Did you know that you can have two 12 volt batteries make 24 volts? Or how about using those same two 12 volt batteries to make 12 volts? It’s true. Depending on your needs, you can easily connect two 12 volts batteries to generate the necessary voltage or increase your battery capacity.
Connecting batteries is a pretty simple process. If you ever put multiple batteries in a radio or a flashlight, guess what? You changed the battery voltage. By putting those batteries from end to end, you connected the batteries in series, increasing the voltage output.
That may sound great, but how do you do this with a 12 volt battery? You can’t just put 12 volt batteries next to each other and expect them to work like a small radio or flashlight battery.
Although the theory is the same for a small flashlight or a large 12v battery, the application and wiring process differs. Since you can’t just connect a 12 volt battery as easily as a flashlight battery, we need to explore working methods that will get you the same results. This post will review how you can make two 12 volt batteries, either 12 volts or 24 volts, including:
- Battery Capacity
- Battery Voltage
- 12 volt system
- 24 volt system
- Wiring in parallel
- Wiring in series
Whether you have a lithium ion battery or a lead acid battery, the following methods apply to get more voltage or battery capacity. These are also likely deep-cycle batteries. Deep cycle batteries are used in boats or when used to get power from an inverter. These are different than a car battery which gives just a quick burst to start the car. Those batteries are not meant to drain down all the way like a deep cycle battery.
Table of Contents
Two 12 volt batteries to make 24 volts
If you have a 24 volt system, you can wire two 12 volt batteries together. The voltage is increased by choosing the correct wiring method, in this case, a series connection. When you connect in series, you connect batteries to get a higher voltage output. In this case, you add the two 12 volts from each battery to end up with a 24 volt battery. In our example above about a flashlight battery, you also double those voltages. Since a flashlight battery is usually 1.5 volts, your net voltage output is 3 volts.
Series Connection
When you connect in series, which can be done with multiple batteries (battery bank), you add up all the voltages of each battery. Doing this will give you a higher voltage than the single battery. For example, if you connect three 12v deep cycle batteries in series, your end voltage would be 36 volts (12v + 12v + 12v = 36v).
This is very similar to electric car motors that operate on higher voltage, such as 400 to 800 volts. The battery bank used is basically smaller batteries wired together in series to increase the voltage.
Wire
Wiring batteries can be a simple process. You will first need a short (about 2 foot) battery cable, sometimes called a jumper. This battery jumper should be the same or thicker as the wires that were already coming to the battery.
A series connection is when you take a wire battery cable and connect the positive battery terminal to the other battery’s negative terminal. You now have a 24 volt system. If you took a multimeter and placed the leads at the negative and positive posts of the open battery posts, you should get a reading of just over 24 volts (assuming they are fully charged).
You then take the open negative battery terminal from one battery and the open positive battery terminal from the other to connect your equipment. Your 24 volt system should function as usual now.
Amps Capacity
We have mostly discussed just voltage and how to increase the battery voltage by wiring in series. And the reason is that when you wire in series, all you do is increase the voltage. There is no increase in the battery amp capacity or amp hours. This means that there is no increase in battery capacity and run time. Whatever your amperage hour rating is for a single battery, it will be the same when connected in series of multiple batteries.
6 Volt Battery
As a side note, it is also common to take two 6 volt batteries, and wire them in series to end up with a 12 volt system.
There are some advantages to having 6v battery setup. For more info, read my post on 6v vs 12v batteries.
Two 12 volt batteries to make 12 volts
We reviewed quite a bit on connecting multiple batteries to get a higher voltage. But what if you want to connect multiple batteries and keep the same voltage? Why would you even want to do that? The reason to connect multiple batteries to have the same voltage is to increase the reserve capacity of your 12 volt system.
This 12-volt multiple-battery setup is very common for RV batteries or solar panels. When you add a second battery to your RV or solar power, you will double the amp hours or battery capacity of power. You shouldn’t increase the voltage since RVs and solar panels operate on 12 volts. However, you can add as many batteries as you want to increase battery life as long as you keep it a 12 volt system.
I have multiple batteries connected to my RV and solar panel system. This allows me a longer time before I need a charge. This is especially helpful if you are running an inverter or are boondocking, where the longer the battery life, the better.
Parallel Connection
Above, we reviewed the series connection that increases the voltage but not the battery capacity. Now let’s go over the parallel connection, which increases battery capacity and not voltage.
Wire
When wiring in parallel, you will need two battery cables, sometimes called a jumper. Each battery cable jumper should be at least two feet long.
This battery cable should be the same or thicker as the wires that were already coming to the battery.
Step 1: Connect Negative Terminals
Take the first jump cable and connect one end to the negative battery terminal and the other end to the other negative terminal on the other battery. So that’s negative to negative.
Step 2: Connect Positive Terminals
Next, take the second jumper cable and connect the positive battery terminal and the other end to the other positive terminal on the other battery. So that’s positive to positive.
Step 3: Connect the Equipment
At this point, each battery terminal should be connected to a wire (negative to negative and positive to positive). The final step is to connect the equipment battery cables to this battery bank. The key is to wire this so the battery acts more like a battery bank and less like two individual batteries.
The best way to make your final connection is to connect your equipment’s negative cable to one of the battery’s negative terminals. And then, connect the positive equipment to the OTHER battery’s positive terminal. Got that? So that’s the negative cable to one battery and the positive cable to the OTHER battery.
Doing this will discharge the battery at the same time, giving you equal distribution of power between the connected batteries. If you did it a different way, for example, connected the equipment negative and opposite cables to the same battery, it would unequally take power from that battery and then make its way to the other battery. This also affects the charge and causes unequal power.
I have much more information on parallel wiring in my post on adding a second 12v battery to an RV.
Final Thoughts
The voltage of a battery can be increased or kept the same by connecting multiple batteries in different configurations. For increasing voltage, connecting batteries in series is the key. This method is commonly used to create 24-volt systems in equipment and other applications requiring higher voltage. On the other hand, batteries are connected in parallel to increase battery capacity without changing the voltage. This is often done in RVs and solar panel setups to increase the reserve capacity and extend battery life. Understanding these wiring methods allows users to tailor their battery setups according to their specific needs and applications.
Tony is an avid camper and RV traveler. He fell in love with camping on his first RV trip with his wife over 25 years ago. Tony loves sharing lessons learned and tips about RV maintenance and safe traveling.
