Battery:

The battery consists of lead peroxide plates (+) and pure spongy lead 
plates (-). 

These plates are separated by insulators and arranged 
in cells within the battery case. The case is then filled with sulfuric 
acid and water (electrolyte). The chemical reaction between the
plates and electrolyte causes the negative plates in the cell to 
have an excess of electrons. When a circuit is completed the electrons
flow to the positive plates.

Each fully charged cell should produce approximately 2.1 volts. Six cells connected in
series would produce 12.6 volts.