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How to conduct a LiFePO4 capacity test?

Not all batteries are created equal and if you want find out how much juice your LiFePO4 battery, or any battery for that matter, can really produce then read on.

In order to check the battery’s capacity, you need to charge the battery completely and then deplete it until it reaches its cut-off voltage. This can be done in two different ways. The batteries are tested at a .2C discharge rate, which is to say 20% of the battery’s capacity. For example, a 100 Amp battery is typically discharged at 20 Amps per hour (20Ah) if it is a 100 Amp battery.

It has to do with the Peukert’s Effect and it is a bit beyond the scope of this article to explain how LiFePO4 batteries are more sustainable and can be discharged at a higher C rate without significantly changing the usable capacity.

THE REAL WORLD LIFEPO4 CAPACITY TEST

In addition to the fact that the real-world tests don’t cost anything, you can use just about any known load source to do them, and they are pretty reliable, there are also several advantages to using them. If you have an accurate amp rating from the load, they are reliable as long as you know exactly how much current they are consuming, so you should make sure you know exactly how many amps your load consumes.

In the downside of this test, it is quite a simple and feel-good test, but there are a few limitations. Aside from the fact that the battery has to remain under close observation at all times, you have to be aware of the exact moment when it stopped outputting power for the calculations.

It is very simple to follow these steps once you have that information.

  1. Fully charge the battery pack
  2. Take note of the time
  3. Use the device until the battery is depleted
  4. Note the time again

Amps x Time (hours) = Amp Hours, which is the formula that is used for this calculation

Then you would be able to get 100 Amp hours (Ah) if you were pulling 20 amps for five hours, and 2 amps for three hours, so you’d get 7 Amp hours (Ah).

The results of a test such as this can be greatly affected by a number of factors, such as temperature, wire size, wire length, etc., all of which play a role in actual results. If you want to know if you are getting what you paid for and if there is anything grossly off, you should investigate further. Real-world tests are a great way to know if you are getting close to what you paid for. They are usually the warm fuzzy feeling that everything is going as planned.

THE METERED BENCH TEST

It is usually more accurate to measure how many amp-hours you are getting from your LiFePO4 battery by using a Hall Effect meter or a dummy load capacity tester. These devices give you both the Watts and Amp-hours that the LiFePO4 battery is delivering.

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