A battery was a device in which energy was stored for later usage.
Batteries were used as an emergency backup to control gravitation and oxygen levels on starships. (TOS: "The Cage")
Gravity control was switched to batteries when the USS Enterprise collided with the galactic barrier in 2265. (TOS: "Where No Man Has Gone Before")
When three lithium crystals burned out on the Enterprise in 2266, the transporters had to be supplemented with battery power. (TOS: "Mudd's Women")
After the Galileo made an emergency landing on Taurus II in 2267, Spock and Montgomery Scott used battery power to electrify the exterior of the hull to drive away the attacking natives. (TOS: "The Galileo Seven")
The Excalbians caused a power failure on the Enterprise in 2269, forcing the ship to operate on emergency battery power only. (TOS: "The Savage Curtain")
Later that year, the Enterprise was pulled into an alternate universe. Captain James Kirk called for emergency battery power to restore the ship's failing life support. (TAS: "The Magicks of Megas-Tu")
The USS Huron also used emergency battery power to operate life support systems. (TAS: "The Pirates of Orion")
When Ferengi attacked the USS Enterprise-D in 2364, battery power was reduced by 30 percent. (TNG: "The Last Outpost")
A duckblind was powered by a 4.2 gigawatt reactor and could be run with battery power for three hours at best. (TNG: "Who Watches The Watchers")
A battery was also used to start old Earth cars, such as a 1936 Ford truck. (VOY: "The 37's")
The Doctor complained "I'm a Doctor, not a battery" when Tom Paris wanted to use his mobile emitter as a power source. (VOY: "Gravity")