Reviving an eneloop with decreased capacity.

[constipated]

I have two AA eneloops that I've been using in an LED torch for about 2 years. It is of the high power variety and can probably drain them in 75-90 minutes although I don't use them continuously often. Once the torch starts to dim often they are completely flat. I don't know if there may have been some reverse charging involved. They are often kept in my car which in summer can easily get to over 40C / 105F during the day.

I have a Maha C9000 charger and on a recent break-in to tune them up. One battery was still around 1920mAH, but the other was about 1800mAH. I am a bit worried about further use causing more reverse charging near the end of the lower capacity cell's life.

What is the optimal way to try and bring back the extra 150mAH capacity. I thought eneloops were a bit better at resisting degradation.

Do repeated break-in cycles really work. I would like some real life accounts before I try this as they are so time consuming.

 

[Mr Happy]

Eneloops are robust, but they cannot resist damage from reverse charging. You need to make sure you never run your light until it starts to dim. By then it is too late and one of the batteries will already be under voltage reversal.

As for recovery, there is probably little you can do. If a break-in didn't recover the cell beyond 1800 mAh that is likely as good as the cell can get at this point. To find out, put that cell back on a discharge cycle at 400 mA and see what capacity it gives after the second charge of the break-in. If it is still 1800 mAh that will be its limit.

Treat it as a learning experience and try to avoid doing the same to a fresh pair of eneloops. Two years is a reasonable lifetime for the original batteries after all.

 

[uk_caver]

I guess it's a trickier issue if the light doesn't appreciably dim even when one cell is exhausted?

 

[45/70]

This discussion points out the importance of matching cells that are used in series.

It's not necessarily enough to match the cells with, for example the results from a "break in" on a Maha C-9000 either, although that can sure help. The best way to match cells for use in series, is to discharge them at a rate that is close to the rate they will be discharged at in the device, then compare capacity. This helps insure that the cells will "drop out" at nearly the same time, and help avoid reverse charging or over discharge of any individual cell(s). With properly matched cells, when the light drops out, it's less likely that cell damage will occur, as they will be dropping out at, or near, the same time.

Just because two cells have similar capacity at a 400mA discharge rate, does not mean they will have similar capacity at an 800mA rate. Also, the capacity in relation to different discharge rates changes over the life of the cell. It pays to periodically check that the cells capacities still match.

Dave