State of Health (SoH)

The equivalent of a fuel gauge for a battery, representing the current electrical energy stored as a percentage of its maximum usable capacity. While SoC indicates immediate energy availability, SoH reflects long-term capacity degradation. A battery at 100% SoC may only deliver 80% of its original energy if SoH has degraded to 80%. SoC is estimated through:

• Coulomb counting (current integration over time)
• Voltage-based lookup tables correlated with open-circuit voltage
• Kalman filtering combining multiple sensor inputs for dynamic correction

The percentage of total capacity discharged during a single cycle, which inversely correlates with cycle life. Deeper discharges accelerate degradation:

• Cycling at 80% DoD may yield 3,000 cycles before reaching 80% SoH
• Cycling at 30% DoD can extend life to 10,000+ cycles
• Partial state-of-charge cycling between 40-60% SoC minimizes lattice stress in NMC cathodes DoD is a primary input to empirical battery degradation models that predict SoH trajectory over time.

An embedded electronic control unit that monitors cell voltages, temperatures, and current to ensure safe operation. The BMS is the computational engine that calculates SoH in real-time through:

• Tracking cumulative charge throughput as a proxy for cycle count
• Measuring internal resistance growth via voltage drop under load
• Comparing current maximum capacity against manufacturer beginning-of-life specifications
• Cell balancing to prevent individual cell over-voltage that accelerates degradation

An empirical or physics-based mathematical representation of capacity fade and internal resistance growth. These models decompose degradation into:

• Calendar aging: Time-dependent electrolyte decomposition occurring even at rest, accelerated by high temperature and high SoC storage
• Cyclic aging: Mechanical stress from lithium-ion intercalation and deintercalation, causing electrode particle cracking
• SEI layer growth: Solid electrolyte interphase thickening that consumes cyclable lithium, the dominant mechanism in early-life capacity fade Physics-based models using Doyle-Fuller-Newman equations simulate internal electrochemical states that empirical models cannot observe.

A measure of the rate at which a battery is charged or discharged relative to its maximum capacity. A 1C rate signifies a full charge or discharge in exactly one hour. C-rate directly impacts SoH degradation:

• High C-rate charging (2C+) generates excessive heat and lithium plating on the anode, causing irreversible capacity loss
• Fast charging at 350 kW for EVs typically operates at 2-3C, requiring active thermal management
• SoH estimation algorithms must compensate for C-rate-dependent voltage sag to avoid underestimating remaining capacity during high-power events
• Fleet operators balance C-rate against operational uptime to optimize total cost of ownership

The ohmic and polarization resistance within a cell that opposes current flow, measured in milliohms. Internal resistance growth is a primary SoH indicator because:

• Increased resistance causes voltage sag under load, reducing usable power
• Heat generation rises quadratically with current (I²R losses), accelerating thermal degradation
• A cell reaching 200% of initial internal resistance is typically considered end-of-life regardless of capacity fade
• Electrochemical impedance spectroscopy (EIS) provides frequency-domain resistance measurements that isolate individual degradation mechanisms