Is there a way to read battery state in kWh?

[Coulomb]

Does the usable capacity have a buffer at both ends or could it be just the bottom though ?

I believe that there is usually a buffer at both ends. Different bad things happen to lithium batteries at either end.

If it's at the bottom end, to prevent complete discharge, then it could be 64kWh at 100%

Theoretically. But if it's reporting usable capacity, then it doesn't matter what end or ends the buffer(s) is(are) at, as you can't use the buffer.

 

[wattmatters]

Last time I charged to 100% SOC my 104S 64 kWh battery pack voltage reported 441 V, an average of 4.24 V per cell. That's pretty much right up there for NMC.

Typical range for NMC is 3.0 to 4.2 V.

At end of a charge to 80% it was 415 V, an average of 3.99 V/cell.

At rest when SOC reported 5% it was reporting 358 V, an average of 3.44 V/cell. That's well above the NMC floor voltage of ~ 3.0 V. While driving just before then it did report 351 V (voltage sag under load) which is an average of 3.375 V/cell.

I'd say the NMC pack BMS has far more buffer at the low end that at the top. The health mode charging to 80% is what provides the buffer at the top end.