This happened to me once when I bought a low quality third party battery. It worked fine for a few weeks, and then it would always not report the percentage left in the taskbar. I could still charge and discharge the battery, it was just hard to know how much juice was left. The best way to check the charge would be to use the advanced power settings from the menu. Tap "start", "settings", go to the middle tab and tap "power" or "battery". If you don't have an advanced power manager preinstalled on your ROM, you'll need to download one separately (it's been so long, I forgot which one I was using).
- 2 Bunny