Home Solar Batteries: How to Distinguish LiFePO4 Cell Grades?

When solar installers and distributors purchase LiFePO4 solar batteries for home use, price is the thing they care most about, because an affordable price gives them more competitiveness in the market. But they seldom seem to think about one question, why do the prices of LiFePO4 solar batteries with the same capacity vary so much?

In the cost of a lithium solar battery, the cell and BMS account for 80% of the cost, and the cost of the BMS does not vary much, so it is the LiFePO4 cell used that has the most to do with the price of the solar cell.

After producing cells of the same specifications, cell manufacturers need to go through a series of testing processes to measure the rated capacity, internal resistance and other parameters of the cells. Based on the test results, they classify LiFePO4 cells into three grades: ABC. Because the material and process of LiFePO4 cell production can not reach the complete one-to-one, there is the same kind of batch and model, there are different capacity segments, such as, the production of 100AH battery:

Grade A - capacity above 110Ah, internal resistance below 60mΩ

Class B-Capacity 100~110Ah, internal resistance 60mΩ~80mΩ

Class C-Capacity below 100Ah, internal resistance above 80mΩ

A Grade LiFePO4 cells

Grade A is the title of the high quality standard of the battery. Grade A LiFePO4 cell rated capacity star reaches the predetermined value, the internal resistance is lower than the specified value, and the appearance and performance are qualified, and its battery material, technology, energy storage, steady charge and discharge, specification parameters, and constant temperature standard are the high quality standard of the industry, and usually sold at a higher price.

A-grade battery cells are generally used by the battery cell factory to place orders directly to the LiFePO4 battery cell factory, which arranges production according to the factory's own production capacity and technical capability, and after a series of battery cell manufacturing processes, finally makes the battery and delivers it to the customer. Before placing an order, the battery production factory will generally communicate the parameters of battery capacity, thickness, length, width, etc. Then the battery made exactly according to the standards of the order is called A product.

B Grade LiFePO4 cells

Any lithium iron phosphate factory production products, there must be a defective rate. Then the defective rate determines the vitality of a factory, in the lithium battery industry, purely by technology, ingredients, etc., the first-line lithium battery and third-line lithium battery factory will not have a big gap, but in the production process and production process, the defective rate gap is very large, first-line manufacturers can do about 2%, while the second and third line may do 5-10%.

Because of this defective rate, B Grade Lithium Battery is created, and there are two types of B Grade Lithium Battery.

Appearance B Grade cells

The appearance of B grade products need special attention is the bump, bump marks, because these are generally caused by internal impurities, if the solar cell working current is large, the bump, bump marks will become the current, heat concentration point, easily lead to heat shrinkage of the isolation film, resulting in internal short circuit, and ultimately affect the life of the entire battery pack.

Package B Grade cells

Package B Grade cells, for soft pack lithium batteries, are generally sold as B Grade cells without leakage, but with encapsulation risk. B Grade cells are sensitive to temperature and are prone to leakage when the sealing edge is opened at high temperature, and also prone to water vapor infiltration in long-term use, leading to battery expansion (in the case of metal-case LiPo, the internal air pressure is too high, which is dangerous when discharged instantly).

Performance B Grade Cells

Performance B grade products, mainly low capacity, low voltage, high internal resistance, etc.. Performance B grade cells have a great impact on the use of energy storage batteries. Performance B grade directly affects the consistency of the battery pack, the same low capacity battery, but also because of low capacity reasons are different, resulting in inconsistent performance after the cycle, and ultimately drag the battery pack.

C Grade LiFePO4 cells

If B grade cells are placed in the warehouse for more than 8 months, if they are still not shipped, they can be called C grade, C grade cells are due to long placement time, his capacity is generally only about 50-60 of the same type of A grade cells, good ones can reach 70%. C grade cells are less safe, in addition, C grade cells are not durable, and then after several cycles of charging, the performance is greatly reduced.

Now we usually also call the LiFePO4 battery pack retired from electric vehicles as C cells, this type of lithium battery pack capacity has declined to only 70 % or even lower, some solar cell manufacturers in order to pursue lower prices, usually use the retired LiFePO4 battery pack to produce solar cells to quickly enter the market. However, this type of battery usually has a lower life span, higher risk, and is heavier, which is very unfriendly to consumers, but consumers usually do not have the ability to identify.

How to distinguish between Class A, Class B and Class C cells?

1. Appearance

A-grade LiFePO4 cells usually do not look defective, B-grade LiFePO4 cells may have bumps, dents and other minor defects, while C-grade LiFePO4 cells have obvious signs of use and do not look brand new. In addition, each cell has a unique QR code as a proof of identity. This also makes it easier for LiFePO4 battery manufacturers to provide after-sales service. While B-grade cells are classified as substandard cells, they usually do not provide warranty. That's why their QR codes are usually removed.

2. Energy Density

Energy density = capacity/weight

Home solar cells made with A-grade LiFePO4 cells usually have a higher energy density. Because the capacity of B and C grade LiFePO4 cells is much lower compared to A grade LiFePO4 cells, more B and C grade LiFePO4 cells are used to make up a home solar cell of the same capacity, which ultimately results in you getting a very heavy home solar cell. So we can compare the energy density of home solar cells to determine if the cell you are buying is an A-grade LiFePO4 cell.

3. Battery Life

Long life is one of the advantages of LiFePO4 cells, but if the solar cell is made up of retired LiFePO4 cells, the life span will be very low, usually only 2-3, so don't easily believe the warranty time promised to you by the supplier, it's just a necessary measure they take to earn your money. Whereas A grade LiFePO4 cells will usually exceed 6000 cycles @ 80% DOD, which means they have a service life of at least 10 years.

4. Self-discharge rate

Self-discharge rates vary by SOC state. For example, the voltage drops faster at 100% SOC than at 50% SOC. So before testing the self-discharge rate, please check the battery specifications about the SOC state to be tested.

During the manufacturing process of Li-ion batteries, a very strict grading procedure is performed on them. Since no manufacturing process can produce a 100% perfect yield, less than 10% of the cells produced do not meet A grade and therefore they are classified as B grade cells. Rejections may be due to cells not matching expected performance or cosmetic defects or both. class B cells also have minimum performance expectations and if not met are further classified as class C. Class C cells are the least expensive cells on the market, they can be used in single cell portable applications, are extremely slow to charge and discharge, and have a short expected battery life.

The solar energy storage market is booming, but this can also lead to a very large number of companies flooding into the industry. In a market with diverse products, distributors and installers should have the minimum ability to distinguish between products in order to bring the best service to their customers, and choosing what price class of product is equivalent to choosing what supplier and service.

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