Classifying the Lithium-ion cells
During the manufacturing of Lithium-ion cells, a very strict procedure is followed for grading them. Since no manufacturing process can produce 100% perfect yield, less than 10% of the produced cells do not meet the standards required to fall under A grade and hence they are classified as B grade cells. The reasons for rejection can be either the cells do not match up to the expected performance or a cosmetic defect or both. B grade cells also have a minimum performance expectation and if they don’t meet it, they are further classified as C grade cells. C grade cells are the lowest priced cells in the market and they can be used for single-cell portable applications operating at a very slow charge and slow discharge rate with lower expected battery life.
Cell quality imported in India and its repercussions
When brand new, B grade cells have about the same capacity (in Ah) and impedance (internal resistance) as A grade cells, which makes it difficult to differentiate between the two. However, B Grade cells are on an average 30-40% cheaper than A grade cells.
If you look at the prices in the international market and compare them with the prices of cells being imported in India, one can easily conclude that a good amount of the NMC and LFP cells being imported in India are B grade cells. A primary reason for this has been severe competition between the battery pack assembly companies to compete against each other on price. Another reason is the pressure from the OEMs to supply battery packs at an aggressive price.
A technical way to know if the cell is B grade is to charge-discharge the cell for a suitable number of cycles depending on the cell capacity, chemistry, form factor and intended application of the battery pack and look at the data. If the capacity fade is higher than what is mentioned in the cell datasheet cycle life graph, it is a B grade cell. And, if the cycling data is true to the values mentioned in the datasheet, then it is an A grade cell.
A few OEMs and battery pack suppliers have experienced issues with using B grade cells because their batteries could not perform up to expectation even during the warranty period, and these companies are slowly shifting towards sourcing A grade cells. However, some new entrants in the battery pack assembly field seem unaware that there exist A grade and B grade cells in the market.
Performance of A Grade vs. B Grade cells
Capacity Fade/Cycle Life – The cycle life of a lithium-ion cell is defined as the number of charge-discharge cycles at 80% Depth of Discharge (DoD) till the retention capacity of the cell comes down to 80% of its original capacity. If the capacity fade of a cell is higher, it tends to have a lower cycle life. B grade cells have a higher rate of capacity fade as compared to A grade cells.
Life – Lithium-ion cells are known for their long-lasting life. The cells degrade and their energy holding capacity reduces over time but they last for a long time, unlike Lead Acid batteries which experience sudden death. B grade cells tend to experience sudden death failure after a certain number of cycles, especially when they are charged and discharged at higher C rates. This does not allow the B grade cells to be reused as second-life batteries and they directly end up at a recycling plant.
Rise in Impedance – Impedance, also known as internal resistance, has an inverse relationship with cell performance. The lower the impedance, the better rate of charge and discharge the cells can go through. EVs demand fast charging and high power discharge, and hence EV Grade cells have lower impedance when compared to Energy Storage Grade cells.
As the cells are charged and discharged, their impedance increases. At some point, the impedance of the cell rises to a level that it becomes unusable for a particular application (such as EVs). At that point, it is disassembled and used as a part of a second-life battery to power applications (such as Energy Storage Systems) using a lower charge-discharge C rating. B grade cells experience a faster rise in the impedance level, and hence they become unusable much earlier compared to A grade cells.
Bulging/Swelling – Bulging of prismatic cells and swelling of pouch cells tend to happen in even A grade cells when they are overcharged, deep discharged or operated at very high temperatures. But the chances of bulging and swelling are higher in B grade cells because their cathode and anode stoichiometric ratio could be off and also due to B grade cells not undergoing proper formation in the first place.
Safety – Since B grade cells do not meet the performance parameters compared to A grade cells, it is not advisable to use B grade cells for fast charging and high power discharge applications such as EVs.
If B grade cells are made to perform at the level of A grade cells, especially in EV applications and combined with a mediocre BMS, consider it a recipe for disaster. It can cause internal cell short circuits due to dendrite formation and lead to thermal runaway. Thermal runaway from NMC chemistry cells can be extremely dangerous as NMC cells tend to catch intense fire, as seen in the cases reported from EV fires from across the globe.
This article was first published in EVreporter October 2021 magazine.
About the author
Rahul Bollini has over 6 years of experience as an international Lithium-ion cells R&D consultant and has closely worked with Indian, American, European and Japanese companies with hands-on experience in Lithium-ion cell engineering and complete fabrication process. He has studied Energy Business and Finance from Pennsylvania State University, USA. He can be reached at email@example.com.
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