See Figure 1
Whenever a vehicle is jump started, precautions must be followed in order to prevent the possibility of personal injury. Remember that batteries contain a small amount of explosive hydrogen gas which is a by-product of battery charging. Sparks should always be avoided when working around batteries, especially when attaching jumper cables. To minimize the possibility of accidental sparks, follow the procedure carefully.
NEVER hook the batteries up in a series circuit or the entire electrical system will go up in smoke, including the starter!
The batteries are connected in a parallel circuit (positive terminal to positive terminal, negative terminal to engine ground). Hooking the batteries up in a parallel circuit increases battery cranking power without increasing total battery voltage output. Output remains at 12 volts. On the other hand, hooking two 12 volt batteries up in a series circuit (positive terminal to negative terminal, and other positive terminal to engine ground) increases total battery output to 24 volts (12 volts plus 12 volts).