To understand battery consistency, you must first understand SOC and OCV.
SOC
SOC (State of Charge) refers to the remaining battery capacity, similar to a fuel gauge in a car, expressed as a percentage (0%-100%). Essentially, it’s the ratio of the battery’s current discharge capacity to its rated capacity.
From a working principle perspective, the charging and discharging process of a battery is essentially the “migration journey” of lithium ions: During charging, lithium ions are extracted from the positive electrode, pass through the electrolyte and separator, and embed into the carbon material of the negative electrode; at this time, SOC continuously rises until it reaches 100% (theoretically, all lithium ions have transferred to the negative electrode). During discharging, lithium ions migrate out of the negative electrode and back to the positive electrode, causing SOC to decrease until it reaches 0% (theoretically, the lithium ions at the negative electrode are completely depleted).
OCV
OCV (Open Circuit Voltage) refers to the battery’s terminal voltage when the current returns to zero after charging and discharging has stopped. Simply put, it’s the “battery voltage after resting”—a crucial signal for measuring the battery’s SOC (State of Charge) because there’s a fixed correlation between voltage and remaining capacity.
However, the OCV-SOC correlation varies significantly among different battery types, which is a key factor causing consistency issues：
The OCV range of lithium iron phosphate (LFP) batteries is between 1.95V and 3.37V, and their OCV-SOC curve exhibits a clear “plateau characteristic”—when the SOC is in the core operating range of 20%-80%, the voltage change is minimal, only about 0.1V. This means that judging the remaining capacity based on voltage can easily lead to inaccurate measurements. For example, if the SOC increases from 30% to 60%, the voltage may hardly change, and the system cannot accurately identify the actual remaining capacity.
In contrast, the OCV-SOC curve of ternary lithium batteries shows a good linear relationship, with the voltage changing evenly and significantly with the remaining capacity. For example, when the SOC increases from 20% to 80%, the voltage steadily rises from around 3.6V to around 3.9V. The system can accurately deduce the SOC from the voltage signal, and consistency issues are relatively easier to control.
What exactly does battery consistency mean? It’s not just about “consistent battery capacity.”
According to industry definitions, battery consistency refers to the degree of consistency among cells in a battery pack in key performance parameters such as voltage, capacity, internal resistance, temperature, and cycle life. Differences in these parameters can have a “butterfly effect”, ultimately affecting the overall performance of the battery pack.
Five Core Dimensions of Battery Consistency
(1) Voltage Consistency
This refers to the voltage difference between cells at the same SOC state. Poor voltage consistency manifests as follows: For example, some cells may show a SOC of 50%, but others may only show 3.2V while others only show 3.0V. This causes cells with higher voltage to fully charge first during charging, leading to premature charging stop; and cells with lower voltage to deplete first during discharging, causing premature discharge stoppage and wasted battery life.
(2) Capacity Consistency
This refers to the rated capacity difference between cells. Even cells from the same batch may have different capacities due to differences in manufacturing processes and materials, resulting in some cells being able to hold 100Ah while others can only hold 95Ah. Inconsistent capacity leads to: smaller capacity cells fully charging first during charging, while larger capacity cells are not fully charged; smaller capacity cells emptying first during discharging, leaving unused capacity in larger capacity cells, thus hindering the overall capacity of the battery pack.
(3) Internal Resistance Consistency
Internal resistance is the resistance to current flow within the battery. The higher the internal resistance, the more energy is consumed during charging and discharging. Over time, cells with high internal resistance will experience greater energy loss, resulting in less remaining charge and a continuous deterioration in consistency.
(4) Temperature Consistency
During battery pack operation, the heating of each cell must be consistent. If some cells have poor heat dissipation and excessively high temperatures, it will accelerate the aging of internal chemical substances, leading to capacity decay, increased internal resistance, and ultimately, compromised consistency. In extreme cases, temperature differences may even trigger thermal runaway, creating safety hazards.
(5) Cycle Life Consistency
Cycle life refers to the number of complete cycles a battery completes from full charge to discharge to the cutoff voltage and then full charge. If some cells in the battery pack prematurely “age” due to poor consistency, it will significantly shorten the lifespan of the entire battery pack—this is a typical example of the “weakest link” effect.
Poor battery consistency has far more impacts than just inaccurate battery life; it can significantly reduce the user experience across three dimensions: battery life, lifespan, and safety, and may even lead to serious problems.
