The following table describes the battery specifications.

Battery type Lithium ion
Nominal voltage/capacity 21.6 V / 2.15 Ah
Max charge voltage 24.9 V
Recommended charge current 2 A
Max charge / discharge current 3.0 A / 2.0 A
Energy 27.6 Wh
Charging duration ~ 5h00
Autonomy ~ 60 min (active use) ~ 90 min (Normal use)

NAO can be used while it is plugged in with the adapter. But the battery will charge only if NAO is powered off and plugged to its adapter.