Basic Battery Information:
The battery that is grounded is considered the first battery. The battery that connects to the solenoid is considered the last battery. Each battery is 12 volts.
Running batteries in series means you connect the positive of the first battery to the negative of the second battery. Each time you connect a battery in a series, your voltage increases by 12 volts.
Running batteries in parallel means you connect the positive of the first battery to the positive of the second battery. This increases the amperage but not the voltage.