Not a car battery, it's an ignition coil from a Ford model T (missing the wooden cover panel). Every model T has four of these - one per spark plug. They take the low-voltage electricity generated by the flywheel magneto and convert it into the high voltage needed to jump the gap in the spark plug and achieve combustion. Inside that tar casing, there are two wire coils: a primary coil, which carries the low-voltage current, and a secondary coil which generates the high-voltage current. Unlike a modern ignition coil, which generates one spark per combustion stroke, the model T's vibrator coils create a series of sparks - the springy brass plate vibrates, breaking and making contact multiple times in a row. This helps ensure that ignition takes place.

The four coils on a model T sit in the coil box, located on the dashboard (for the 1926-27 model years, it was moved to a mount above the engine). In early cars, the coil box was made of wood; later, it was switched to steel.

The coils were originally made by both Ford and the K-W Ignition Company, but in 1923 Ford assumed production of all coils. That year they began stamping the Ford logo on the top of the vibrator assembly, which means yours was made after that date.