SOH versus SOC: The Vital Difference
Before diving into algorithms, let's clear up the single most common confusion among prospective EV buyers:
State of Charge (SOC): How full the battery tank is right now, expressed from 0% to 100%. If you plug the car into a charger for two hours, SOC rises. It tells you today's immediate fuel level.
State of Health (SOH): The maximum size of the tank today compared to when the battery was brand new. If a pack had a usable capacity of 60.0 kWh when leaving the factory, and after seven years can only store 51.0 kWh, its capacity-based SOH is roughly 85%.
When a car's dashboard shows "100% charged", that means it has reached 100% of its current, degraded capacity (its current SOH ceiling), not 100% of the energy it could hold on the day it rolled out of the factory.
How On-Board Systems Calculate SOH
There is no tiny sensor sitting inside the lithium pack that measures "health" directly. State of Health is an estimated algorithmic figure continuously calculated by the vehicle's Battery Management System (BMS). Different manufacturers use different mathematical approaches:
- Coulomb Counting: The BMS tracks every amp entering during charging and leaving during discharge. By measuring total charge throughput between known voltage points, it estimates usable capacity.
- Open Circuit Voltage (OCV) Resting Curves: When the vehicle is parked asleep for several hours without temperature fluctuations, the cell voltage stabilises. The BMS looks up this resting voltage on calibrated chemistry tables to recalibrate its baseline.
- Impedance Spectroscopy & Resistance Models: By monitoring how sharply cell voltage sags during heavy acceleration spikes, the computer models internal cell resistance.
Why Methodology Matters
Because SOH is a software model rather than a direct physical dipstick, the conditions of the reading matter immensely. If a car has only been driven for short three-kilometre trips between 40% and 60% charge for eight consecutive months, the BMS may not have observed an full top-balancing charge or an open-circuit rest cycle at low SoC.
In such scenarios, the computer's estimate drifts. We have seen vehicles display a pessimistic 86% SOH, which then recalibrates upward to 91% after a continuous slow charge to 100%, followed by an uninterrupted six-hour sleep. Conversely, quick dealer OBD resets can momentarily mask battery drift until a complete driving cycle exposes weak cell groups.
The Context Spectrum: Why 85% Isn't Automatically "Bad"
A frequent question at our diagnostic desk is: "What SOH number should make me walk away?" There is no universal answer because vehicle purpose and price dictate value:
| Vehicle Context | Typical SOH Range | Practical Practical Implication |
|---|---|---|
| Compact Urban Commuter (7-9 yrs) | 76% – 83% | Completely functional for 70-100 km daily European city runs; steep market price discount. |
| Mid-size Family Estate / SUV (3-5 yrs) | 88% – 94% | Minimal motorway range penalty; normal expected calendar and moderate cycle fade. |
| High-Mileage Fleet / Taxi (2-4 yrs) | 82% – 89% | Higher DC charging stress; requires cell delta check to verify uniform module wear. |
Potential Limitations of SOH Metrics
Relying solely on one aggregate SOH percentage hides crucial granular faults:
- Cell Balance Disparity (Delta V): A pack might average 88% overall capacity, but if one individual cell group drops into critical low voltage under acceleration before the others, the BMS will throttle propulsion or enter limp mode early. The overall SOH didn't warn you; the cell delta would have.
- Buffer Manipulation: Many automakers build "top buffers" and "bottom buffers" into usable capacity. In early years, normal chemical degradation eats into the hidden top buffer without reducing the user-visible range, making SOH appear flat until it suddenly steps downward.