We know that a car's battery is composed of many battery cells. Generally, modules are composed of battery cells, and the modules form a PACK.

For example, the Tesla Model S's battery system is carefully composed of 7104 18650 lithium batteries. The Xiaomi SU7 Max battery pack is composed of 198 battery cells connected in series. With so many battery cells, if one of them is faulty, it's easy to carry the whole into the ditch. As a familiar saying goes, "A piece of mouse dung will kill a pot of porridge". So the consistency of battery cells in power batteries is very important.

1、 The harm caused by poor consistency

Poor consistency of battery cells in power batteries can cause various hazards, directly affecting battery performance, safety, lifespan, and overall vehicle reliability. The following is a specific analysis of hazards and mechanisms:

① Capacity degradation and battery life decrease

Shortboard effect: In cells with poor consistency, the unit with the lowest capacity will be the first to fully charge or empty, resulting in the overall available capacity of the battery pack being limited by the "weakest cell".

Example: If the capacity of a certain battery cell is 10% lower than others, the entire battery pack capacity may be lost by 5% to 8%, significantly reducing the vehicle's range.

② Safety hazards: thermal runaway, fire and explosion

Risk of overcharging/overdischarging: When the voltage is inconsistent, some cells may be forcibly charged to overvoltage (>4.3V) or discharged to undervoltage (<2.5V), causing lithium dendrite growth, piercing the diaphragm and causing internal short circuit, ultimately leading to thermal runaway.

Uneven heat distribution: Cells with large internal resistance or temperature differences overheat locally during charging and discharging, forming "hotspots" and accelerating heat diffusion.

③ Significantly shortened battery life

Aging acceleration: Cells with poor consistency are subjected to uneven loads during cycling. For example, repeated overdischarging of low capacity cells can accelerate the decay of electrode materials, potentially shortening their lifespan by 30% to 50%.

Chain reaction: The failure of a single battery cell (such as a sudden drop in capacity) will force other battery cells to compensate, exacerbating overall aging.

④ Failure of Battery Management System (BMS)

SOC/SOH estimation distortion: BMS relies on cell parameter consistency for precise management. If the difference in battery cells is too large, BMS may not be able to accurately estimate the remaining state of charge (SOC) or state of health (SOH), and may misjudge the battery status.

Overload of balancing function: BMS needs to frequently balance cells with significant differences, resulting in increased energy consumption and limited effectiveness, ultimately leading to overload or failure of the balancing circuit.

2、 How to screen out cells with poor consistency

The following are the methods and steps for consistency screening of battery cells:

① Voltage matching: That is, OCV testing measures the open circuit voltage (OCV) of the battery cell after it has been idle (usually 24-48 hours), and eliminates cells with voltage deviations exceeding the set threshold (such as ± 5mV).

② Capacity sorting: Obtain the actual capacity of the battery cells through charge and discharge tests, and divide them into capacity intervals (such as ± 1%) to ensure that the difference in battery cell capacity within the same battery pack is minimized.

③ Internal resistance screening: Measure the DC internal resistance (DCR) or AC internal resistance (ACR), and remove cells with high internal resistance or large dispersion (such as internal resistance deviation>3%).

④ Self discharge rate (K value) test: After fully charging the battery cell, let it stand for 7-14 days, calculate the self discharge rate (K value) based on the voltage drop, and screen for cells with similar K values (such as K value difference<0.5%). The role of K=(OCV1- OCV2)/(t2- t1) K value testing should not be underestimated.

⑤ Aging and cycle testing

High temperature aging screening: Place the battery cells in a high-temperature environment (such as 45 ℃) and monitor the voltage decay rate to screen for cells with consistent aging characteristics.

Cycle life pre-test: Perform a small amount of charge and discharge cycles on the battery cell (such as 50 times) to eliminate cells with abnormal capacity decay or sudden internal resistance changes.

When selecting power batteries, the quality of the battery cells will greatly affect the performance of the entire battery pack, so it is necessary to conduct strict inspections of product quality and suppliers in the early stage. As a professional lithium battery supplier，Be Power is committed to providing customers with high-quality and customized solutions；

We are the number one Chinese battery supplier delivered to automotive OEM in Brazil.

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