Hi Dan,
I do the same as you, and use the Maha C9000. One main advantage the Maha has over other chargers is the IEC standard capacity test, for those unfamiliar with it, it takes at least 36 hours to complete each test.
I've had similar luck with NiMH cells now going 7 years on the oldest, all in use. They my older NiMH were not optimally charged and those that survived are as low as 50% capacity, but the devices I use them on have such low consumption, I get 1 mo of use out of it [ shavers, electric toothbrushes etc.,]. Those that were speed charged on a 1 hour charger didn't make it past 100 charge cycles.
I've also resurrected a set of 4 AA cells over 10 years old found in an forgotten bag, and recovered about 50% of its mAH capacity. However, the self discharge rates were tremendous, about 2 days to fully discharge so they were eventually discarded, but it demonstrates that the Maha restoration charge-discharge cycles procedures work.
The calendar life of NiMH chemistry is not well studied, but if you find my old threads, Cobasys claims the chemistry, if charged and discharged optimally, can exceed 10 years easily, they claim up to 30 years. I had a brief archived discussion with a Varta battery engineer on element14.com and he suggests Varta NiMH can obtain 10 years, but again, no papers to make solid reference.
Per Varta, they claim the cells are being used for backup power for an auto based cellphone emergency system installed in cars, and are designed for a minimum of 5 year life.
Toyota Prius first generation have packs of NiMH still going strong at 10+ years old, and is widely reported particularly in regards to the resale of older Priuses.
These anecdotes suggest that the calendar life of NiMH can easily exceed standard LiPo and ion, if charged and discharged optimally. Its a key characteristic in design when deciding to use NiMH or Li chemistry. LiPo has a calendar life of under 4 years, with a continuous decline in mAH regardless of charge cycle, but it provides far high capacity for weight. LiFeP04 may fix that, but Chinese label LiP04 cost about $4-5, about the same as 2 eneloops, but in the right application the NiMH will last 3-7x longer.
I have a pool of around 80+ NiCd/NiMh batteries that I'm taking care of, both at work and at home.
Since I got my Maha C9000 charger, I'm also keeping a spreadsheet containing a log of the available capacity per battery (as reported by the Break-In mode of the charger which follows the IEC standard for determining the available capacity).
Some of the findings from this activity are as follows:
- 12 Eneloop (AA and AAA) cells are at or exceeding (105%) the typical capacity
- Around 30+ Sanyo NiCd 700 mAh cells that have around 8years are still at 80-85% capacity. These are the batteries used by the DECT Siemens phones used at work
- 12 cheap Lenmar LSD cells are performing very well for now (100% capacity), but they are fairly new
- Very old (9years?) GP NiMh batteries are still quite OK: at around 75% capacity, but it looks like they developed a rather high internal resistance
- NiCd/NiMh rechargeable batteries delivered with solar lights that you can buy very cheaply these days are a joke: even new cells are at somewhat 75% of the rated capacity (which is very low btw. Ex: 600 mAh for an AA cell)
- Oldest NiCd batteries are some cells that I've found in emergency light units. They are rated for 700 mAh and still keep about 56% capacity. No idea about the age though (20+ years I would hazard)
- Only definitive loss is a rather nice Sanyo HR-3U 1600mAh cell that developed internal shorts (a possible failure mode with NiMh cells). I forgot two cells in a CD MP3 player somewhere during 2003 and only found them recently. The cells were totally discharged and stayed like this for a long time. One of the cells recovered nicely (80% capacity) but its sister was dead. I have tried to zap the internal shorts with high voltage pulses. It sort of worked because the battery was able to keep a charge only for a short period (hours).
Cheers,
Dan
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