1. Which battery it takes
Most modern meters use a CR2032. Some older ones take a CR2016 or a pair of AAs, and a few use a proprietary pack, so check the manual or the old battery before you buy.
2. How often it changes
A CR2032 gives a meter several hundred readings, depending on how much the meter uses the radio and the backlight. The clock, the memory and the test strips all draw from the same cell.
3. Change it when the warning appears
Most meters show a battery symbol or a warning on the display before the cell is dead. Changing it when the warning appears, rather than when it fails, keeps your log complete.
4. Your readings are not lost
A flat battery doesn’t wipe a modern meter. The results sit in memory, and changing the cell usually leaves them alone. Set the date and time again if the display looks wrong.
5. Take a spare to appointments
Keep a fresh CR2032 with the meter and its case. Running out of blood before you run out of battery is far more likely, and you can’t test without the cell.