I'm not sure if the battery is dead. But when my laptop is unplugged and running on the batteries it only lasts 5 minutes despite the battery being 100% charged...
I don't think its dead because you have to use the battery a lot and recharge it a lot. I've had my laptop for 2+ years and I almost never use it when it isn't plugged in. I've maybe used it 20 times without being plugged in.
It sure sounds like the battery cannot hold a charge. 100% charge does not imply that the Amp-Hour rating is per new battery specs. after charging. Shorted cells within the battery could be the cause. Lithium batteries are never designed to be flattened (fully discharged), and permanent damage could result if flattened.
Since the battery does seem to achieve 100% charge after discharging, the charging circuit is fine. It's the battery that is not up to par. Try a new battery.
I agree completely. Whats more, a standard laptop battery will not "live" for much more than a year. If you have had it for 2 and lived with it on charge you will have worn that battery down substantially. You can usually get laptop batteries for about $50 on ebay (depends on type) so look around. Wholesalers are generally cheaper than retailers also.
Was your battery attached to the laptop? It sounds like you overcharged your battery if you used it for 2+ years with your laptop plugged in.
This happened to me with my ThinkPad T40. I used it for about 6 - 9 months plugged into the A/C outlet with the battery installed. Afterward, the battery only lasted 10 - 15 minutes. After numerous fully charging / discharging over several months the battery was able to hold a 2.0 - 2.5 hour charge; that's roughly a 40% - 50% capacity before I overcharged the battery.
My advice is to disconnect the battery if you are going to be using it plugged in for a long period of time. If you are the type that may travel on a moment's notice, then I would fully charge the battery and then disconnect from the laptop. Once a week connect the battery to the laptop and recharge it for a short period of time. 15 - 30 minutes should be more than enough, then disconnect the battery again.