Ever got such a problem like this! Maybe the battery is broken, try to borrow others battery!
I got it real soon and it wasnt the battery..it was a bug in firmware due to a bad ported patch...
So just reflash a CLEAN firmware (just CG1 of the RGF in this case) and that should solve it
And, BTW, it isnt "Invalid Battery" all the time.. I used it like that for a week until I got bored and reflashed...anyway - you need to plug it out-in like 7-10 times and wait to see what happens...
Also! When you unplug the charger...it stays Invalid Battery, doesnt return to normal? (that was my case)