Battery sometimes fails to properly charge

I've had this computer for a couple years now, and it's still running strong. The battery still gives me 4-5 hours charge, which is fantastic. The only problem is that sometimes the battery refuses to charge when my laptop's plugged in and the battery still has a good amount of charge (somewhere between 90% and 50%). Also, if I'm charging the battery from a lower charge, it sometimes refuses to finish charge - it says "Finishing charge" (or something like that, I'm not looking at it right now). What do you guys suspect the problem is?