In the last months I've used the FT-817 on the field and noticed that the Li-Po battery pack wasn't lasting as much as I expected. I had to choose: to build or not to build a device to measure the battery capacity? I opted for buying one and the choice was XH-M239 from aliexpress.
It tests one cell at a time with about 500 mA discharge current. It's slowish, but it mimics the consumption of a laptop I dare to say.
I set the stop voltage at 3,4 V because that's in the range for my intended use (3S "12V" pack) and all cells tested scored a very low rating! The best reached 1350 mAh, and many were below 500 mAh. None of them is brand new from a reputable source, so that was somewhat expected.
Nevertheless I started questioning XH-M239 readings. With a DVM, it's easy.
The measured battery voltage does match my DVM (that was checked against a reference voltage source).
The measured battery current does not match! See:
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| The real current is lower. Battery under test is connected to the screw-on terminals. |
The actual current flowing out of the battery is lower! The capacity is overestimated by 17%! As long as the time interval is correct. I will repeat the measurement at a higher battery voltage, in case we are hitting some sort of non-linearity. The measurement circuit uses a 0.020 ohm resistor and a "A53A" SOT-23-5 device, which is a voltage detector.
Alright. I don't need to guarantee the capacity of my battery packs, I simply want to build "good performers" with these second hand 18650 cells, so the higher mAh the better.
