Gene, there are two batteries, one internal that can not be removed and one in the top turret, when using the scope the internal battery is used first, when it gets low it will shift to the turret battery. I have been running about 2 1/2 hrs on the internal and then about 2 hrs. on the turret battery, I do not run the scope on all the time just when hogs come out. It only takes 2 seconds to turn on and Nuc the scope and I'm ready to shoot. I like the battery system on the Thermion, I can charge the batteries while on the rifle thru a usb port or take it off and charge it, I have a QD mount that allows me to move from my 6.8 to my 6.5 Grendel with no change in zero, I just have to go internally and swap my profile setting, I have five profiles that I can set up to five different weapons on...again takes about two or three seconds to make the change