Sometimes less voltage can mean practically no light at all.

The Fenix L2D LED I have takes two AAs. It burned about 2 hours on full blast with batteries provided with the light, getting close to manufacturer's claims. On my yesterday's commute, with rechargeable NiMHs it burned maybe one hour, tops.

I'm thinking one of three reasons, or maybe combination of all three: first, the NiMHs I used were old, heavily used, and no-brand. They're probably close to expiring. Second, they were only 2100mAh, so less time should be expected, although I don't know what capacity normal alkaline batteries have. Third, it was coldish at -5C. My rear LEDs (powered by similar NiMHs) worked fine, but loss of power is not so dramatically visible with them.

I now have a pair of Energizer 2650mAh rechargeables in the light, and I'm waiting to see how long they last. I also have a couple of GP Powerbank NiMHs and I'm thinking of buying a pair of Hama NiMHs, as those seem to be highly regarded among photo-enthustiasts around here. I guess I'll see soon enough if brand name makes a big difference. None of the mentioned brands is cheap though.

--J