Series vs. Parallel Battery Connections

Batteries in Parallel. When you connect two batteries together in parallel the output voltage remains the same as the original voltage of the electrical system. The benefit is that the reserve capacity is almost doubled.


I typically do this in antique fire truck applications using my 6-volt alternator and two 6-volt Optima Batteries. That combination provides plenty of reserve battery power to run the flashing lights and siren while still providing plenty of reserve capacity for engine cranking.


To connect two batteries in parallel… connect the positive terminals of both batteries together then connect to the original positive connection of the charging system. You want to do the same with the negative battery cable. When you go to the auto parts store to have the battery cables made, be sure take the measurements of the distance between the battery posts. Tell them you are connecting two batteries in parallel and you need the cables to look like the cables they use on golf carts. One gauge cable works best for both the positive and negative cables. Both batteries should be the same size and have the same rating.


Batteries Connected in Series – when two batteries are connected in "series" it is done most often to increase voltage. To build a "series" battery circuit, connect the positive of the first battery to the negative of the second battery, then on to the 12-volt positive connection of the charging system as it was originally. Connect the negative post of the first battery to the positive post of the second battery then connect to ground as before. Connecting two 6-volt batteries in series will yield 12-volts.


Batteries should be the same size and have the same rating. The total reserve capacity of the batteries will remain the same even though the battery voltage increases. Electric golf carts are a good example of an application that use batteries wired in series to increase voltage. Electric golf carts, are powered by drive motors that are typically 36 volt and require six (6-volt) batteries connected in series.


To print a copy of the Series-Parallel picture diagram click here.

Fifth Avenue Facts

Randy designed and built the first 6-volt alternator in 1987. Up to that point there was no such thing as a 6-volt alternator, only modern 12-volt alternators were available.