resellers in an attempt to limit warranty replacements. This is great for trucking fleets and automotive applications but early on I was lead to believe these could be a good predictor of state of health / SOH for marine batteries too.. Boy was I mislead, at least in regards to Ah capacity, which is what really matters to boaters.. This particular tool is a 4 figure product… Don’t get me wrong I love this tool and it serves many other purposes but predicting Ah capacity or SOH (state of health) as related to usable battery capacity, ummmm…well…………????

So how did it do? As can be seen I accidentally programmed it for 810CCA when the battery is really rated at 800CCA. This really matters little because we are trying to figure out where the testers says this battery is in relation to the factory rating. If I had been printing a test receipt, for a customer, I would have changed it from 810 CCA to 800 CCA but not a big deal for this.

The Midtronics EXP-1000HD showed this battery performing at approx 109% of its factory rating or BETTER THAN NEW. While this may be true for short duration cranking amps it is an utter failure at predicting usable Ah capacity because we know for a fact this battery is down 30% of its as new factory 105Ah rating at just 73.4Ah’s… Ouch……

Factory CCA Rating = 800CCA
EXP-1000HD Results = 871CCA
Percentage of Factory CCA Rating = 109%
Factory Rated Ah Capacity = 105 Ah
New Tested Ah Capacity = 104.5 Ah
Actual Ah Capacity When Compared to Midtronics = 73.40 Ah

For fairness this battery was still in my water-bath which is used to control battery temp to 77F during testing. The Midtronics measures battery temp as can be seen from the screen shot and the battery is at 77F just as it should be for a 20 hour capacity test..

Argus 500 Screen Shot

Next up is the Argus 500. Unfortunately Argus does not exist any longer because Midtronics sued them out of business for stealing proprietary information and using it in the Argus product line. Based on that it is no surprise at all that the Argus came it with a 1 CCA difference of the Midtronics or 870 CCA vs. 871 CCA.

Factory CCA Rating = 800CCA
Argus 500 Results = 870CCA
Percentage of Factory CCA Rating = 109%
Factory Rated Ah Capacity = 105 Ah
New Tested Ah Capacity = 104.5 Ah
Actual Ah Capacity When Compared to Argus = 73.40 Ah

Results for comparing Ah capacity to CCA = FAIL!!!!!!!!!!!!!!!!

04 Impedance Testers

05 Impedance Testers

Centech Screen Shot

Just for grins I was in Harbor Freight one day and I grabbed the Centech conductance tester. It was under $100.00 but is, shall we say, sloppy when compared to the Midtronics or Argus.

In this shot the Centech/HF tester is making this very tired battery appear to be a rock star. A battery that is down 30% of its factory rated Ah capacity is anything but a “rock star”. If buyers of this tester believe it to be true, their actual capacity could be WAY off.

Factory CCA Rating = 800 CCA
Centech Results = 1018 CCA
Percentage of Factory CCA Rating = 127%
Factory Rated Ah Capacity = 105 Ah
New Tested Ah Capacity = 104.5 Ah
Actual Ah Capacity When Compared to Centech = 73.40 Ah

Results for comparing Ah capacity to CCA = FAIL!!!!!!!!!!!!!!!!

I honestly used to believe I could learn to translate the impedance data to Ah capacity but every time I though I had it nailed, a curve ball was thrown at me.

  • If you want to know the cranking ability of a battery, these testers can be very useful.
  • If you want to know your batteries SOH as related to Ah capacity fogedaboutit.

So how can you use one? I still suspect we can draw “some” conclusions from an initial baseline to a future test using the same tester and the same batteries. I have developed lots of these sorts of data-sets for my Argus and Midtronics, but they keep proving me wrong and this is very, very frustrating.

The bottom line is that impedance testers simply do not translate well to Ah capacity, and on low rate discharge house banks, Ah capacity or usable capacity is ALL THAT MATTERS.. I wish they were more accurate because it would make my life much, much easier, but I am sorry to report, they are not.

I have no qualms with the Midtronics for predicting cranking capacity, and suspect it is rather accurate. The Harbor Freight / Centech on the other hand, I would urge using caution in taking it at face value for cranking ability….

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