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NiMH break-in charging

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pcm81:
I am doing some break-in charging of A size NiMH cells. These to be exact: https://www.batteryspace.com/nimhrechargeablecellsanyoasize12v2700mah1pc.aspx
I 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

GLouie:
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.

pcm81:

--- Quote from: GLouie on December 27, 2022, 05:13:04 pm ---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.

--- End quote ---

I 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.

GLouie:
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.

IanB:
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.

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