Eight V is pretty low, I'd do a battery load test. A fully charged lead acid battery is about 12.66 so 12.5 isn't bad for state of charge.

If the battery tests good, I'd follow the inj circuit and maybe apply a jumper to battery power then crank. MAKE SURE you are applying power to the positive side of the injector, it you apply positive power to the ground side of the injector you will burn up the engine computer.

Measuring voltage tells you battery state of charge ( how _full_ the gas tank is )

A load test tells you capacity of the battery ( how _big_ the gas tank is. )