While it is technically feasible to mix different brands of LiFePO4 batteries, this practice presents significant challenges and risks that can impact performance, safety, and longevity. Factors such as subtle variances in voltage, capacity, and internal resistance can impact the performance and longevity of a lithium battery setup. So, mixing various lithium battery LiFePO4 models under strict conditions might work, it is not recommended due to the increased risk of performance degradation and potential safety issues like overheating or thermal runaway. This article provides a detailed understanding of the factors involved.

Performance Inefficiencies

Voltage Variations: When batteries have different nominal voltages, they do not discharge evenly. The higher-voltage batteries must adjust to match the lower ones, which places unnecessary strain on the cells and can lead to cells damage over time.

Capacity Mismatches: Batteries with varying amp-hour (Ah) ratings discharge at different rates. Smaller capacity batteries deplete more quickly, reducing the system's overall efficiency and raising the risk of over-discharge, which can cause permanent damage to the weaker batteries.

Internal Resistance Variation: Different manufacturing standards lead to varying internal resistances. Higher resistance in some cells limits their current flow, which reduces the total power your system can deliver.

Safety Risks

Over charging and Over discharging: Connecting batteries with varying capacities or internal characteristics can cause some batteries to over charge or over discharge. Overcharging may result in overheating, and over discharging can permanently damage the batteries.

Short Circuits and Thermal Runaway: Incompatibilities between batteries can raise the risk of electrical malfunctions, which may lead to short circuits or thermal runaway. Such situations can cause overheating and fires.

Load Distribution Issues: Variations in physical design, such as cylindrical versus prismatic configurations, can lead to uneven load distribution. This imbalance increases the risk of mechanical or thermal failure.

Reduce Battery Life

Battery Management System (BMS) Inefficiencies: BMS units are engineered to monitor and safeguard uniform battery packs. However, mixed battery setups can confuse the BMS, resulting in improper cell balancing and accelerated wear across all cells.

Uneven Aging: Batteries age at varying rates due to differences in usage and chemical composition. Mixing old and new batteries can place extra stress on the older ones, thereby reducing their lifespan even more.

Impact On System

Inconsistent Output: Uneven discharge rates lead to unstable voltage and current output, which can damage sensitive electronics.

Increased Maintenance: Constant need to monitor and balance mixed batteries adds to the complexity and cost of maintenance.

The Risks of Mixing Old and New Batteries

Reduced Lifespan: Mixing old and new batteries causes the newer ones to overwork to make up for the weaker performance of the older ones. This imbalance shortens the lifespan of all connected batteries.

Increased Costs: Though initially appearing economical, the rapid degradation of batteries leads to frequent replacements, which ultimately costs more than purchasing a new and reliable battery pack from the start.

Safety Concerns: Old batteries often have higher internal resistance, which can lead to overheating or uneven charging when used in mixed setups. Such issues pose safety risks and may undermine system reliability.

Best practices

Use identical batteries: Ideally, use batteries from the same manufacturer, same model, and same age.

Match voltage and capacity: Ensure all batteries in the bank have the same voltage and capacity (Amp-hour rating).

Check for consistency: Before connecting, measure the open circuit voltage and ensure they are within 0.1V of each other. Equalize them with a controlled charge if necessary.

Avoid mixing new and old batteries: Even if from the same brand, a significant age difference between batteries can cause issues.

Match cable lengths: When wiring to bus bars, ensure the "round trip" cable length (from the battery to the bus bar and back) is the same for all batteries to promote even current distribution.

Selecting the appropriate LiFePO4 battery can greatly affect the performance and safety of your energy system. Although mixing old and new batteries or combining different brands might appear convenient and economical, this practice frequently leads to increased costs and lower efficiency of the system over time. It is best to use batteries of the same brand, model, age, and capacity.