Method of Grading for Lithium Ion Battery

The concept of grading:

In a fixed requirement environment, When the lithium battery is fully charged, Release of electricity under certain conditions, The amount of power released from the battery at this point is the capacity of the lithium-ion battery. Differentiation of lithium-ion batteries according to capacity, It's grading.

Purpose of grading:

1.   Distinguish between capacity-qualified products and capacity-unqualified products.

If the capacity meets the requirements, it is a qualified product. If the capacity is lower than the specification, it is a unqualified product.

2.   One of the means of classifying and grouping lithium-ion batteries.

Selection of monomers with the same capacity and internal resistance, Such monomers with the same performance form a battery pack.

Inconsistency in battery capacity can cause inconsistency in the depth of discharge of each individual cell in the battery pack. Batteries with smaller capacity and poorer performance will reach the full state of charge earlier, causing batteries with large capacity and good performance to fail to reach the full state of charge.

Vantage: simple and convenient

Disadvantage: The method is a static measurement method, which does not reflect the differences in the actual application of changes and has limitations.

Method of grading

1.  Discharge capacity method

Lithium-ion batteries are fully charged under certain conditions and then fully discharged at a certain current,

Discharge current * time, is the discharge capacity of lithium-ion battery.

Advantage: Capable of accurately and comprehensively reflecting the performance of lithium-ion battery discharge capacity, etc.

Disadvantage: Longer time, affecting productivity

2.  Charge capacity method

Lithium-ion batteries are charged to SOC1 under certain conditions, Then follow a charging method to reach SOC2,Calculate charging capacity between SOC1-SOC2,

Compare the above relationship between charging capacity and final capacity of lithium-ion batteries, Estimate the actual discharge capacity of a lithium-ion battery.

Advantage: Short time and high productivity

Disadvantage: Existence of bias and misjudgement.

3.  Open circuit voltage method

Li-ion batteries are charged at constant current to a certain SOC，determine the relationship between open circuit voltage and discharge capacity, discharge capacity deduced from open circuit voltage.

Advantage: Short time and high productivity

Disadvantage: Low accuracy of judgement, Not suitable for high-precision grading.

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