Sony DR-BT21iK Mode D'emploi page 13

Checking the remaining battery
When pressing the POWER button while the headset is turned on,
the headset indicator (red) flashes. You can check the remaining
battery by the number of times the headset indicator (red) flashes.
Headset Indicator (red)
3 times
2 times
1 time
Note
You cannot check the remaining battery immediately after the headset is turned on,
or while pairing.
When the battery is almost empty
The headset indicator (red) flashes slowly automatically.
When the battery becomes empty, a beep sounds and the headset will turn off
automatically.
Status
Full
Mid
Low (needs to charge)
1
US