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Compass Marine How To | all galleries >> Welcome To >> LiFePO4 Batteries - Thoughts & Musings > Capacity Testing - Create A Baseline
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Capacity Testing - Create A Baseline

With expensive batteries such as LiFePO4 I believe it is a wise idea to create a baseline capacity figure that you can repeat at least once per year. For the fist 11 capacity tests on this bank I used the set up pictured here. For loads I used an inverter and ceramic disc heater plus an incandescent bulb to get the current to approx 100A. This represented a load of approx .25C. To count Ah's removed from the bank I used a Victron Ah counter.

This set up worked and so long as the cells were tested at the same temp, using the same equipment, the test was fairly repeatable with some level of accuracy. On the first capacity test after the initial top balance the bank yielded 425 Ah's at a .25C load.

With this initial capacity test I now had a solid baseline from which to monitor changes over time. I ran a capacity test approx every 50 cycles and the most recent was at cycle 550.

In order to run a capacity test you will need a load plus a way to track Ah's. You will also need a way to track individual cell voltages and cut the test off when voltage drops on the lowest cell to 2.8V.

It is not important to test these cells at a .25C, .5C or 1C rate because the use on a vessel, as a house bank, will draw considerably less than that. Using higher C rates just makes the testing shorter in duration.

There will be some slight changes in capacity between say a .1C and a .5C test because these cells do have some Peukert effect, but this will be nowhere near as pronounced as it is with lead acid batteries.

Do yourself a favor and create a baseline so you know what you actually have! It's kind of tough to program an Ah counter when you really have no idea of the actual capacity other than what the manufacturer tells you. There have been numerous reports of folks not getting the "rated" capacity in their cells and without a baseline you really have no way to know whether you started with less capacity or you caused premature capacity loss, through your treatment or mistreatment of the bank.

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