Are you going to store your cash register away for a while? Then beware! Most machines have an internal backup battery that stays charged when the register is plugged in. When you unplug the machine, or you have a power cut, the registers memory (all of your products, store name, keyboard functions, files, and other system settings) are held via the battery. If you leave the machine turned off for more than 1 to 2 months, most registers will lose all programming, and you have a big job on your hands. (If it is dead, or has scrambled looking characters when you plug it back in, then data loss is likely).
What you need to do is take it out periodically, and plug it in (make sure the display is lit). I recommend leaving it on for 1 day each month to keep the battery topped up. If you haven't had the battery changed within the last three years, that is a good idea too, as they lose their capacity over time. If you have an older battery in your machine, then charge it more often, say once per week.
If you have any questions regarding the batteries, please just post a message in this forum.