Need new battery it seems...

[Mux213]

Rats,

I'm getting good at pushstarting my car hehehehe..

Since yesterday I keep running into my car not starting. I still got power but not enough to make the starter go round, it tries but dies.... Yesterday I still had that after a drive when I park the car I could start it right up again but after it stood still for 15 mins no joy. But since today even after a drive it just wont start.

Tonight I had the battery out of the car and in a charger for about 4 hours but that didn't do any good so I guess that rules out a problem with the alternator

So its either the battery or the starter itself. Anything I can check to make sure its the battery that's gone? I need to buy a new battery anyways cause I need one for the RX7 but rather make sure I need one first...