substitutebattery
nounUK /ˈsʌbstɪtjuːt ˈbætəri/
US /ˈsʌbstɪtuːt ˈbætəri/
A substitute battery is a spare battery you use when the first one runs out.
I packed a substitute battery for my camera so I could keep taking pictures all day.
OTHER MEANINGS
2. A battery used in place of the original one when it is unavailable or broken.
The mechanic fitted a substitute battery while the original was being tested.
3. A person or thing that fills in for another in a role or job.
She acted as a substitute battery for the team's regular coach during the tournament.
4. A temporary power source used in an emergency.
During the blackout, the hospital switched to a substitute battery to keep the monitors running.