Whistler WIC-2409C - Manual (Page 8)

Plug the adapter cable into the power socket on the monitor, plug the
other end of the adapter cable into the end of the power adapter. Plug
the power adapter into a 110v AC outlet.
- 6 -
The built-in Li-battery will charge after the adapter is plugged in.
The red indicator light will illuminate during charging and will turn off
after charging is complete.
The monitor can be used while charging.