I really do not like blinkies that take alkaline batteries, because I am always tempted to leave the batteries in and get the last little bit out of them before replacing, and that sometimes leads to getting to my destination and finding out that I was down a taillight (one of the reasons I always run at least two).
With rechargables, I just top them off as part of the normal charging regime (two cameras, headlight, taillight or two, bluetooth headset) and they're always pretty well topped off.
Depending on the light, if it stopped taking/holding a charge I'd probably pop it open and see if it's a size I can get. It's usually not too hard to at least come close on size and solder in a new LiIon.
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Work: the 8 hours that separates bike rides.