As has been suggested, it's most likely that you are experiencing the typical behavior of Energizer 2500s, unless you charged the cells up immediately before using them, then much of their capacity was probably lost to self-discharge...
if you contact energizer via email and complain about their crappy cells they will send you a $10 coupon towards any energizer or everready product. I did not feel the least bit bad about complaining about these cells as I feel they are crap cells in every respect and should not be sold to unsuspecting consumers as all they tend to do is generate a negative attitude towards the concept of rechargeable cells in the general public.
The other possibility is that the headlamps runtime is falsely advertised.. this would not surprise me or anyone on this forum.
Another possibility is that the headlamps runtime is based on the diminishing output provided by Alkaline cells, which will drag out a long but diminishing output in many designs, but with NIMH cells, the headlamp *might* probably provide a more "regulated" output through the run as a result of the discharge characteristics of NIMH, and cause the light to not run as long, but maintain more steady output levels through the run. This is all hypothetical and depends on the specific headlamp. Good regulated flashlights will almost always run longer on a good set of NIMH than they will on alkaline cells.
Eric
I suspect a combination of the 2.
SilverFox,
I commute to work on a 2 wheeler. I need to learn more about batteries and charging. Last year I bought a LaCross H-900 charger and have been using Energizer 2500 NiMH cells. I also have some LaCrosse cells that came with the charger.
After a full refresh the LaCrosse lasted for quite a number of trips to and from work. Fearing they were about to die, I replaced them with what I thought was a set of fully charged Energizers. The charger reported ~2500 mAh after the charging cycle was complete. The Energizers lasted only about two or trips (90 minutes) in a headhight advertized to last 5 hours on fresh cells.
I've always been uncertain on what charging rate to use for these AA batteries. Can you help? Any idea why the Energizers died so quickly? Someone in another forum said the Energizer 2500 was the worst battery produced. You've done a lot of testing. Any commets? The recommendation was to use the Maha Powerex 2700. I didn't see that cell tested in a test report of your, which I believe was a bit dated. How do you view the Powerex 2700?
Several questions so I'd appreciate any insight or knowledge you can share. Thanks.