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TO REWORK

• Voltage:
• V < 2.5V cells should not pass to next step (update: not true)
• Cells < 2.0V should go to recycling straight away
• We could later try to revive > 2.0V cells if considered profitable despite the risks
• “Li-ion cannot dip below 2V/cell for any length of time. Copper shunts form inside the cells that can lead to elevated self-discharge or a partial electrical short. (See BU-802b: Elevated Self-discharge) If recharged, the cells might become unstable, causing excessive heat or showing other anomalies. Li-ion batteries that have been under stress may function normally but are more sensitive to mechanical abuse.” (source : Battery University)

• Internal Resistance:
• IR > 90 mOhm is rejected (value depends on cell model)
• A lot of articles mention the “200% increase” criteria

📰 Review of scientific papers:

Research papers never give precise screening figures to make cell batch based on IR.

Internal resistance matching (…) and impacts on battery pack cycle life

2.2 Ah cylindrical LiFePO4 cells

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20% difference in cell IR between cells in parallel ⇒ 40% reduction in cycle life (4.5C)

20% difference for a typical 25mΩ value is equal to 5mΩ

A new method for Li-ion battery uniformity sorting based on internal criteria

Feasibility of using second life EV batteries

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A 20% drop in capacity and a 200% increment in internal resistance are indicators for battery replacement

The only precise figure we’ve got is: a 20% difference in cell IR between cells in parallel causes a 40% reduction of cycle life. 5% mismatch ⇒ 10% reduction. 3% mismatch ⇒ 5% reduction.

Forum quotations

cell matching, internal resistance?

With ir you can get away with cheaper testers, but you will need to find the fabric specifications on the cellsir. If there optimum ir is for an example 25, and you measure 50mohm, that cell wont do you any good anymore.

https://secondlifestorage.com/index.php?threads/cell-matching-internal-resistance.8106/

• “If you stick to the fabrics optimum ir value and you take a max deviation of 10% you will be good” ⇒ FOR IR VALUE NOT MATCHING

• Difference under 1mΩ How to match cells for a battery pack before assembling the pack CMX FAQ (cmxbattery.com)

IR tester configuration

Use example : https://www.youtube.com/watch?v=X0sdpfQHmOY

Parameters :

With these parameters, the machine seems to be used in a pack assembly line, on new cells.

IR for new cells should be in a known range (20~30mOhm here). Their voltage should also be in a certain range (here 3.1~3.5V). Out of these ranges are rejected (grade 5) or retested to be used on other packs.

It also verifies that the cells are in the right orientation before assembly

Then 4 voltage groups are formed. Inside each group, all cells are in the 20~30mOhm range and within +/- 50 mV, which means they can be connected in parallel.

Final balancing will be done by the BMS during charging of the finished pack.

Other parameters :

https://www.youtube.com/watch?v=2hKtrbkgv5k

🔎IR-V testing machine selection (1)