understanding worn 18650 cells, testing capacity / resistance etc - Page 1
Forum section: electronics / beginners

author: carpetpaul2129
author_rank: Contributor
timestamp: February 28, 2022, 07:45:15 pm
content: so, I'm very new to this, interested indeed, but I've a lot to learn. I'm on with a couple of projects where I've been replacing the battery packs in some small devices, attempting to test the old vs new cells to see how they might show that they're worn out etc.I repaired a cordless tyre inflator which housed 3x 18650 of semi-unknown brand, charged voltages were 4.16v / 4.17v / 4.17v. Using a 1ohm resistor, I checked the voltage drop and found high resistance in all (compared to every other cell I have) they were 120milliohm / 110 milliohm / 140 milliohm. I then connected a 0.22ohm resistor ** and timed how long it took for them to drop to 2.5v ( I think that's about 16.82 amps ? ) they lasted 22 seconds each (these three same cells also had measured 1344mAh / 1278mAh / 1200mAh when I measured the capacity on my terrible Chinese IMAX B6 clone (stops at 3volts), I think the most it can discharge is at 0.4Amp ? - is there any conclusion I can draw from this other than the fact that these batteries can't handle much load ?Next up I have two sets of 6 cells, the 6 which were removed from my old Dyson vacuum battery pack, charged voltages 4.19v / 4.19v /4.18v / 4.18v / 4.19v / 4.15v. resistance was 80milliohm for 4 of the cells, a 91milliohm, and 98milliohm. then using the 0.22ohm resistor, 4.18v - 2.5v most of the cells took on average 9min40sec, with one cell lasting 10min 35sec. This (apparently) if my maths is correct, is about 2600mAh. cells are Sony VTC4 which should have a capacity of 2100mAh when new (and I bet not at 16 Amp either !) - what went wrong ?Lastly I have the new set of 6 cells, this time Sony VTC5, which should have 2600mAh, charged voltages 4.17v / 4.19v / 4.19v / 4.18v / 4.18v / 4.17v, resistances 94 / 66 / 83 / 94 / 100 / 94 Milliohms. runs times for .22ohm / 16.82 amp were all remarkably consistent at between 12min12sec to 12min 52sec, seemed to work out at an average of 3470mAh, clearly wrong, and again at very high drain (the cells were nice and warm when finished) these cells I also tested on the ImaxB6 and got an average capacity of about 1700mAh using around 0.4amps and stopping at 3v** the 0.22 resistor idea was born out of me wanting to (try) to load the cells in the same was as the Dyson vacuum does, I think it's 350 watts from 6 series cells. my thoughts were that this was 16amps ?questions (if I may) :-1, Either I'm just simply doing something wrong, or is the resistor not 0.22ohm ?2, When is a cell depleted ? does this depend of the load that it's trying to run ? all of the cells in the experiment bounced back once rested, and now they're at around 3.3v - since they won't run that load any more, then they're flat ? but 3.3v isn't quite flat ? if they were in something with less load then they would continue on ? - is this effect anything to do with cells not giving out as much capacity to a heavy load ? not giving as much total capacity ? it's hard to get my head around a cell that's flat for one purpose, but at the same voltage, wouldn't be considered flat for an easier load ?3, I can't measure the resistance of the resistor, I'm not sure I know how, but if we knew that the VTC5's do measure 2600mAh, then, on the one hand, we could work the math backward to arrive at the resistance of the resistor, if indeed it's deemed to be somehow incorrect. Only issue I that I've measured using 0.22ohm, and surely at this amount of load, those cells should not have been able to give out the optimal capacity as per the spec sheet.forgive my rabble, but any comments most appreciated, as everthanks for readingPaul

author: ogden
author_rank: Super Contributor
timestamp: February 28, 2022, 07:56:32 pm
content: Quote from: carpetpaul2129 on February 28, 2022, 07:45:15 pmYes. Capacity differ for high current versus low current discharge. Check datasheet, discharge curves at different currents - you will see.

author: tunk
author_rank: Super Contributor
timestamp: February 28, 2022, 08:26:13 pm
content: Forum member HKJ has tested quite a few 18650s:https://lygte-info.dk/