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AJC® Batteries

Series and Parallel Battery Connections

When powering a device, sometimes, you just need more voltage or more capacity, or both. Other times, a larger battery just won’t fit in the battery space provided. You don’t always need to purchase a behemoth battery, you can connect two or more batteries to get the power you need. Connecting batteries may better allow you to work within the space limitations and still get the battery power needed.

Let’s start with a few battery definitions so we are speaking the same language.

  • AMP Hour is a unit of measure for a battery’s electrical storage capacity. The standard rating is an amp rating taken for 20 Hours but some manufacturers use different standards.
  • Voltage represents the pressure of electricity. Some applications require more “pressure,” meaning higher voltage.
  • Battery Bank is the system created by connecting two or more batteries, regardless of the method.

There are two ways to connect multiple batteries: series connection or parallel connection. Most battery chemistries handle either type of connection, but sealed lead acid batteries have been the battery of choice for creating high voltage or high capacity battery banks for many years.

Series Connections

Two or more batteries connected in a series increase the voltage of the battery system, but the amperage, or capacity stays the same. Two 6V batteries that have a rating of 10 Amp hours connected in a series will produce 12 volts but still only 10 Amp hours.

To connect batteries in series, you connect the positive terminal of one battery to the negative terminal of another until the desired voltage is achieved. Don’t cross the remaining open positive and open negative with each other. It will short circuit the batteries and may cause damage or injury. Use another set of cables to connect the open positive and open negative terminals to the device you are powering.

Parallel Connections

Batteries joined in parallel will increase amp-hour capacity but the voltage will remain the same. Connecting batteries in parallel will increase the amount of time you can power your equipment, but will not allow you to power anything above the standard voltage output.

To connect batteries in parallel, the positive terminals are connected together via a cable and the negative terminals are connected together with another cable until you reach the desired capacity.

Serial and Parallel Connections

You can also increase both the voltage and the capacity by connected at least four batteries both serially and in parallel. This gives you a higher voltage battery bank that also has a longer run time for your application. It is common practice for applications such as electric vehicles and large UPS systems. There are different ways to connect the batteries to achieve the increased voltage and capacity.

With four batteries, you can create two series that are connected via a parallel connection, or two parallel banks connected by one serial connection. Either way results in the same voltage and capacity gains.

Charging Battery Banks

Batteries connected in a series have no effect on the Amp hour capacity of the battery bank, so when charging, focus on voltage. The charger needs to satisfy the charging requirements of the batteries in the series. For example, two six volt batteries, serially connected to create a 12 volt battery bank need to be charged with a 12 volt battery charger to meet the needs of both 6 volt batteries.

Once you have determined the correct charger, connect the positive output of the charger to the positive terminal of the first battery. Then connect the negative output of the charger to the negative terminal of the last battery in the series. It will take the same amount of time to charge the series as it would take to charge one battery.

When charging batteries that are configured in a parallel combination, you need to take into account the increased Amp hour capacity that will result from the new configuration. This is because when you are charging in parallel, you are not recharging the voltage of the system, but rather the amp-hour capacity. Multiply the time it takes to charge one battery by the number of batteries to arrive at the amount of time it will take to charge the battery bank.

One method of charging batteries connected in parallel, is to connect the positive output of the charger to the positive terminal of the first battery. Connect that positive terminal to the positive terminal of the second battery. Continue until all batteries are connected. Now do the same with the negative output of the charger, and the negative terminals of the batteries.

If you have a battery bank that is connected both serially and in parallel, it gets a little more complicated. Connect the positive output of the charger to the positive terminal of the first battery, and that gets connected to the positive terminal of the second battery. Connect the negative output of the charger to the negative terminal of the third battery, then connect that negative terminal to the negative terminal of the fourth battery. Last, connect the negative terminals of the first and second batteries to the positive terminals of the third and fourth batteries, respectively. 

Some Battery Bank Best Practices

To get the most reliable energy from your battery bank, always use the same type of battery, ideally from the same manufacturer (we recommend AJC batteries). Use batteries that are all the same voltage and capacity, and when the need arises, replace all batteries in the bank at the same time. A weak battery will discharge first, reducing the time between charges, and the weak battery will complete its charge first, meaning it will be more inclined to overcharge while the other batteries are charging. The battery bank is only as strong as the weakest battery you use in the application, and weak batteries will shorten the life of all of the other batteries in the bank.