How NMC 811 (2170) loses capacity
NMC 811 loses capacity through a mix of SEI growth, cathode surface reconstruction and micro-cracking of nickel-rich particles. Resistance climbs faster than in LFP, which matters as much as capacity for second-life duty.
At about 260 Wh/kg and a nominal 3.7 V per cell, NMC 811 (2170) sits in a specific trade-off between energy density and durability that shapes how it ages in service.
Finding the knee before it finds you
Fade is gradual through most of vehicle life, but high-nickel cells can reach a knee earlier if they were fast-charged heavily or held at high state of charge in heat. Module-level testing catches cells that are close to it.
The practical test is trend rather than a single reading: a module whose resistance is rising faster than its neighbours is heading for the knee, even if its capacity still looks healthy today.
| Measured SOH | Grade | Typical route |
|---|---|---|
| 85% and above | A | Grid-tied or commercial storage |
| 75–85% | B | Telecom backup, solar buffering |
| 65–75% | C | Low-duty backup, short warranty |
| Below 65% | D | Discharge and refining |
Signals worth tracking in the field
Fleet telematics can flag packs that are ageing faster than the rest of the fleet long before they retire. Pairing that data with a bench test at retirement makes grading faster and pricing more accurate.
- Usable capacity at a standard rate and temperature
- DC internal resistance per module
- Impedance spectrum shape from EIS sweeps
- Self-discharge over a rest period
- Temperature history and fast-charge share