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.
There are a couple of ways you can check capacity, both involve fully charging the battery and then depleting it down to it’s cut-off voltage. Typically, tests are done at a .2C discharge rate, which is to say 20% of the capacity of the battery. So, if you have a 100Ah battery, you typically want to discharge it at 20 Amps per hour (20Ah). LiFePO4 batteries are a bit more robust than lead-acid and you can discharge at a higher C rate without much change in the useable capacity. It has to do with Peukert's Effect and a little beyond the scope of this article.
The Real World LiFePO4 Capacity Test
The advantages of the real-world test are that it doesn’t cost anything, you can do it with just about any known load source, and they’re fairly reliable. They’re reliable, as long as you have an accurate amp rating from the load, so be sure you know how many amps the load your using is consuming. Disadvantages are that you have to keep a close eye on the battery so you know exactly when it stopped outputting power for the calculations. This is also not a very technical test with specific numbers to report, it’s more of a feel-good test.
With that, the steps are simple.
- Charge the Battery to 100%
- Note the time
- Put the load on the battery
- Wait for the battery to be expended
- Note the time again
The formula is Amps x Time (hours) = Amp Hours
If you know with certainty, you were pulling 20 amps for 5 hours, then you got 100 Amp hours (Ah).
2 Amps for 3.5 hours, then you get 7Ah.
Be aware, there are a lot of environmental factors that can affect a test like this, temperature, wire size, wire length, etc., all play a factor in actual results. The real-world test is a really good litmus test to know if you’re getting close to what you paid for and typically provides the warm fuzzy that things are as they should be. Anything grossly off should be investigated further.
The Metered Bench Test
Using something like a Hall Effect meter or a dummy load capacity tester can be a more accurate way to measure how many amp-hours you’re getting from your LiFePO4 battery. They typically give you Amp hours and Watts.
Each of the above devices will show with pretty good accuracy, what the capacity of your battery is. Each product has advantages and disadvantages. The last, the dummy load tester is probably the most accurate and flexible option. You can simply connect the battery, set an Amp draw, reset the counter and walk away. When the battery shuts off, the tester stops the clock. It will even retain the info if the power goes out. The other two are not bad, nor overly complicated, but results and opinions vary on their accuracy, so buyer beware and read the reviews on Amazon.
The general steps for this LiFePO4 capacity test are similar to the first:
- Fully charge the battery
- Reset your test device
- Put a load on the battery
- Wait for the battery to hit the low voltage cut-off
- Check your results
If you’re doing a capacity test, be sure to charge the battery until the battery reaches 100%. Then discharge the device until the battery is fully depleted. If you’re using a LiFePO4 reBel Battery with Bluetooth, you will be able to see through the App when you are 100% as well as when you are at 0%.
When the test is complete, you should have a reading at or above what the battery is rated for. You may see slight variations based on temperature, wire sizing, wire length, and other factors. If you’re more than 5% off the rated capacity, try a couple more cycles as sometimes “kinks” are worked out by simply cycling the battery. Less than 5% variations can generally be attributed to environmental factors.