In 2005 Sanyo introduced Eneloop NiMH rechargeable batteries. Part of their marketing was that they are already charged when you get them and you can use them right away. The reason for this is they charge them at the factory and that the batteries have “low self discharge” (LSD), in other words it takes them much longer to lose a charge sitting on a shelf than other NiMH batteries. Sanyo claimed that Eneloops will retain 85% of their charge after a year. I bought some last year and have no reason to doubt the claim. I have two HP calculators that eat through AAA batteries so I wanted something rechargeable but also something with a fairly long shelf life. I thought it would be good to have a set for my Archos Jukebox too since I have been charging its batteries separately instead of by using the AC adapter it came with (gets very hot and can’t be good for the batteries).
I found an exhaustive review of Eneloop batteries at Candlepower Forums. Eneloops still seem to be the standard, but most other companies have them too: the Duracell Precharged, Rayovac Hybrid, Kodak Precharged, and Maha Imedion. Given that Sanyo, Kodak, and Duracell’s batteries have similar performance, appearance, and are made in Japan, it is thought they are the same thing. All of the LSD batteries have lower capacities than other NiMH batteries (typically 2000-2200 vs. up to 2900 milliamp-hours (mah) for regular NiMH cells). The tests confirm that these things really work and many people at Candlepower swear by them. However if the cell isn’t made in Japan, the results seem to go downhill. Made in China cells lose their charge faster, with Chinese Duracells having measurably worse performance than Japanese Duracells. The Maha Imedions (made in Taiwan) have a higher initial capacity but discharge a little faster than Japanese cells. Battery Junction has an in-house battery brand called Titanium. Their Titanium Enduro LSD’s seem similar to Rayovacs, are made in China, but early ones started at about 50% capacity and after repeated break-ins would work their way up to 90%. They’re cheap though: $6 for 4 (AA or AAA) with a free plastic case and a 5% discount (“cpf2006”), though shipping is high.
Therefore these batteries can be good for flashlights where you want power, but it might be a while. However the Eneloop batteries lose charge much faster when they are hot, so they are not a good option for a flashlight kept in a car. Imedion batteries are supposed to be better at dealing with the heat.
However, some people at Candlepower have pointed out that the Everex (same company as Maha) and Sanyo 2700 mah batteries seem to discharge pretty slowly too. Not as slowly as the LSD batteries, but because they are starting out with more oomph in the first place, they can afford to lose more. So after a year, they might lose a third of their charge, which will put them at 1800 mah, slightly more than 2000 mah Eneloops that will have only lost 15% to be at 1700 mah. So that kind of puts a different perspective on things. One guy did testing by running through 100 discharge cycles in a row on both Eneloops and Sanyo 2700’s and noticed that Eneloops stay at roughly the same capacity while the conventional NiMH dropped off (still pretty slowly).While the 2700 started with 29% more capacity, after 397 cycles the Eneloop would (theoretically) have higher capacity. His sample size was only 1 Eneloop and 1 Sanyo 2700, so it’s hard to say if those results would hold up. I’m just amazed he ran the test 100 times!
The other perspective is that 4 Eneloops cost about $11 and for the same price at Walmart you can get 30 Energizers. So for things like clocks, remotes, and indoor/outdoor thermometers that only need to be changed once a year or so, it is probably more economical to get alkalines (7 alkalines cost the same as 1 Eneloop, so you won’t break even until you charge the Eneloops 7 times).