How to Charge Two Batteries in Parallel: Step-by-Step

Charging batteries in parallel can be an effective way to ensure a steady and reliable power supply, whether you're working with RVs, boats, solar systems, or other outdoor power setups. However, it's important to understand the process and follow best practices safely and efficiently.

In this article, we'll guide you on charging two batteries in parallel, explain key considerations and safety tips when batteries in parallel charging.

What Does Charging Batteries in Parallel Mean?

Charging batteries in parallel refers to connecting two or more batteries in such a way that the positive terminals are linked together, and the negative terminals are also connected. This setup allows you to increase the total available capacity (amp-hours) while maintaining the same voltage as a single battery.

When charging 2 12V batteries with 100Ah in parallel, for example, the voltage remains at 12V, but the available energy storage doubles to 200Ah. This is especially useful for applications that require a larger energy reserve, such as RVs, boats, or off-grid solar systems.

(Two Redodo’s 12V batteries in parallel)

Things to Note Before Charging Batteries in Parallel

To safely charge two batteries in parallel, make sure these batteries are allowed to be connected in parallel. They need to meet the following conditions:

• With the same battery type (e.g., two 12V lead-acid or two 12V LiFePO4 batteries)
• With the same battery capacity (Ah) and BMS (A)
• From the same brand (as lithium battery from different brands has their special BMS)
• Purchased in near time (within one month).

How to Charge Two Batteries in Parallel: Step-by-Step

Charging two batteries in parallel is a straightforward process, but it requires careful attention to wiring, battery condition, and charger specifications. Here’s a step-by-step guide to ensure you’re charging batteries in parallel correctly:

Step 1: Wire the Batteries Together

• Connect the Positive Terminals: Use an insulated cable to connect the positive terminal of Battery 1 to one terminal on the positive bus bar. Then, connect the positive terminal of Battery 2 to a different terminal on the same positive bus bar. This setup creates a total positive input on the bus bar.
• Connect the Negative Terminals: Similarly, connect the negative terminal of Battery 1 and the negative terminal of Battery 2 to a same negative bus bar, which creates a total negative input.
• Use a bus bar as the total positive or negative input, rather than a single battery terminal. This way helps distribute the current more evenly in parallel connections and prevents the terminals to overheat even melt.
• Ensure the connections are tight and secure, as poor connections can lead to voltage drops or charging inefficiencies. Also, ensure that the cables you use are rated to handle the maximum current of the battery system. 

Step 2: Choose a Suitable Battery Charger

When charging batteries in parallel, it's important to use a charger that is compatible with the total capacity of your battery bank. For example, if you have two 12V 100Ah batteries in parallel, you'll be charging a 12V system with a combined capacity of 200Ah. Be sure the charger can handle the total battery capacity without overcurrent.

• Lithium vs Lead-Acid: If you’re using lithium batteries, ensure your charger is specifically designed for lithium-ion or LiFePO4 batteries, as these have different charging profiles compared to lead-acid batteries.
• Charge Rate: Check the recommended charge rate for the specific batteries you're using. It’s recommended to use 0.2C of charge rate to charge multiple lithium batteries.

Step 3: Connect the Battery Charger

• Positive Lead: Connect the positive lead of the charger to the main positive input.
• Negative Lead: Connect the negative lead of the charger to the main negative input.
• Begin the Charging Process: Double-check all connections to ensure tight connection and properly insulated. Start the charging process by turning on the charger once all connections are verified.

Step 4: Monitor the Charging Process

While charging, it’s crucial to monitor the battery voltage and ensure both batteries are charging evenly. It's recommended to use a battery monitor or a voltage meter to check the voltage of each battery periodically. 

Step 5: Disconnect After Full Charge

Once the batteries have reached their full charge, firstly disconnect the AC input of the charger, then disconnect the charger and the batteries. Be sure to disconnect the charger from the batteries before removing the cables.

Safety Tips When Batteries in Parallel Charging

Charging batteries in parallel involves handling electrical equipment that can be hazardous if not done correctly. Here are some safety tips to follow:

• Protective Gear: Always wear gloves and safety glasses to protect yourself from battery acid and potential sparks. This is especially important when working with lead-acid batteries, which may release hazardous fumes or leak.
• Ventilation: Charge batteries in a well-ventilated area, especially when dealing with lead-acid batteries. Batteries can emit flammable gases, and proper ventilation reduces the risk of a dangerous buildup.
• Environment: Keep the charger and battery away fromwater, heat sources, sparks, flames, flammablegas,and hazardous chemicals. Never use it in a thunderstorm.
• Avoid Charging Issues: Unplug the input/output cable of the chargerimmediately once the charger or battery is found to be abnormal or damaged during charging.

When changing lithium batteries in parallel, a BMS is crucial for maintaining safety when charging batteries in parallel. It helps monitor battery health, prevent overcharging, and protects against short circuits.

Redodo batteries are equipped with an advanced BMS that offers multiple protection against overcharging, over-discharging, short circuits, overcurrent, and overheating, ensuring the safe operation of each battery during charging or discharging.

Conclusion: Charging 2 12V Batteries in Parallel

Charging batteries in parallel is a practical solution for those who need increased energy storage but want to maintain the same voltage level. By following the proper wiring techniques, ensuring battery compatibility, and using the correct charger, you can safely and efficiently charge two 12V batteries in parallel.

For more expert advice on battery management, feel free to subscribe our newsletter and get extra 5% new user discount. Explore our range of 12V LiFePO4 lithium batteries designed for your reliable power needs.

Related Reading:

How to Connect Batteries in Series and Parallel?

Batteries in Series vs Parallel: Which is Better

How To Charge LiFePO4 Battery Safely?

How to Charge a Deep Cycle Marine Battery (Step-by-Step)