Unexpected capacity loss of batteries in series - Page 1
Forum section: electronics / general

author: karlng
author_rank: Newbie
timestamp: October 02, 2024, 02:09:24 pm
content: Due to the high self-discharge rate of LiPo batteries, I am considering switching to NiMH, which has a lower self-discharge rate. I have chosen to use three NiMH cells in series, as their combined voltage is about the same as a single LiPo cell, making them a suitable replacement in my application.However, I'm experiencing an unusual issue when testing the capacity of Eneloop Pro (NiMH) batteries. When testing a single cell, I get the expected capacity of about 2500 mAh, as specified in the datasheet. But when testing three cells in series, the apparent capacity drops significantly to around 1700-2000 mAh. Here are the key details:Test setup: all batteries were fully charged, capacity test by using the battery function of a LOAD device (Rigol DL3021) with constant current discharge at 100mA and stop voltage at 0V.Single cell test: ~2500 mAh capacity, normal discharge curve3x series test: ~1700 mAh capacity (for the second test), ~2000mAh (for the first test), with unusual step-like voltage dropsWhat could explain this significant capacity loss in the series configuration? I'm particularly curious about the step-like voltage drops observed only in the series tests.Has anyone encountered a similar issue or can suggest potential causes or additional tests to isolate the problem?Any insights or suggestions for further investigation would be greatly appreciated!I will attach the results as images below. Thanks for all your help and support.

author: MK14
author_rank: Super Contributor
timestamp: October 02, 2024, 09:53:41 pm
content: Quote from: karlng on October 02, 2024, 02:09:24 pmThey (NiMH batteries), don't do well, if you over-discharge them. Discharging them down to zero volts, can damage them, and limit their life.If that is how you tested (discharged down to zero volts), the three batteries in series, then there would be a serious risk, that not only would you be over-discharging them, but one or more of them, could end up seeing a reverse voltage (across it), that could even more dramatically damage/destroy the battery(s).See here:https://www.panasonic.com/global/energy/products/eneloop/en/faq.htmlQuoteQuoteI'm not sure exactly what the safe voltage is, but I think going below (while discharging) around 1 volt, is not good for the cells charge/discharge life expectancy.The above link, seems to recommend, not going below around 70% to 75%, of their capacity.Quote

author: IanB
author_rank: Super Contributor
timestamp: October 02, 2024, 10:09:35 pm
content: Quote from: karlng on October 02, 2024, 02:09:24 pmOdd. LiPo, being lithium ion chemistry, should have a low self-discharge rate. Pretty close to zero, unless the protection circuit is imposing a parasitic drain. But OK.QuoteNo, that's not an exaggeration. What you are doing is a good way to damage NiMH cells.The stop voltage for a single cell should be no lower than 1.0 V. For cells in series, the stop voltage should be higher, even 1.1 V per cell.As mentioned by MK14, if you try to discharge a string of cells down to zero volts, you will reverse at least one of the cells and irreversibly damage it. After you have damaged it, it will permanently have a lower capacity.Your best option would be to start with three fresh, undamaged cells and repeat the discharge test, with a stop voltage of 1.1 V per cell each time. You should then find much more consistent capacity measurements.Quality cells like Eneloops have manufacturer guides and data sheets available. There is much good information in there about how to charge them, discharge them, and care for them.

author: karlng
author_rank: Newbie
timestamp: October 08, 2024, 08:11:50 am
content: Many thanks to MK14 and IanB for your advice on avoiding discharge to 0V.As IanB says, LiPo has a low self-discharge rate, but I've had some issues with buying some battery LiPos from China with a much higher self-discharge rate. NiMH has a much lower self-discharge rate, so I'm going to test the performance and capacity.The LiPo I'm using now has a capacity of 2400mAh, while the 3x NiMH serial has the same capacity (2500mAh). My system requires a voltage of more than 3V. I'm just testing the capacity. I'm concerned that the capacity when testing a single battery is much better than when I test it 3x serial NiMH. Do you guys have any ideas about it?

author: IanB
author_rank: Super Contributor
timestamp: October 08, 2024, 01:14:44 pm
content: Quote from: karlng on October 08, 2024, 08:11:50 amMost likely there is something wrong with your test procedure. If you test each of the three NiMH cells individually, you will obtain three capacity readings. If you then test the same three cells in series, you should obtain a capacity about equal to the lowest individual capacity of the three cells. This is assuming you use the same termination voltage of, say, 1 V per cell each time.Also, when you test the capacity, you need to ensure each of the cells is fully charged in the same way before doing the discharge. A good way to achieve a full and consistent charge is to charge at 0.1 C for 16 hours.

author: bookaboo
author_rank: Frequent Contributor
timestamp: October 08, 2024, 03:14:52 pm
content: Likely there was some damage caused by the first test to 0V.The other thing to keep in mind is that in 3S you will have 3x the internal resistance, although at ~1/25C discharge it shouldn't have too great an effect, perhaps not enough to notice, but if the results with fresh cells are the same, perhaps it's worth considering.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 08, 2024, 06:55:36 pm
content: There is no such a thing as "lipo".Li-ion batteries have very low self-discharge unless they are faulty. And if they are faulty, they are possibly dangerous. Don't buy random crap.Don't try to solve a problem which does not exist.