Γεια σου / Hi Theodora,
You have a battery that is designed to provide an alarm signal, particulary when mains power is lost - power outage, fault in the power circuitry - not because of these conditions but because the result is failure of the refrigeration system to be able to cool. So the freezer will naturally heat up and you will get, for example, an overtemperature alarm - say above -70°C when you are working at -80°C.
The fact that you see 'low battery' means that:
- Your alarm battery is faulty itself in not accepting a charge, or
- The charging circuit is not working to keep the battery charged, or
- The backup battery was not connected properly upon installation.
For the latter case, it would not be accepting charge and would slowly lose charge over time. Of course, if you have not had an emergency condition (probably the case) then it would not be used, hence the 18 month delay before seeing the signal. It would only discharge more rapidly during a fault condition of the freezer.
I hope that this helps,