Hi
I'm not sure if this is a bug or whether Rockbox's battery management is better than Sandisk. Having had my Sansa Clip on charge for approx two hours, it is reporting the battery as 94% (or icon, one pixel away from full). When I boot into Sandisk's firmware, it is reported (graphically) as full.
It is possible that the battery is loosing charge now (quite old) but thought I would mention.