Left-hand slot is meant to hold the room key - you insert it there to enable power in the room -- the idea being that you remove the room key when you leave and the power all shuts off. However, if you have batteries charging, you need a way to keep the juice flowing when you leave with your room key. Job done!
May 01, 2015@07:37

K5__0051
1/50 sec at f / 4.0@43 mm
ISO 6400
PENTAX K-5
smc PENTAX-DA* 16-50mm F2.8 ED AL [IF] SDM