Dell Charger Poser

[Prudie]

Here's a poser for the techies - I have a Dell laptop with a genuine Dell charger. I now find that when I plug in the charger I get a warning to connect a genuine Dell charger. Partner also has a Dell laptop and gets the same message with my charger. This is the odd bit though, having charged to 100% with his charger (and I always run laptop with lead to the mains) this faulty charger will hold it at 100% indefinitely but if I let the battery run down the charge will not increase.
I know it's faulty but why is the battery staying at 100%?

[Hammer]

Dell chargers have 3 wires, positive and negative to give you your juice, and a third wire that sends a message saying "i'm a genuine charger". It's possible that this third wire is cracked somewhere so that the message can't get through.

The laptop will let an unofficial charger (or one without the genuine message getting through) power the laptop, but not charge the battery.