Top Balanced but way off after discharge?

[Shale MacGregor]

So I top balanced 4 cells to 3.5V with all very similar IR, then performed a ~ .3C discharge with them in 4s with a Xianxing type smart BMS. I discharged down to 10v and the final voltage of each cell was

1- 3.13V
2- 2.12V
3- 3.01V
4- 2.58V

The pack hit ~ 95% of its rated capacity but one cell was way lower than I would want to discharge.

What am I missing?

 

[NVS]

I would top balance to 3.65v and let the batteries set a day then top off to 3.65 again. Then run your capacity test. Set your bms to low voltage protection on each cell to 2.5v. You can't tell by voltage alone where each battery is on the charge curve, so you have to take to max voltage to truly top balance.

 

[MisterSandals]

The pack hit ~ 95% of its rated capacity

So you nearly top balanced, got 95% capacity and are wondering why your cells are at different voltages at the very bottom?
If you bottom balance they should be much better at low charge state.

 

[Sojourner1]

Do a proper top balance to 3.65vpc while in parallel then when doing your capacity test while they're in series be sure you can stop discharging when any cell hits 2.5v.

 

[Bob B]

The low voltage cutoff shouldn't be based on pack voltage .... it needs to be done based on lowest cell voltage.
2.5 is as low as you want to go.

I would charge back up and see if the cells are still top balanced. If they are, the difference of voltage at the bottom is an indication that your cells are not the same capacity.

 

[time2roll]

Best to stop at 2.5 for any cell. While in use you may want to stop at 12.0 pack voltage or a bit higher. Not sure why you held back from 3.65 balance and then let the cell go all the way to 2.12 volts... is the spec on these 2.00 volts minimum?

You can top balance again to 3.65 to see what goes.... otherwise capacity is what the low cell is missing. 95% is not exactly uncommon.

 

[Gazoo]

So I top balanced 4 cells to 3.5V with all very similar IR, then performed a ~ .3C discharge with them in 4s with a Xianxing type smart BMS. I discharged down to 10v and the final voltage of each cell was

1- 3.13V
2- 2.12V
3- 3.01V
4- 2.58V

The pack hit ~ 95% of its rated capacity but one cell was way lower than I would want to discharge.

What am I missing?

I am curious why the LVD didn't kick in when the lowest cell reached 2.5 volts? I have tested my BMS to make sure the LVD is working properly and it kicks in at 2.5 volts. Also that's a very high delta. Why didn't you charge to 3.65 volts? My delta isn't all that great either being .5 volts when my lowest cell reaches 2.5 volts. But that's to be expected unless the cells are capacity matched and some have had better results than I have with non capacity matched cells.