Why is my power adapter not charging the battery?

The other day, my MacBook Pro 17" (A1151) shut down after being used for only a few minutes. The battery indicated at least 90% power left. When I got home and plugged the power adapter in, it powered up, but would not charge the battery. (the battery indicator displays power by adapter and not charging battery). If the adapter is unplugged, it shuts down immediately.

I reset the SRM as instructed (no results). The battery is recognized by both Coconut Battery and the "about this Mac". Both indicate the battery has about 85% of the original capacity.

Thanks in advance.

Matt