Panasonic FZ-G2 Series Operating Instructions Manual page 48

Confirm that the battery indicator lights green; this means that the built-in bridge battery has enough remaining power required for a hot swap.
When the battery indicator doesn't light; Shutdown, Hibernation and while the AC adaptor is connected to the computer.
I M P ORTANT
In the case of a hot swap, finish replacing the battery within a minute. Depending on the built-in bridge battery
l
status, the computer may enter hibernation within 1 minute. Replace the battery pack as soon as possible.
4. While sliding the battery latch (J) with the detach lock (I), lift the battery pack (K).
5. Insert the battery pack (L) until it touch to the secured position.
Be careful about the battery pack direction.
In case that the projection does not match the dents of the computer, detach the battery pack once, then slide it while pushing it lightly to avoid
floating.
6. Slide the detach lock (M) to the locked position.