Can a headlight battery be reconditioned if it won't hold a charge?
The battery is probably shot but the battery pack can be rebuilt.  An outfit like "Batteries Plus" (in the US) can do it.  Or you can generally do it yourself if you're fairly clever (though getting it professionally rebuilt is better because they have the right spot welder, etc).

(It would be nice if the light mfgrs would use standard tool batteries rather than their own designs.  It would be fairly easy to build an adapter to hold one in a water bottle cage, eg, and then replacements would be readily available and you could use the tool charger.)