extreme team 72v 150 lipo battery pack low voltage

[augiedoggy]

Originally Posted by Pat911

Hi Augiedoggy,

0% SOC is 2.5v.

These cells might be at or below 0.67v. That is well below 0% SOC and in the area where permanent irreversible damage HAS been done to the cells.

Yes, I know you’re going to question how a cell can be below 0% SOC so I’ll try to explain how capacity is measured and calculated for lithium cells. I’ll keep the discussion to LiFePO4 but applies equally to other forms of lithium and even FLA cells, albeit with different top and bottom voltages

1. Cell is fully charged to 3.65v
2. Cell is discharged to 2.5v and discharging stopped
3. Amount of energy drawn from the cell during this process is it’s rated useable capacity. This is the capacity that is advertised for the cell. So an advertised 100Ah cell will have 100Ah available from 3.65v down to 2.5v.
4. There is minimal energy (maybe 1%-3%) still remaining in the cell at 2.5v but this is not useable energy and is not counted toward the cells capacity because trying to use it will irreversibly damage the cell.

Referring back to the other thread where this came up, a 100Ah cell has a full 100Ah useable capacity and using all 100% of this useable capacity will not damage it. What will damage it is if you try to use (say) 103Ah of a 100Ah cell, bringing the capacity to -3% (yes I know, not mathematically possible but hopefully you get my point). Doing so will bring the voltage of the cell below 2.5v, permanently damaging it.

The point of the BMS is to stop any possible discharge once any one cell in the battery pack gets down to 2.5v, hence preventing damage to that, and any other cell.

Cheers
Pat.

Thanks for the explanation I knew the bms would or should shut off the pack at about 2.5v but I had a hard time with the math behind 2.5v out of 3.65v max being 0%.. when I deal with higher voltages at work such assumptions would get me killed. If I discharge a capacitor I fully discharge it ..
Anyway .. back on topic.

[Pat911]

Yep, I thought that’s where the confusion was.