To produce a 12 volt output, two 6 volt deep cycle batteries must be wired in series (positive to negative). Each 6 volt battery pair operates as a single 12 volt battery. You can then connect each 6 volt pair in parallel with another pair to create a larger 12, 24 or 48 volt battery bank. This is called a series parallel configuration.
You're probably thinking why not just use 12 volt batteries to begin with? That's a good question. I believe that's where we're heading due to the growing popularity of solar, wind and lithium battery technologies. While we migrate in that direction, 6 volt batteries remain a popular option for off-grid energy storage. Here's why.
Why use 6 volt batteries?
Six volt deep cycle batteries are sold in many places and have powered equipment like electric golf carts for many years. They are true deep cycle batteries that can be combined together to power 12, 24 and 48 volt systems. That's the short answer.
Here's the long answer. First, some context.
Batteries are categorized as either starter, deep cycle, or marine batteries based on their intended use:
- Starter batteries are used to start your vehicle. They are designed to provide a lot of power for a short period of time before being recharged by your car's alternator. Rated in Cold Cranking Amps (CCA) their primary use is for starting your engine.
- Deep cycle batteries are used mostly in off-grid power systems. Boats, RVs and off-grid homes draw their power from deep cycle batteries throughout the day and evening. The batteries are discharged to a deeper level before being recharged by solar panels, wind turbines or generators. Deep cycle batteries are rated in Amp Hours (Ah) and are designed to withstand deeper discharging and recharging. You might see six volt batteries labeled as golf-cart batteries. Twelve and twenty four volt deep cycle batteries may be labeled as solar batteries. If you see a CCA rating in addition to a Ah rating, then it's a marine or dual-purpose battery.
- Marine (or dual-purpose) batteries are a hybrid between a starter and deep cycle battery. They are used to start your engine while also accommodating many deep discharge and recharge cycles. They are rated in both Amp Hours (Ah) and Cold Cranking Amps (CCA). Marine or dual-purpose batteries are often lumped into the same category as deep cycle batteries which causes much debate and long threads in online forums. Many do not consider them true deep cycle batteries while some do. If your RV came with a single battery, then chances are it's one of these.
So why do many RV owners prefer six volt batteries? Six volt batteries are always deep cycle batteries. There is no confusing a six volt battery for a hybrid or marine grade battery. Six volt flooded lead acid batteries also have only three cells to maintain compared the six cells found in a 12 volt battery.
Which is better, 12 volt or 6 volt deep cycle batteries? That's entirely up to you. I hope I've given you enough information here to make that decision. Personally, I don't think it matters. As long as your using true deep cycle 12 or 6 volt batteries, the outcome should be the same. If you disagree, please leave a comment and let us know why.
Connecting Six Volt Batteries in Series / Parallel
Start by connecting two 6 volt batteries in series to make the equivalent of a 12 volt battery.
Connecting 6 Volt Batteries in Series
HOW: Connect positive terminal on battery A1 to negative terminal on battery A2. Repeat this on batteries B1 and B2.
RESULT: The outcome of connecting batteries in series is the that voltage output is added together (12 volts total). The amp hours remain the same.
Example: Two 6 volt batteries rated at 100 amp hours and connected in series will produce 12 volts for 100 amp hours.
Two six volt batteries wired in series are needed to create a 12 volt battery bank. But what if you want to increase that capacity? Adding more six volt battery pairs is the solution.
Connecting 6 Volt Battery Pairs in Parallel
HOW: Connect positive output terminal on battery pair A to positive on battery pair B. Connect negative terminal on battery pair A to negative terminal on battery pair B.
RESULT: The outcome of connecting each pair in series is that the output voltage remains the same (12 volts) while the amp hours are added to the overall capacity.
Example 1: Two pairs (each producing 12 volts for 100 amp hours) connected in parallel to another similar pair will produce 12 volts for 200 amp hours
Example 2: Create a 450 amp hour 12 volt battery bank with four 6 volt batteries (demonstrated in the video using four 6 volt 225 amp hours batteries)
- Wire the first pair of batteries (A1 and A2) in series to output 12 volts at 225 amp hours
- Wire the second pair of batteries (B1 and B2) in series to output an additional 12 volts at 225 amp hours
- Wire the two pairs together in parallel for an overall output of 12 volts at 450 amp hours
WANT MORE POWER? To increase your amp hour capacity even more, simply add more pairs of 6 volt batteries. For example, adding an additional six volt battery pair will add an additional 225 amp hours for a total of 700 amp hours at 12 volts.
You'll Need a Battery Monitoring System
A battery monitoring system is the only way to accurately track your battery usage and determine how much reserve capacity is available. Unfortunately, your RV probably didn't come with a good monitor. Most RVs come with a level meter with few lights that represent the current battery voltage. This is not a true indication of how much reserve capacity you have available.
The state of charge (SoC) of your battery bank is like a gas gauge for your battery bank. A good battery monitor will tell you how much power you're taking out and how much is going back in. You'll get an exact measure of your available capacity as a percentage of full.
Without this information, you may be damaging your expensive new batteries by drawing them down too low.
It should go without saying that you should follow proper electrical safety practices while working with electricity. For starters, be sure to disconnect all electrical power from your RV before disconnecting or connecting batteries. If you are inexperienced or unsure of your abilities, you should have the work done by a professional.
You should also install proper overcurrent protection (terminal fuses, circuit breakers) and battery disconnect switches on load-serving positive connections to protect you and your equipment. This is not the focus of this article, but I felt compelled to mention it.
Here's a list of AGM batteries, battery cables, switches, circuit breakers and and related components that I recommend.
Lead Acid Batteries are Heavy
Large lead acid deep cycle batteries (LA, SLA, AGM) can be very heavy. They contain a large amount of lead for storing all of that energy. Each 225 Ah battery I installed weighs 70 pounds. All together, my battery bank weighs roughly 280 pounds. To support that weight in my storage bin, I built and installed a steel support bracket (see photo below) that bolts directly to the frame of the RV. So keep the weight in mind when choosing the location for your battery bank.
I Have Four of These Six Volt AGM Batteries Connected In Series/Parallel to Get 450 Amp Hours of Capacity
How Much Do Lithium Batteries Weight?
Lithium batteries weigh about 1/3rd the weight of a lead acid battery with the same Amp Hour capacity. That and other factors are what make lithium batteries very attractive for RV use. Eventually I believe all of our RVs with large battery banks will be lithium powered, but certainly not yet. I'll elaborate on this later in another article.
Cable Thickness and Length
Keep in mind that when wiring batteries together, you are essentially creating a very large battery. You want the energy to flow freely between each battery across the cable connections with little to no resistance. This is why I used very thick cable for this. In the video I used #1/0 gauge cable. You can find pre-assemble cable in short lengths like like this one on Amazon.
It is also a good idea to keep your interconnecting cables the same length. Use the shortest cables possible. Doing so creates an even and efficient flow of energy across all batteries which ultimately aids in charging and overall battery efficiency.
More Information You May Need
- Why You Need a Good RV Battery Monitor
- Use a Power Inverter to Generate AC Power
- How Much Battery Capacity Do You Need?