Your SAM4s ER-5200M cash register has an internal battery and it will need replacing from time to time. We are not talking about AA or AAA batteries here, this is a small battery pack that is soldered onto the main logic board.
If your battery has died because you left it unplugged for too long or perhaps you lost power and the machine no longer works. If it is because you left it unplugged for too long try plugging in for 24 hours and perform a memory all clear before replacing. You will need to re-program the register from scratch if you don't have a back up. If you lost power for a few minutes and lost your program the battery is dead and should be replaced and the machine re-programmed.
This is not a repair the average end-user can make. the leads from the battery pack must be soldered in place after removing the old one. It is not hard if you know how to solder electronics (not copper pipes with a torch) this soldering is with an iron.
This repair should only be done with the register unplugged. You will do serious damage to your register if you make this repair with the power plugged in.