Picking good 8 out of 9 cells - Page 1
Forum section: electronics / general

author: liviu.gelea
author_rank: Contributor
timestamp: April 02, 2023, 09:21:54 pm
content: Hello everyoneI bought 932700cells I'm planning to turn into a battery bank (4s2p). I bought 9 since I didn't trust them much and they were pretty cheap. In fact I bought 1 first and tested its capacity to above its rated 6000mAh ( it held above 6150mAh) and only then ordererd 8 more thinking I would exclude the worst one from the pack.Now... since I bought them I didn't get the chance to test them all and kept them in storage for a year.Recently I found that one of them had dropped to a mere 0.7v or started this way, who knows. I tried to "recover it" and charged it for a few hours at 2mAh until 2.8V, then using around 1A until full. Then tested it and it was 6139mAh. I so far tested a second one to 6245mAh and I'm in the process f charging all of them.My question is what kind of tests I can do besides capacity to determine if it's safe to pair them inside a 4s2p bank and is it likey for a cell to be bad if it's the right capacity and has reasonable internal resistance?For testing resistence I'm thinking I should do an AC test since I have a function generator and an oscilloscope besides a pretty good multimeter.Should I store them at full charge and worry if some drop more than others with time? should I just pair highest with lowest capacity in paralel to equalize the capacity of each 2p ?Thanks.

author: trobbins
author_rank: Frequent Contributor
timestamp: April 04, 2023, 10:28:01 am
content: Seems like you did a very practical test already - self leakage.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 04, 2023, 10:58:49 am
content: I agree, you already did one of the best possible tests. Only seriously faulty cells self-discharge like that. It may indicate a safety issue as well. Get rid of that 0.7V cell.Probably the others are crap too, so proceed at your own risk.If you want to test more, maybe try to discharge at full discharge current you expect to see, and compare if some cells heat up too much.Capacity differences are also problematic because in series configuration, the worst cell capacity (Ah) becomes the capacity for the whole pack.

author: liviu.gelea
author_rank: Contributor
timestamp: April 19, 2023, 07:52:04 pm
content: Hello, I did some more testscolor photo version:I think I either am incapable of measuring internal resistance or these cells are bullshit fake.I bought these from Romania, but looking up the code written on the cell I found what I think is the original seller in China:https://www.made-in-china.com/showroom/7692f304771b8487/product-detailsXyJNHGCpORw/China-MSDS-CE-Approved-Rechargeable-3-2V-LiFePO4-Battery-Cell-32700-6000mAh.htmlFor measuringDC resistenceI set my multimeter to measure peak voltage, I attached my multimeter leads to a1 ohm3.3Ohm 100w resistor and shoved the leads on the battery. Insert some Ohm's law while assuming internal resistance negligible when calculating current and got those hideous numbers.For measuringAC resistenceI set my function generator in my oscilloscope input a 1kHz square wave into the batteryusing this circuit. But I had issues. Unlike in my simulation, changing the resistor value near the capacitor doesn not correlate linearly to the measured output voltage. I got very similar values for both 1ohm resistor and 3.3 ohm one. This made me think my function generator is incapable of supplying sufficient current.Still, the datasheet claimsAC Impedance At AC 1000 Hz ≤8.0mΩThis should make me think the batteries are bad but look how consistently over the rated 6000mAh every cell is. Even the one that got discharged to under 0.7V.I want to build a pack for a remote controlled rover pulling 2 amps at most. At 2 amps the batteries barely raise in temperature. Can I use these for anything at all?Thank you.

author: trobbins
author_rank: Frequent Contributor
timestamp: April 19, 2023, 11:43:30 pm
content: For the AC resistance setup, did you do a benchmark test with say a 0.01 ohm current shunt in place of the battery ?For DC resistance it looks like you are using the peak measured voltage across the 3.3 ohm resistor to calculate the peak current - that measurement of peak current doesn't need to assume anything. What you may be assuming is that the battery voltage stays constant, and doesn't droop by way of the internal resistance and the nominal 1A discharge current, as a way to then calculate internal resistance. As you are using a spreadsheet for results, then its very simple to remove that assumption in the final result value.Perhaps reduce the number of digits in your calculated values, as it looks like you only have about 2-3 digits of effective resolution. Perhaps you also just took a single sample measurement for each battery, rather than say 5 samples per battery and then averaging across the samples. For the DC resistance testing you could similarly add in series say a 0.01 ohm current shunt and repeat 5 sample tests and see if the calculated internal resistance increases by circa 0.01 ohm.