o2 irregular charge intervals

[deathevent]

hey everyone, I been having some problems charging my o2. It all started like 2 months ago, one day I just turned it on after a "complete" charge and it only had 2 bars of battery. The same thing continued for the next few days, even though I left it all night charging it only charged until it had 2 bars left(if you tried to charge it again after that it would only charge for a few minutes and after that the red light would turn off, it was like the battery was fully charged but then the O2 would only show 2 bars of battery left when turned on).

One day it suddenly fixed and started charging completely again. However, since a week ago this same problem arised again, and even worse. Last night with only 1 bar left I connected it and after 2 minutes the red light turned off. Sometimes it charges for 5 minutees or for 2 hours but I haven't been able to completely charge it(Haven't tried using the usb cable yet though). One day i tried to charge it again and again after every time the red light came off and I was able to get to 4 bars(almost complete charge) but the problem is that i can't get a complete charge plugging it just one time.

Does anyone have any idea of what could be happening? could it be that the charger is damaged, or else the O2?(worst case )