Can Lithium Batteries with Different Capacities Be Connected in Parallel?

Connecting lithium batteries in parallel is a common practice to achieve higher voltage and capacity, widely used in applications such as power tools, electric vehicles, and energy storage systems. However, in practice, not all batteries are suitable for direct parallel connection. Specifically, when the capacities of the batteries differ or other parameters are inconsistent, there can be several potential safety issues. Therefore, understanding the factors to consider when connecting lithium batteries in parallel is essential.

1. Basic Requirements for Connecting Lithium Batteries in Parallel

The most crucial factor when connecting lithium batteries in parallel is ensuring that the battery parameters are consistent. These parameters include:

• Capacity: Capacity refers to the amount of energy the battery can store. Batteries with higher capacity can provide longer operating times, but their charging and discharging characteristics may differ from those with smaller capacity.
• Open-Circuit Voltage: The open-circuit voltage is the voltage of the battery when no load is connected. Batteries connected in parallel must have the same or very similar open-circuit voltage. Otherwise, imbalanced currents between the batteries can occur, which may lead to overcharging or over-discharging.
• Internal Resistance: Internal resistance refers to the resistance to current flow inside the battery. Batteries with higher internal resistance will generate more heat and may cause overheating, affecting the battery’s lifespan and safety.

Batteries should only be connected in parallel when these parameters are as similar as possible, ensuring that the parallel connection is safe and effective in improving system performance.

2. Potential Issues with Connecting Batteries of Different Capacities in Parallel

When the capacities of the batteries differ, even if other parameters are similar, several potential problems can arise, as outlined below:

1. Imbalance During Charging
   If the parallel batteries have different capacities, the smaller capacity batteries will charge faster than the larger capacity ones. This can cause the smaller batteries to reach full charge first, putting them in an overcharged state. Overcharging leads to an increase in the battery’s electrolyte and may cause side reactions, which could age the battery, cause leaks, or even result in fire or explosion. The risk of overcharging is higher if the batteries are not equipped with a protection circuit (BMS). Even with a protection circuit, the smaller battery may still be overcharged for prolonged periods, leading to chemical reactions inside the battery that compromise its safety.
2. Imbalance During Discharging
   During discharge, smaller capacity batteries will deplete their energy faster than larger capacity batteries. As a result, the smaller batteries may become over-discharged, which significantly shortens their lifespan and can cause leakage or damage. If the battery remains in an over-discharged state for too long, internal chemical changes can occur, which may lead to fire or explosion.
3. Reduced Battery Lifespan
   Constant overcharging or over-discharging significantly shortens a battery’s lifespan. Additionally, imbalances in the voltage and current during the charging and discharging cycles can decrease charging efficiency, further affecting the battery’s performance and lifespan.

3. Safety Issues and Risks

Connecting batteries with different capacities in parallel can also pose serious safety risks. Overcharging or over-discharging a battery in a short time can cause overheating, swelling, leakage, or even explosion. If the batteries are sourced from disassembled power banks, the parameters (such as capacity, voltage, and internal resistance) may be inconsistent, and it is unclear whether the batteries have protection circuits, increasing the risk of using them. Therefore, it is strongly advised not to assemble these batteries for use by yourself to avoid potential safety hazards.

4. How to Avoid Safety Risks?

To avoid safety hazards when connecting lithium batteries of different capacities in parallel, the following measures are recommended:

1. Ensure Consistent Battery Parameters: Make sure that the capacity, open-circuit voltage, and internal resistance of the parallel batteries are as similar as possible. If possible, choose batteries of the same model and from the same production batch.
2. Install a Protection Circuit (BMS): A Battery Management System (BMS) can effectively monitor the charging and discharging process, preventing overcharging, over-discharging, overheating, and other issues. For safety, it is recommended that all battery packs are equipped with a protection circuit.
3. Regularly Check Battery Status: Regularly inspect the battery’s voltage, temperature, and operational condition to ensure that the batteries are not overcharged or over-discharged and that no swelling or leakage is occurring.
4. Avoid Self-assembling Batteries: If the origin of the batteries is unclear or you are unsure whether the batteries have protection circuits, it is best to avoid self-assembling them. Especially if the batteries are disassembled, their internal resistance and capacity may not be consistent, which can pose significant risks.

5. Conclusion

Although connecting lithium batteries in parallel can increase the capacity and output power of a battery pack, it is crucial to ensure that the parameters of the batteries are consistent to prevent imbalances that may result in safety issues. The use of a protection circuit is essential when connecting batteries in parallel, as it helps prevent overcharging, over-discharging, and other risks, ensuring the safety of the battery pack. To avoid potential safety risks, it is strongly recommended not to assemble batteries of different capacities, particularly when the battery source is unclear. With proper design and management, the stability and safety of the battery system can be ensured.