NiMH break-in charging - Page 1
Forum section: electronics / general

author: pcm81
author_rank: Regular Contributor
timestamp: December 27, 2022, 04:18:54 pm
content: I am doing some break-in charging of A size NiMH cells. These to be exact:https://www.batteryspace.com/nimhrechargeablecellsanyoasize12v2700mah1pc.aspxI am using SkyRC MC3000 to cycle them.On all of them I get around 2500mAH on discharge cycles. On some of them the charge cycles fail to terminate with -dV set at 3mV and Temp goes up to about 40C-45C but does not trigger end of charge. Happened on 2/4 cells that i left on charge/discharge cycle last night. Two are now discharging, the other two kept charging until 3600mAH limit threshold was reached and charging terminated due to that. Is this a known thing with NiMH? Should i keep cycling those that failed to terminate or drop them and try to break in some other units? I need to find a set of nearly identical cells to make a battery, but have about 2x as many cells to choose from than i need for the battery.Thanks

author: GLouie
author_rank: Regular Contributor
timestamp: December 27, 2022, 05:13:04 pm
content: I don't know the SkyRC charger, and have only used Eneloops.But, I wonder what your charge current is, or exactly what charge or break-in mode you use? From the manual, it appears that "break-in" mode just charges at 0.1C for 16 hours. I have heard that NiMH chargers often have trouble detecting termination at lower than 0.5C.

author: pcm81
author_rank: Regular Contributor
timestamp: December 27, 2022, 05:21:16 pm
content: Quote from: GLouie on December 27, 2022, 05:13:04 pmI am charging at 0.5A which should be 0.2C for full nominal capacity. Might be time to bump it up though... The initial capacity started around 200 mAH so i started at lower charging current around 200mA, then went up to 500mA....I read that break-in can take 5-10 cycles for quality cells and 10 to 50 cycles for crappy ones.

author: GLouie
author_rank: Regular Contributor
timestamp: December 27, 2022, 05:36:07 pm
content: I would try some experiments at between 0.5C and 1C to try to find reliable termination. IMO, cell heating/charge speed preference would be the final factor if termination is reliable.I don't understand why the cells would start at 200mAH capacity, which is un-Eneloop like.

author: IanB
author_rank: Super Contributor
timestamp: December 27, 2022, 05:39:09 pm
content: You have two choices with NiMH. Either charge for a fixed time at a low rate like 0.1C, or use -dV termination at a high charge rate like 0.5C or above.Trying to use -dV at a rate like 0.2C is not going to be reliable, and it depends a lot on the design of the charger.A good strategy to recover underperforming cells is to charge for 16 hours at 0.1C, then a slow discharge at 0.2C, then repeat.

author: pcm81
author_rank: Regular Contributor
timestamp: December 27, 2022, 05:42:57 pm
content: Thanks all for input. Going to try 1A charge.I did not expect the cells to be not-broken in either, but that is how they came in...

author: IanB
author_rank: Super Contributor
timestamp: December 27, 2022, 05:45:24 pm
content: If you read the data sheet, it recommends conditioning the cells by charging at 270 mA for 16 hours, then discharging at 540 mA down to 1.0 V, then repeating this cycle until the capacity stabilizes.Also, for normal fast charging it recommends a charge current of 2100 mA with a -dV of 10 mV.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 27, 2022, 05:50:17 pm
content: Quote from: pcm81 on December 27, 2022, 04:18:54 pmI remember -dV (negative delta V) being the end of charging criteria for CdNi, while for NiMH the charging is considered completed at 0dV (zero delta V).If possible, set the charger to stop at -dV = 0mV for NiMH.Even so, after many cycles in a battery the dV detection might become unreliable. That usually happens after 1-2 years of usage for my AA/AAA. Still good to use for a couple more years but for low current loads only, and charger will overheat them until the timeout occurs. That is why I charge the old batteries with a slow charger that ignores the voltage and it just pumps the same current for 16 hours straight.An alternative way to detect when an old battery is full (while still using fast charging) could be the sudden raise in temperature. When the battery can not store any more energy into its chemistry reactions, most of the energy pumped by the charger will have to go somewhere, so it will transform into heat and thus will appear a sudden raise in the battery's temperature.

author: floobydust
author_rank: Super Contributor
timestamp: December 27, 2022, 05:53:17 pm
content: I think -dV 3mV is too small. Tenergydatasheetshere are 5mV, to 10mV for large 4300mAh cells.Charge should have also terminated due to high voltage, at 3,600mAh it must have been high, over 1.60V?For an Tenergy Centura 8.4V unit "Please discharge to the 7.0V end voltage with 0.2C before charging the battery unit."I think this means initial use i.e. discharge to 1.0Vpc at 0.2C which is news to me.

author: pcm81
author_rank: Regular Contributor
timestamp: December 27, 2022, 06:00:36 pm
content: Initially at faster than 150mA per cell discharge rate, the voltage would drop to 1V basically like after a minute, which is why i figured they were not broken in. Also, initially charging at above 150mA would drive charging voltage up to 2v, which again pointed at cell needing a break in. Since i could only guess the actual capacity, in the non-broken in state, i started at 150 mA charge/discharge with dv detection and 3600mA and 16 hour cutoffs for few cycles.Will now try 1A charge rate with -dV termination to see if it behaves well. It did on 2 out of 4 cells last night.1A charge rate has just successfully detected and terminated charge on 2 bats that were full from last night. Hopefully that is the answer.

author: IanB
author_rank: Super Contributor
timestamp: December 27, 2022, 06:05:54 pm
content: Quote from: RoGeorge on December 27, 2022, 05:50:17 pmNo, this is not the case. Check the data sheet for the cells linked in the first post:https://www.batteryspace.com/prod-specs/A2700.pdfThey show a typical charging strategy of 2100 mA (0.8C) with a −10 mV delta V for termination.The negative delta V happens when the rising temperature at full charge causes a drop in the internal resistance of the cell. At constant current, when the cell EMF reaches a maximum and the internal resistance drops then the cell terminal voltage also drops. The drop in voltage is proportional to the charging current, so a 10 mV drop at 2100 mA would be a 5 mV drop at 1050 mA, or a 2.5 mV drop at 500 mA.Since the OP was attempting to charge at 500 mA and looking for a 3 mV drop, it is no wonder the termination was unreliable.

author: pcm81
author_rank: Regular Contributor
timestamp: December 27, 2022, 06:11:49 pm
content: Quote from: IanB on December 27, 2022, 06:05:54 pmThank you! This makes sense. I did not know that -dV is linearly proportional to charge current.

author: pcm81
author_rank: Regular Contributor
timestamp: January 01, 2023, 03:18:22 pm
content: Has anyone seen a case where battery temperature goes up but -dV does not get triggered even though it should?I am charging at 1.5A, have -dv set at -3mV and have few cells that terminate due to over-temp, set at 45C rather than due to -dv. Most cells terminate good at those same settings by -dV.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 01, 2023, 04:12:00 pm
content: Take a look at this project for the ultimate NiMH charger design:http://www.ultrasmartcharger.com/It also helps to not fully charge (except occasionally for balancing) or fully discharge the cells. That's how Prius batteries regularly last well over 10 years.

author: pcm81
author_rank: Regular Contributor
timestamp: January 01, 2023, 04:35:32 pm
content: Quote from: NiHaoMike on January 01, 2023, 04:12:00 pmI am using Sky RC MC3000, which "should" have good temperature and voltage monitoring.These are brand new cells i am breaking in to capacity match them in a battery assembly.

author: IanB
author_rank: Super Contributor
timestamp: January 01, 2023, 07:04:17 pm
content: Quote from: pcm81 on January 01, 2023, 03:18:22 pmGenerally, they would be bad quality or underperforming cells. They might be rescued by a break-in cycle (charging at 0.1C for 16 hours, discharging down to 1.0 V at 0.2C, then repeating). If they are not rescued this way, then you should probably put them in the reject pile. You don't want to use them.This failure to terminate normally would tend to occur if the cells have an abnormally high internal resistance due to age (if old) or some problem in manufacture (if new). The minus dV will only occur if the cell is fully charged at the time the temperature goes up. If the temperature starts to go up before the cell has reached a full charge then the voltage will continue to rise due to charging and the over temp limit will be reached first. Cells like this with a high internal resistance will not only fail to charge normally, they will also have poor performance in a battery.It goes without saying that when selecting cells for a battery pack you want to reject any cells like these that do not perform to a good standard.