Originally Posted by
bicycleflyer
I'm going to go against the grain here.... Go with Li Ion. Like one poster above, at one time I had both. The AAs were always giving me trouble. It seemed i never could keep four good batteries. It only takes one bad one to ruin your ride. I had a very nice charger and name brand batteries, but that did not help. It never failed, between 30 and 45 minutes the light would go into power saver mode. My only savior during those rides was a set of extra Lithium batteries I kept in my bag... in a extra battery holder. When Dinotte introduced the 400L, I decided to upgrade my light system. While I was at it, they upgraded my AA to Lion (free) and I just used the spare battery that came with the 400L... My life has been a whole lot easier since then.
Something definitely odd there. I use a set of pretty old AAs in mine, 2350 mAH, and I run them two days both ways, 45 minutes each way, before they go into low battery mode (still not power save mode, still bright and flashing, just with the red light on in back).
If you have a charger that does two batteries at a time, replace it. Ditto "timed" chargers (doesn't sound like you have that). Only smart chargers with -dV sensing and a separate channel for each cell are good. Otherwise, as you say, you can have one bad cell mess you up.
I also keep an eye on the batteries as they charge; if one is still charging when the other 3 have finished, I put a mark on it with a sharpie. If one cell consistently gets marks on it (3 or 4 marks say) I toss it in the recycling bin.
When it gets really cold, I charge after every ride, but the NiMH still work for me even down below zero.