Instant 'reserve battery power' message when unplugging power cable

Since this morning I've got this issue with my macbook pro. When I unplug the power cable, the os instantly shows a 'you're running on reserve battery power' message. Even though it's already been charging for hours. However,
the mbp doesn't shutdown after a few minutes when I leave the cable out, so it's just the message that's wrong. Has anyone heard of this before? A solution would be much appreciated.

Not heard of this before, but I'd give resetting the SMC a go.
Intel-based Macs: Resetting the System Management Controller (SMC)