Active V3 Transponder

When the transponder reports a battery status of 2.4V at room temperature (20°C), it is good enough to be used in the cold as well. With the V3 Transponder there is no need to test the battery in the cold (like it was before with V2 transponders). If the transponder reports 2.3V or less, it should not be used in the cold, and you will need to replace the transponder soon.

Active V2 Transponder

Almost empty batteries will start to create problems in the cold. When a battery is too low it may result in a reset, this leads to lost stored passings and deactivated tracking. A reset can best be detected by looking at the wakeup count - if it reports a lower value than before, the transponder has reset (a continue counting from 2000 shows that the restart is caused by a low battery). 
 
This would be a typical empty battery transponder. It was able to generate a first passing with a regular Wake Up Count (WUC). Then it reset after sending the prewarn (which generated a rescue passing) during a passing. On the next WUC it restarts with 2001.

How to test if the battery is almost empty:

1. Put your transponders in a normal fridge cooling them down to about 5°C (40°F). You will need to leave them there for several hours for the transponders to actually cool down. Scan your transponders - if you see battery values 2.3V and below at around 5°C (40° F) - they will need to be replaced.
2. Only if you are timing winter sports: put your transponders in a freezer overnight to make them cool down, then check if they still detect. Be aware: they are expected to stop detecting below -10°C (14°F) when they have under 50% battery. They will still be perfectly fine for normal temperatures, but if expect temperatures below -10°C on your event, you need to use newer transponders.