Nimh cells- getting old or in need or maintenance?

[Comdrpopnfresh]

I've got some rayovac ic3 15-min rechargable batteries that have lasted me well. Lately, though, they tend to discharge a bit faster when sitting unused. I suspect this isn't what is actually going on, rather, they aren't charging to full capacity. Is there anything I can do to restore performance without buying anything?

 

[pm]

About the only thing that I've heard recommended is charge cycling. This Duracell NiMH page mentions it - pull down the PDF for massive info:
http://www.duracell.com/oem/re...ble/Nickel/voltdep.asp

?Voltage depression from repeated partial discharge can be reversed through
charge cycling (see figure below).

Charge cycling is back-to-back repeated full charge/discharge cycling - usually a function done in a high-end NiMH charger, but can be done by hand. It usually takes ~20+ cycles to see the effect go away.

 

[aceO07]

Try what pm suggested.

Keep them cool while they're recharging. The heat from rapid 15-minute charging will kill them faster.

My charger has some nice modes including refresh battery (discharge/recharge until max capacity) and detect capacity. I really like that I could check up on my really old rechargeable batteries.

 

[Comdrpopnfresh]

work a try I guess- the energizer 15-minute nimh cells, the only ones still on the market i believe, only recharge 150 or so times. But those IC3 cells were said to handle over 1000 recharges. What sort of drain (as in; standard device in need of batteries) would one put AA's or AAA's in to effectively perform a discharge? I always leave lots of clearance under the charger for them- and it has a fan built in, so preserving free access to cold air helps a lot.

 

[Comdrpopnfresh]

Okay- I'm going to do a few cycles and keep track of discharge times. I plan on initially charging the cells, placing them in a freezer, and then putting them into an electric shaver and running them down. I figure the current draw from the shaver isn't as low as a flashlight, which would take hours or days to run down, but not quick+high loading to drain it quickly but incompletely. The charger does a good job of keeping the cells from getting hot, thought I'm uneasy about them getting as warm as they do, and then directly discharging them- seems like this would only quicken the building of internal resistance. That's why I'm going to put them in the freezer- to allow them to cool down, but quickly enough that any self-discharging while waiting is negligibly small.
This whole thing seems rather ghetto- is it at least going to do more good than harm?

 

[pm]

I fly electric model airplanes and up until a couple of years ago when I went lithium-ion I used to use NiMH for all of my batteries. I have a charger that can be programmed to cycle batteries, but to be completely honest, I never had much luck with the technique. I used a large house fan blowing over the area to cool the cells. It wasn't very ghetto, but I also only recall a couple of packs that I recovered with this technique - the majority just died over time and my attempt to save them by cycling didn't seem to help at all. I agree with Ace007 - although I'd forgotten about it since lipoly cells don't heat when you charge them - that keeping them cool is key.

I don't mean to suggest something that will waste time - this is definitely the recommended approach to recover cells. But I've never had much success at it. I figured that using my packs once or twice a week for a year or two, I got my money's worth out of them and sent them off to recycling reincarnation and bought new cells.

 

[aceO07]

I've been 'refreshing' a badly drained NIMH battery the last 2 days. This was a 2600mAh battery that I never used before. It was so drained that the charger wouldn't even charge it and I had to trick it to start charging. After the first 'refresh' loop finished, it said it was around 900mAh. I 'refresh' looped it again and it managed to get back up to 1700mAh. I'm doing it again and hoping for more.

fyi, 'refresh' option on my charger does discharge/recharge cycles until the current cycle's measure capacity is not greater than the last cycle's measurements. It's not perfect, but it seems to work decently. Since I know what the 'max' capacity is supposed to be, I just do it again until it's as close to the max as I think it will go.