W520 connected to dock, battery dies & system shuts off

[Arf the Lab]

I've had my W520 for well over a year now. It is always here in my office, connected to its dock w/170W power supply. And it has always been working harmoniously in said setup. At least until a few days ago. Lately, I notice the battery light on the laptop is constantly flashing orange. And eventually, after some hours' usage, the machine shuts off entirely. Needless to say this isn't OK, and I need to find a solution. The dock connection is solid, power + USB from the dock are all working a-OK, etc. So I'm at a loss.

Any help appreciated, thanks.

[RealBlackStuff]

Replace the battery.

[Arf the Lab]

Done. And working fine now. Thanks.

[jcvjcvjcvjcv]

Replace the battery.

Ok, but why did the system shut off? The battery is supposed to have no activity when the whole thing is connected to AC? Or does it attempt continuously to charge it and overheating some circuit?