After reading Egosbar's post, I checked the d5200 manual. Your clock can go for about one month and it takes three days of having the main battery installed to recharge this internal battery. In theory, this should cause the main battery to drain down a bit quicker when reinstalled as it recharges the internal battery. I don't know if the extra drain on reinstalling would be significant. My d3100 is seldom used these days. I have stored it with and without battery. I find it annoying to pick it up and have it flash me to set the clock so I now I just leave the battery installed.