I have a Nikon D5100, which I bought in 2010. Whenever I turn on the camera, it says that the clock has been reset. I read in the manual that the backup battery needs to get recharged for three days for holding charge for about one month. Now, the question is that should I leave the camera powered on for three days to charge the backup battery or should I just put a recharged EN-EL 14 in the battery compartment without powering on the camera?
You can put a charged EN-EL 14 battery and the internal battery should recharge as it charges both from an AC adapter or a battery.
I usually leave the battery on the camera anyway, even though the manual says you should remove it if you don't use the camera for a long time. I like to be able to pick it up and start shooting right away :)