This issue started happening not so long ago. It takes too long to charge my T420 battery now. ACPI even says 90 hours, or 50 hours remaining to fully charge etc, then the AC disconnects and reconnects.
$ acpi Battery 0: Charging, 70%, 44:24:49 until charged
Battery's lifetime is still pretty good, however. When I do some heavy tasks, the AC power dumps down to zero.
$ cat /sys/devices/platform/smapi/BAT0/power_now # idle 19921 $ cat /sys/devices/platform/smapi/BAT0/power_now # heavy tasks 0
Here's TLP's output. I'm using Arch Linux.
edit: I have tested the Thinkpad with battery removed. It doesn't seem to respond bad at all, it doesn't suddenly shut down or anything like that.