Guide to Connecting Batteries in Parallel Properly

When it comes to expanding battery capacity, connecting multiple units in parallel is a common approach. But in practice, doing it properly requires careful attention to safety, battery compatibility, and wiring techniques.

In this guide, we’ll explore not just the basic steps, but also the underlying principles, practical tips, and common mistakes to avoid. By the end, you’ll have a clear understanding of how to connect batteries in parallel safely and efficiently.

What happen if you connect batteries in parallel

When batteries are connected in parallel, the overall capacity and current output of the battery bank increase, while the voltage remains constant. Each additional battery contributes to the total energy storage, effectively extending backup time within the same voltage system.

For example, wiring two 12V 100Ah batteries in parallel results in a 12V 200Ah setup. This configuration stores more energy, allowing appliances or equipment powered by a 12V inverter to run significantly longer before needing a recharge.

 

Pros and Cons of connecting batteries in parallel

Advantages:

• Expands total storage capacity and system runtime without altering voltage, allowing longer energy supply and improved flexibility for different load requirements.
• Reduces the load on each individual battery and ease the strain on older batteries by expanding the available capacity of the bank.
• Provides redundancy, ensuring that if one battery fails, remaining units continue supplying power, lowering the risk of complete system shutdown or power loss.

Disadvantages:

• Requires precise wiring and proper balancing to prevent uneven charging or discharging.
• Additional cables and connections increase system complexity, raising the chance of loose joints, corrosion, and unwanted heat buildup during high current flow.
• Higher current draw at lower voltage means thicker cables and more voltage drop, which can be inefficient for larger power appliances.
• Space limitations may occur since batteries must be positioned close together, complicating installation in areas with limited physical room for placement.

Rules for Connecting Batteries in Parallel

• Always connect batteries of the same chemistry, voltage rating(12v,24v,48v), and amp-hour capacity to advoid uneven charging, faster degradation, and potential safety hazards.
• Before wiring batteries in parallel, ensure they are balanced to the same state of charge (SOC). Large voltage differences may trigger damaging current surges in the system.
• In parallel battery setups, branch cables (battery to busbar) and main cables (busbar to controller/inverter) within each group should have equal length and thickness. This ensures consistent resistance and balanced battery discharge, preventing any battery from being overloaded.
• Install batteries close together to minimize cable length, resistance, and power loss. At the same time, leave enough space for proper airflow to prevent overheating and maintain safe operation.
• Avoid mixing old and new batteries in a parallel bank. A weaker or older battery can reduce the performance of the entire system, leading to uneven discharging and a shorter overall lifespan.
• Choose battery cables with the proper wire size to handle the maximum expected current. Parallel connections increase current flow, so using wires that are too thin can cause excessive voltage drop, heat buildup, and safety risks.
• Install a fuse and circuit breaker on the positive terminal of each battery. This protects the system by preventing one battery from discharging excessive current during a short circuit or failure.

How to wire batteries in parallel

Wiring batteries in parallel must be done carefully to ensure safety, efficiency, and long-term reliability. Follow these steps to build a properly balanced parallel battery bank.

Step1. Plan the parallel battery connection diagram

Since parallel battery connections require careful planning, it’s recommended to first draw a diagram. This helps map the layout based on available space, number of batteries, inter-battery distances, and cable routing, reducing uneven current flow and easing installation.

Tips for parallel battery diagram

• Never daisy-chain batteries in parallel, as this can create uneven load distribution and shorten battery life.
• For a simple setup with just two batteries, connect them in parallel using a diagonal connection. Connect all the middle terminals together, positives to positives and negatives to negatives, then use the first battery’s positive and the last battery’s negative for the load or charge controller. This ensures balanced current across both batteries.
• When connecting more than two batteries in parallel, using a battery busbar is always the best way to balance load distribution by providing equal length, low resistance paths from each battery to a centralized connection point with one for all positive terminals and another for all negatives.

While a busbar is the optimal choice, there are some alternative parallel wiring methods for four batteries, which we will discuss along with important considerations in another article.

Step2. Size wire to connect batteries in parallel

Choosing the right battery cable size is critical for both safety and efficiency.

In a parallel battery bank, the wires between batteries or from batteries to the busbar carry only the current from individual units. These wires should be sized to handle the maximum current of a single battery.

By contrast, the wires running from the battery bank or busbar to the inverter or charge controller must carry the total current drawn by the load or the maximum output of the inverter. Therefore, they need to be thicker to safely handle the full load and minimize voltage drop.

Example of wiring two batteries in parallel

Let’s assume two 12V 100Ah LiFePO4 batteries are connected in parallel, each with a maximum discharge current of 100A, powering a 2000W inverter. You will need 4AWG and 2AWG wires as follows:

• Inter-battery connections: For connections between the batteries, a typical cable length of 0.3–0.5 meters is sufficient, and 4AWG (25mm²) copper wire is usually adequate.
• Connection to the inverter: The wire from the battery bank to the inverter must carry the combined current of both batteries under load. A thicker wire, such as 2AWG (35mm²) or larger, is recommended to ensure safety and maintain efficiency.

For the entire battery system, both the positive and negative main wires should be of equal length. In addition, all inter-battery wires and battery-to-busbar wires should also be of equal length and properly crimped to ensure balanced current flow and prevent uneven charging or discharging.

Step3. Balance the batteries before connecting them in parallel

Before connecting batteries in parallel, it is important to ensure that all units are balanced and at a similar state of charge. This helps prevent excessive current flow between batteries, which can damage weaker cells and reduce the overall lifespan of the battery bank.

• For lead acid battery, balancing is usually achieved by fully charging each battery individually before parallel connection. Periodic equalizing charges are recommended to prevent sulfation and capacity loss. Ideally, the voltages of all batteries should be roughly the same, within 0.05–0.1V of each other, to ensure even current sharing.
• LiFePO4 batteries can often be balanced without a dedicated balancer. Our technicians usually achieve this by disconnecting the batteries from the charging source and letting them rest in parallel for about 3 hours, allowing higher-voltage batteries to naturally charge lower-voltage ones until their voltages align. If needed, individual batteries can be fully charged and discharged once or twice to reset the BMS and ensure correct voltage readings. The BMS also manages charge at the cell level to prevent overcharge or deep discharge.

Step4. Finish the batteries parallel connection

Once safety checks and balancing are complete:

1. Disconnect all charging sources and loads before working.
2. Use properly crimped lugs and tighten all connections firmly.
3. Connect all positive terminals together and all negative terminals together, either via diagonal connection (for two batteries) or busbars (for multiple batteries).
4. After wiring, recheck connections for tightness and polarity.
5. Finally, connect the positive and negative outputs of the parallel bank to the inverter, charge controller, or DC load.

It is strongly recommended to install fuses on each battery’s positive terminal as well as on the main line to provide reliable protection against short circuits.

FAQs on connecting batteries in parallel

What is the best wiring method to balance the load in parallel batteries

The most effective method is a busbar system, which provides a simple, clean layout and ensures equal-length, low-resistance paths for balanced current flow. For a two-battery setup, a diagonal connection is also a practical option.

Do I need to fuse batteries in parallel

Yes. It is strongly recommended to place a fuse or circuit breaker on the positive lead of each battery before it connects to the busbar or main line. This protects the system in case of a short circuit or if one battery develops an internal fault, preventing the other batteries from dumping current into it.

How many batteries can I safely connect in parallel?

Most manufacturers recommend limiting parallel connections. For example, PowMr 12V batteries can be paralleled up to 4 units, while 48V batteries support up to 15 units. The more batteries you add, the harder it becomes to maintain proper balance, so using busbars, thicker cabling, and fuses becomes increasingly critical in larger systems.

Can I mix LiFePO4 and lead acid in parallel?

No. Mixing different chemistries in parallel is not recommended. Lead-acid and LiFePO4 have different charging voltages, discharge characteristics, and internal resistances. Connecting them together can lead to imbalance, reduced lifespan, and potential safety risks. Always use batteries of the same type, capacity, brand, and age for best results.