Most likely the battery as they don't have a full balancing circuit, so some of the cells can be over discharged, or over charged, eventually killing them.
Cheap to get another though. Quality is a bit iffy.
I've built a 6xAA units using off the shelf
Maplin 6 AA battery packs with rechargeables, then shrink wrapping the MS connectors. I use these on my rear Magicshines - does cut run time in half though, but not much of an issue with the rears - with the front lights it would be a different question. The AA's then need charging in a standard charger.
You can now get new batteries that are fully protected, but they cost a fair amount.