How to Wire Batteries in Series and Parallel at the Same Time

In energy systems like large off-grid solar arrays or high-performance RVs, you need both higher voltage and greater capacity. This is achieved with a wiring method called a series-parallel connection.

This powerful configuration allows you to build a custom battery bank that precisely matches your system's demands. While the name sounds complex, the process is logical and systematic. This guide will walk you through exactly how to wire batteries in series and parallel at the same time, using clear, step-by-step examples for 4, 6, and 8 battery series-parallel setups.

 

What is batteries series-parallel connnection

Wiring batteries in series and parallel is the combination used to increased both voltage and capacity the battery bank. With this arrangement, you not only increase the voltage but also double the capacity, making it suitable for systems that require both high power output and longer runtime.

For example, four 12V 100Ah batteries can be wired in a series-parallel configuration. First, two pairs of batteries are connected in series, creating two sets of 24V 100Ah. Then, those two sets are connected in parallel, resulting in a final bank of 24V 200Ah. 

How to wire batteries in series and parallel (4 batteries example)

Next, let’s use four batteries as an example to demonstrate how to wire them in a series-parallel configuration. This step-by-step guide will show you how to group the batteries, connect them, and includes a circuit diagram for illustration.

Safety precautions for series and parallel battery connections

1. Wear personal protective equipment (PPE)
   Always wear insulated gloves and safety goggles to protect yourself from electric shocks, acid spills, or sparks. Avoid wearing metal jewelry, watches, or rings, as they can accidentally touch battery terminals and cause a short circuit.
2. Use proper tools and work area
   Use tools with insulated handles to prevent accidental short circuits. Make sure the work area is dry, clean, and well-ventilated. This reduces the risk of electric shock and prevents the accumulation of flammable gases, which is especially important when working with lead-acid batteries.
3. Ensure battery compatibility
   Only connect batteries of the same age, type, voltage, and capacity. Mixing different batteries can lead to imbalance, reduced performance, or even damage. Also, ensure all batteries are at a similar state of charge before connecting them.
4. Use proper battery cable
   The wire between batteries in series can be smaller, carrying only a single string’s current. Wires connecting series strings in parallel must be larger to handle the total current. Always size wires for the maximum expected current to prevent overheating, voltage drop, or fire hazards.
5. Install overcurrent protection
   Use battery fuses for each battery string and main parallel connections to protect the battery bank from overcurrent. Ensure that the protection devices are rated appropriately for the voltage and current of your battery bank.
6. Follow proper wiring sequence
   Connect batteries in series first, then combine the series groups in parallel. Double-check all connections before powering the system to avoid reverse polarity or miswiring, which could cause damage or accidents.

 

Step1. Group the batteries

Using four 12V 100Ah batteries as an example, arrange them in pairs with equal spacing between each unit and between the two strings. This layout ensures that all connecting cables remain the same length, which helps maintain balanced current flow and stable system performance. When grouped this way, the four batteries form two battery banks, each consisting of two batteries.

Step2. Connect batteries in series

For each of your paired battery groups, take one jumper cable and connect the positive (+) terminal of the first battery to the negative (-) terminal of the second battery.
The main terminals for each string are now the open negative (-) terminal of the first battery and the open positive (+) terminal of the second battery. You have now created two independent 24V, 100Ah battery strings.

 

Step3. Install overcurrent protection

As mentioned earlier, each parallel string should be individually fused to prevent a faulty string from dangerously drawing current from the healthy one.
Install battery fuses on the positive leads of each string. These fuses should be sized based on the maximum discharge current of the batteries in the string (check the manufacturer's datasheet) and the ampacity of your wire.

Step4. Connect batteries in parallel

With your two 24V strings built and fused, the final step is to connect them in parallel to double the capacity. Use cables of identical length for these connections.

• Connect the positives: Use a jumper cable to connect the fused positive terminal of String1 to the fused positive terminal of String2. You will attach your system's main positive cable to this connection point (often a battery busbar).
• Conect the negatives: Use another jumper cable to connect the main negative terminal of String1 to the main negative terminal of String2. Your system's main negative cable attaches here. #Now, you have successfully created a single 24 Volt, 200 Amp-hour battery bank.

 

How to connect 6 batteries in series and parallel

There are two common ways to wire six batteries in a series-parallel configuration. The method you choose depends on your target voltage. We'll assume you are using 12V, 100Ah batteries.

Option1. Creating a 24V 300Ah bank (2s3p)

1. Group: Arrange the six batteries into three pairs.
2. Series Connection: Create three separate 24V, 100Ah series strings by connecting the positive of the first battery to the negative of the second in each pair.
3. Parallel Connection: Fuse each string's positive lead. Then, connect all three fused positive terminals together and all three negative terminals together. This results in a 24V, 300Ah bank.

Option 2. Creating a 36V 200Ah bank (3s2p)

1. Group: Arrange the six batteries into two groups of three.
2. Series Connection: Create two separate 36V, 100Ah series strings. In each group, connect Battery 1 (+) to Battery 2 (-), and Battery 2 (+) to Battery 3 (-).
3. Parallel Connection: Fuse each 36V string's positive lead. Then, connect the two fused positive terminals together and the two main negative terminals together. This results in a 36V, 200Ah bank.

 

How to connect 8 batteries in series and parallel

With eight batteries, you have even more flexibility to hit specific voltage and capacity targets, common in large solar installations.

Option1. Creating a 48V 200Ah bank (4s2p)

1. Group: Arrange the eight batteries into two groups of four.
2. Series Connection: Create two independent 48V, 100Ah series strings.
• String1 (Batteries 1-4): Wire these four batteries in a single series chain (positive-to-negative). The output is 48V.
• String2 (Batteries 5-8): Do the same for the second group of four batteries.

3. Parallel Connection: Fuse the positive lead of String A and the positive lead of String B. Connect the two fused positive terminals together to a common bus bar. Connect the two main negative terminals together on a separate bus bar.

You now have a robust 48V, 200Ah battery bank, ideal for powering high-efficiency inverters.

 

Option2. Creating a 24V 400Ah bank (2s4p)

1. Group: Arrange the eight batteries into four groups of two.
2. Series Connection: In each group, connect two batteries in series (positive-to-negative) to form a 24V, 100Ah pair. This gives you four identical 24V, 100Ah pairs.
3. Parallel Connection: Fuse the positive leads of all four pairs, then connect them together at a common positive bus bar. Do the same for the negative leads on a negative bus bar.
   

 

FAQs about batteries in series and parallel

How do I correctly charge a battery bank that is connected in series and parallel?

Choose a charger with an output voltage equal to one series string (e.g., 48V for 4×12V in series) and a charging current capacity that matches the total of all parallel strings. Ensure all batteries are identical, balanced, and monitored for safe charging.

 

Can I mix old and new batteries in the same series or parallel bank?

It is highly discouraged. Mixing batteries of different ages, capacities (Ah), or even brands can lead to severe imbalances. The newer, stronger batteries will overcharge the older, weaker ones, while the weaker ones will drain the stronger ones. This drastically reduces the lifespan of the entire bank and can pose a safety risk. For best performance and longevity, always use identical batteries purchased at the same time.

I connected my batteries in parallel, but one seems to be draining faster. Why?

This is a common issue often caused by unequal cable lengths or poor connections. For a parallel setup to be balanced, the electrical resistance from the main positive/negative terminals to each battery must be identical. Best practice is to use cables of the exact same length and gauge for all connections. Also, connect your load's positive cable to the first battery in the bank and the negative cable to the last battery. This "diagonal" connection method helps balance the charge and discharge across all batteries.

 

For a LiFePO4 (lithium) battery bank, does the BMS manage balancing in series?

Yes, but its capability is limited. A quality Battery Management System (BMS) actively balances the cells within a single battery pack and provides protection. When connecting multiple lithium batteries in series, each battery's internal BMS works independently. If one battery's cell group reaches high-voltage cutoff during charging, its BMS will shut it down, stopping the charge for the entire series string, even if other batteries aren't full.

 

What happens if one battery fails in a series vs. a parallel setup?

The outcome is very different. In a series circuit, if one battery fails (creating an open circuit), the entire string stops working, like a single broken bulb in old Christmas lights. In a parallel circuit, if one battery fails (but doesn't short), the remaining batteries will continue to supply power, although at a reduced total capacity. However, a failing battery can also drain the healthy ones, so it should be identified and removed promptly.

 

Is it better to use two 6V batteries in series or two 12V batteries in parallel for my RV?

This is a classic debate in the RV community, with a strong consensus favoring two 6V batteries in series. 6V deep-cycle "golf cart" batteries are specifically designed for repeated, deep discharges. They typically have thicker plates and a more robust construction than standard 12V marine/RV batteries, leading to a much longer lifespan under heavy use. While the wiring is slightly more complex, the durability and longevity benefits generally outweigh this for serious RVers.