The phrase "extra battery" refers to an additional power source that you can use for your electronic devices when the original battery runs out of charge. Full definition
Once you fire up this cute cat, you're good to go for 16 hours of extra battery life. (travelandleisure.com)
I would recommend keeping plenty of extra batteries for your flash lights, since they are small and cheap. (humblemechanic.com)
This is a unique looking true Bluetooth headset that comes with extra batteries so you don't have to wait once the used batteries get drained off. (gogi.in)