Battery weird - seems to be phone but no obvious hardware issue

I have a galaxy s2 and bought a new battery recently (samsung one) thinking it would solve an issue which I thought was my battery.
Its very hard to describe but will do my best (have checked other threads but nothing quite like this..):
The battery will always say charging when plugged in but will sometimes increase in charge and other times not. I can charge overnight sometimes and it will say charging 7% and in the morning be completely dead and will not even show the charging animation (I have fixed by using my girlfreinds phone to charge it a bit and then it will be fine for a bit).

I thought might be charger - but my girlfreinds s2 charges fine on it - her phone works fine on both batteries and charges both fine.

Seems to be something to do with my hardware and I thought it may be the charging port - but not sure as it does still say charging without having to wiggle or anything like that so that suggests not the port.
The other thing that suggests something other than the port is I have had a battery in my girlfriends phone with it on, then plug it in and it said charging 37%, take it out and put in my phone with it plugged in and turn the phone on and it will say charging 4%...

Does anyone have any ideas as to this? Any things to try?

Thanks heaps in advance!