ThinkPad T420s (and many other laptops) support a UltraBay battery instead of a CD or HDD drive.
If You boot FreeBSD with this battery inside, everything works ok, but If You boot without it and then insert it as laptop is running already its not used by FreeBSD.
The error says its not configured:
% acpiconf -i 1
acpiconf: get battery info (1) failed: Device not configured
Similar problem when You switch that UltraBay battery to other ultrabay battery, the change is not detected.
I have also seen this on my T530 with the extra battery attached via the bottom docking port.