Go into the scanners menu to the Settings. There you have battery options. Select Set battery Low and Set Voltage. Set that to the lowest value.
Now run the scanner and when it stops working, it will probably say something about not possible to record if you have recording enabled, or it starts to act up, then note what the display say about the battery voltage. Add 0,1 volt to that value and enter that as the Battery Low value in the menu.
Batteries can have different operating voltages and one guys scanner might work down to 3.2v and another guys scanner migh starting to fail at 3,7v
So proper test of you own battery can be a good thing to do. Especially if you want to set an early warning to have time to stop a car at a convenient place and connect the charger cable without the scanner dying and all temporary avoids are restored and possible all other settings if there's no time for the scanner to save the current setup to the SD card.
/Ubbe