The FIT is operated from two AA Alkaline battery cells. When operated with the LCD backlight turned off, the FIT can operate for approximately 50 continuous hours. Continuous operation with the LCD backlight on is approximately 20 hours.
There is a battery test under the Systems menu to determine when to replace the batteries. To replace the batteries, turn the power switch off. Remove the two AA battery cells and replace with two AA Alkaline cells, while noting the proper polarity of the battery holder.