Sounds like your battery developed an internal short-circuit. Rechargeable batteries do that a lot. It often helps prolong their life if you let them run down and recharge them once a week or so. Leaving them on 'float' charge permanently isn't the best thing to do.

I hate Dell. Due to company policy I was forced into buying 25 machines from them. Not one lasted 12 months without problems, some even had 2 keyboards in that time.