Totally agree on the battery thing - sounds like the issue.
However, before you splash out on a battery I'd recommend checking you charging base. Make sure the contacts are clean (on the phone as well), and that the light on the phone comes on when you dock it. Once you've established the actual charging isn't at fault, then it can only be the battery.
The batteries are available on eBay, or of course at your dealer. From memory they're about £15-20 ish..
Lee
BeoWorld - Everything Bang & Olufsen