These days I'd not be surprised if in order to force more cash out of you something within the unit is volatile, and its lost on powerdown, then you have to buy a new one.
The operating system is probabaly on flashrom, it'll be an I'm alive code that is stored in battery kept ram or something.
Jumper a pair of wires from another battery and quickly replace it, be quick as a large current will flow from a good to a dead battery.
Also to spoil your day there could well be a timer, or more apt a calendar within the unit that states how long it'll work for if the battery lasts longer than the manufacts thought (or you replaced it).
Reading the code will probably be difficult as most devices these days have read protect, as mentioned, which almost undoubtedly will be turned on.