All rechargeable battery manufacturers love to boast about their product's current capacity (mAh). But there is a dirty little secret that they don't want you to hear: self-discharge rate. Simply put: a fully charged NiCd or NiMH cell will gradually lose its stored energy over time. Technical papers I have researched typically put the self-discharge rate at 10-20% per month for NiCd cells, and 20-30% per month for NiMH cells. This kind of self-discharge rate is usually acceptable in applications such as digital cameras.
I bought 8 of those Energizer 2500mAh rechargeable NiMH batteries over one year ago. At first, I was very happy about the large current capacity offered by those batteries. But within a few months, I started to notice that they die very quickly in my digital camera. In fact, a set of Sony 2300mAh NiMH batteries I bought one year earlier seems to last much longer when used in the same camera.
I recently did some controlled experiments (using the LaCrosse BC-900 AlphaPower Battery Charger) and found out what's wrong: The Energizer NiMH batteries have very high self-discharge rate. After fully charging all 8 cells and left them on the shelf for one week, five of them lost over 30% of their charge, and the other three lost about 20%. In comparison, the set of older Sony batteries only lost around 10% over the same one-week period.
So what this really means is: if I charge up those Energizer 2500mAh batteries and leave them in my camera for three weeks, they will become totally exhausted. I found this kind of self-discharge rate completely unacceptable, therefore I strongly advise against buying those batteries.
[Update on April 9, 2007]
I have hardly used those Energizer 2500mAh cells in the last few month. Now they have deteriorated even further. Five of them can't even hold their charges for more then a day.
Instead of the Energizer 2500mAh cells, I recommend buying the Rayovac "Hybrid" 2100mAh cells. They have very low self-discharge rate (see my review on "RAYO 4PK AA") and are cheaper than the better-known Sanyo eneloop 2000mAh cells. Kodak also sells a "Pre-Charged" NiMH cell with exactly the same spec as the Rayovac Hybrid.
The bottom line: Low-Self-Discharge NiMH cells are your best choices. There is absolutely no point in taking chances with older generation 2500-2700mAh NiMH cells now.
[Update on Oct 20, 2010]
The product image of Energizer Rechargeable AA has changed. It no longer advertises '2500mAh'. I went to my local W*mart to check out the actual batteries. All Energizer rechargable AA cells are now 2300mAh. Evidentially, Energizer has 'rollled back' battery technology to 2004. This is actually a good move because the lower capacity cells do not suffer from hideous self-discharge rate. Contrary to popular belief, a 2300mAh cell is not any less capable in delivering high power than a 2500mAh cell. Just that the run time will be about 8% shorter. Energizer NH15BP-4 Rechargeable AA Battery 4 Pack
I've been using NIMH batteries from SANYO, NEXCELL, POWEREX and AccuPower for over 6 years. Use them for everything from high current drain digital cameras and high-end digital audio recorders to CD players, bathroom scales and flashlights.
I rely on two of the excellent MAHA MH-C401FSB smart 2-rate chargers. These charge and monitor each battery independently, not in pairs. Energizer 2500 mAH units were fine for about three months. Then I noticed that devices would be calling for new batteries with little to no use. This has never happened with other AAs NIMH, including those with less than half the advertised capacity. The MAHA charger also started reporting that some of the Energizer AAs were not taking a charge at all.
Kept finding that typically one out of four were the problem. Many of the 32 AA Energizers I bought have had problems. Repeatedly have had one battery run out of charge while others still test very strong. Felt pen dots put on discharged batteries showed it was the same batteries time and time again. Have reverted in some cases to 1600 mAH SANYO OEM batteries put in service in Feb 2001 and those provide longer and more reliable performance!
In my considerable experience with these 2500 mAH AA Energizers they have been a very big disappointment and, regardless of warranty, cannot be relied upon.
I bought a bunch of Energizer's AA ACCU NiMH rechargeable batteries to power a halogen bicycle light and LEGO Mindstorms robots. Their performance has exceeded my expectations.The bicycle light seems as bright as it ever was with alkaline batteries, affording good visibility during night time cycling. And stopping play---er, experimentation---with the LEGO robots for recharging has yet to be a problem.However, I do have to issue this warning: these batteries are slightly fatter than standard AA batteries. I tried using them in a Mini Mag Light. After I got one battery in the light's barrel, I realized---too late---that I'd never get the batteries back out. I ended up destroying the Mini Mag Light in order to retrieve the batteries.If your application is cramped for space, beware! If they do fit, expect good energy for a good long time. - Battery Aa Rechargeable - Energizer - Battery - Green 3'
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