Series VS Parallel Battery Connections...Which Is Which?

Series vs. Parallel Battery Connections
You have all heard the rules about connecting more than one battery together to either get more reserve power or more more voltage. The confusion lies in knowing which is which and understanding the results. Lets start by understanding how the battery cable connections are different between to two.

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. This is common in RV's and trolling motors on small fishing boats where you need lots of reserve capacity to run small electrical loads like interior lights and accessories. Typically you will see two deep cycle batteries in these applications. Deep cycle batteries are made to be discharged / recharged slowly over and over and are designed to deliver a steady output over multiple hours.

Parallel battery connections can also be used in automotive applications. I use two Optima 6-volt automotive batteries along with my 6-volt alternator in applications like antique fire trucks and ambulances That combination  provides plenty of reserve battery power to run the flashing lights and siren during a parade while still providing plenty of reserve capacity for engine cranking.

Each Optima 6-Volt Battery is a thousand cranking amps! So connecting two in parallel along with one gauge battery cables results in 2000 cranking amps! That will start about anything!

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, which is usually the starter solenoid or starter button on the starter. The connections on the battery end of the starting system are the same as they were originally, there is just a little more help in between.

You want to do the same with the negative battery cable, all of the negative posts are connected together. And as you learned earlier if you connect the ground end of your battery cable to the starter mounting bolt or as close to the starter as you can get..(not to the painted frame or engine block) your vehicle will start much easier, because you have then created a direct path between the battery and the starter.

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 your 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 because now you have twice the current to deliver. Both batteries should be the same size and have the same rating. Adding a second battery will double your reserve capacity or the length of time you have before all of your battery current is gone.

Batteries Connected in Series – when two batteries are connected in "series" it is done most often to increase voltage and efficiency, (just like when the car manufacturers went from 6-volt to 12-volt). 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 the ground end to a starter mounting bolt or as close to the starter as you can get it.

Your goal is to create a direct path between the battery and the starter. You want both battery cables to be one gauge or bigger because now you have twice the current to deliver.  Connecting two 6-volt batteries in series will yield 12-volts. The negative side will work the same as the positive. And connecting two 12-volt batteries will result in 24 volts. Some antique tractors had 24 volt electrical systems, many of the early John Deere tractors for example.

Batteries connected in series should be the same size and have the same rating. The total reserve capacity of the batteries will remain the same even though there is a second battery, because each battery is independent of the other but working towards the same goal. 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 volts and require six (6-volt) batteries connected in series to obtain the 36 volts.

As we learned elsewhere in this blog when converting from 6-volt to 12-volts, when you double the voltage, the amperage load goes to half, so in the case of the 36 volt motor in a golf cart, the higher voltage makes the electric motor more efficient and will require less amperage from the battery than it the electric motor was 12 volts. That explains why you sometimes see a 48 volt golf cart.  Those carts (Club Car) is a popular brand will climb hills better and have more power. Many have been converted to off road applications.

The difference in those 48 volt golf carts is how they get to 48 volts. That can happen one or two ways, either (4) 12-volt batteries or (6) 8-volt batteries.

(Now you know where those 8 volt batteries come from that I tell you NOT to put in your antique to try to get it to start better)

You know from reading this blog that the (6) eight volt batteries will have more reserve capacity (more power) than the (4) 12-volt batteries, because the 8-volt batteries contain more lead (more surface area inside the battery so more output). It makes the golf cart cost more initially... and also when it comes time to replace the batteries five years down the road. But think of the fun you had in between!

That is also true in 6-volt vs 12 volt applications, two 6-volt batteries will have more reserve capacity than one 12-volt. Again two 6-volt batteries have twice the surface area so twice the output). That is why you will see off road construction equipment with two 6-volt batteries connected in series instead of one 12-volt battery. The extra cranking reserve will make it easier to start the diesel engines.

As for that 48 volt golf cart withe the 8-volt batteries...that extra efficiency will insure the cart can be driven all 18 holes without a dead battery. That extra reserve may be necessary if the players get extra thirsty, and require more trips to the club house to retrieve adult beverages.

So remember parallel battery connections are the same as you are used to...positive to positive and negative to negative and the resulting voltage will NOT change. The benefit is that you get extra reserve because now you have two batteries to drain instead of one. In applications with a heavy electrical load like an antique firetruck this is almost a must.  To remember parallel... think two parallel lines on the highway, they are both the same at the end as they are at the beginning.

Think of the series battery connections, as like baseball. Every battery connection builds voltage just like every player in baseball is scoring points. The more players that cross home plate the higher the score. In Series battery connections the more battery posts the cables connect to... the higher the voltage. In the example of the 36 volt golf carts described above...the battery cables (both the positive and the negative) will each cross 6 terminals to get the resulting 36 volts.

So...what if forget your lesson here and connect the 6-volt batteries in your golf cart in parallel instead of series? It is doubtful your 36 volt golf cart would even move... because the electric motor would only have 1/6 of the current it needs to operate. Even if it did operate... you wouldn't get very far.

The only reason I bring that up is that every spring I have one or two customers call after they put new batteries in their golf cart and it "is still dead."

Yep...you guessed it... they connected their golf cart batteries in parallel instead of series, They were ready to declare their new batteries defective! Then they realized that the chance of all 6 batteries being defective is pretty slim. It was a simple fix in both cases, they both had made the same mistake, and got the same results.

Here is a simple chart to remind you how to connect battery cables for Series AND Parallel applications. Now you know which is which and why...your welcome!