One of the things I offer my customers is 20 hour capacity testing for their expensive AGM or GEL batteries. Some even take advantage of it for flooded deep cycle. The only true way to test a lead acid battery, for use as a deep cycle bank, is to physically capacity test it using industry standard testing procedures. The 20 hour test is most representative of the loads used on boats and thus a true 20 hour capacity test is what I use.
Many years ago I built an Ah capacity tester using an Ah counter, relays and DC loads but the accuracy was not as good as I would like because the DC load needs to stay steady the entire duration and this means manual manipulation. As the voltage decays the current you initially set at 12.7V changes, thus changing the batteries discharge rate. With an non steady discharge rate, on lead acid batteries, this results in less than stellar accuracy. With LiFePO4 holding the current steady, while capacity testing the bank, is less critical because of the low Peukert effect.
A few years ago I had Mark Grasser, of Mark Grasser DC Solutions, build me a custom DC constant load tester for Ah capacity testing batteries at the 20 hour rate. This device worked well but still I wanted better control, easier set up and better overall accuracy.
I finally invested in a lab grade DC electronic load with battery testing capability. What a world of difference! The accuracy of these devices is amazing but they are not inexpensive.
At capacity test #12 (cycle #550) I switched this battery bank over to the new DC electronic load tester. All testing from this point forward will be done using this device. My new baseline was established at cycle #550 and the bank delivered an astounding 419.2 Ah's at a 30A constant load after 550 cycles!
I hesitate to compare this capacity figure to the previous 11 capacity tests but it was very close to them and the bank delivered 419.2Ah's. From any 400Ah rated bank with 550 cycles on it this is something I would have never believed, had I not done the testing myself.
This bank is proving my initial skepticism as perhaps being wrong. While the discharge rate was lower using the DC electronic load the Peukert effect on this bank is also low. What really matters, or should, to myself or anyone using these as a house bank, is how many Ah's can you get at your average house loads? The answer for this bank is in excess of 400Ah's even at 550 cycles in...
I do believe I have lost some capacity in those 550 cycles, but not much. The last test using the old testing rig was 423 Ah's.. Tests using the old method & equipment yielded anywhere from 421Ah's to a high of 426Ah's. This test was at 30A, not 100A, but with a higher accuracy piece of equipment and no inverter, just a pure DC load. Still this bank delivers more Ah capacity at a 30A load, which is multiples more than our average on-board load, and that is good! We have 419 Ah's of capacity, at a 30A constant load, after 550 cycles. I'm a happy camper!!
Pictured here is the 400Ah bank a 60A lab grade power supply for charging and the 40A lab grade DC electronic load tester. Most of the smaller wires, in this spaghetti mess, are voltage sensing leads...