cell phone weirdness

[eve11]

I have a motorola razor and lately it's been working fine except it's decided I have an "Invalid Battery" and it tells me this every once in a while. Now my battery power's running low and I plugged it into the charger, but again, it doens't show any charging progress or anything, it just says, "Invalid Battery". What exactly does this mean and does anyone know how to fix it?