Ok. How do you know that a discharge to 2.5V has no long term effect on the cell? You could surely only establish that with a long term (4000 cycle?) comparative test of cells discharged to 2.5V vs those never discharged below 2.9V (for example).
Every cycle, partial or full, has a long-term impact on cycle life. My point is that if you're trying to establish actual performance, you need to test per the cell spec.
In addition to
Gazoo's comments. LFP discharge floor USED to be 2.0V standard, and some cell manufacturers retain it. It was raised to 2.5V for improved cycle life and the fact that there was little capacity below 2.5V.
The real danger is in overcharge. It actually damages things.
I also personally have 76 CALB 40Ah cells that were installed in a Gen 2 Prius Plug-in-hybrid system (aftermarket). These were abused in this application for 2.5 years in the Phoenix area with no cooling system. The system was removed from the car and placed into storage.
All cells discharged to 0.6V over a long period of time and were held there for months (owner neglected them). The monitoring board on each cell continued to draw microamps from the batteries until they were more than completely discharged. After fully charging them, they all tested to between 60 and 75% rated capacity. What was interesting is that these 76 cells test almost identical to the 5 or so loose cells that had been stored without a board on them and had 3.30V. These cells were used "spare" cells sent by the system builder.
I recently re-tested 10 of them after a year in storage. Before storage they were fully charged. After a year, they retained 97% of the prior charge, and repeat capacity tests were the same as a year prior.
Again, fears of discharge to 2.5V are unfounded. Yes. You will lose one of your 2000+ cycles when this happens. It also helps to remember that ever single cycle of any kind reduces cycle life/capacity, and at the end of it's cycle life, it will still have 80% of its capacity remaining and still be perfectly usable.
