The Diagnostic Challenge
Unlike a petrol tank, which has a fixed physical volume, a battery's capacity is an electrochemical reality that changes dynamically based on temperature, age, and discharge rate. Ascertaining a highly accurate State of Health (SoH) is surprisingly difficult. It requires the Battery Management System (BMS) to interpret indirect electrical measurements and run them through complex mathematical models.
Coulomb Counting
The most fundamental method of tracking battery state is coulomb counting. This involves placing a current sensor in series with the battery pack and literally integrating the current over time (amps multiplied by hours). By tracking every single electron that enters during a charge and exits during a discharge, the BMS can estimate remaining capacity.
However, coulomb counting suffers from drift. Sensor inaccuracies accumulate over time. If a battery is only partially charged and discharged over many months—a common occurrence in city driving—the BMS loses its baseline. To correct this, the system must perform periodic resets by looking at the Open Circuit Voltage (OCV).
Open Circuit Voltage (OCV)
When a battery is at rest (no current flowing in or out for several hours), its voltage corresponds directly to its State of Charge. The relationship between OCV and SoC is mapped out on a voltage curve specific to the battery chemistry.
- NMC Chemistries: Have a sloped voltage curve. The voltage drops steadily as the battery discharges, making it relatively easy for the BMS to pinpoint the SoC just by reading the resting voltage.
- LFP Chemistries: Have an extremely flat voltage curve. An LFP cell might read 3.3V at 80% charge and 3.28V at 30% charge. This makes OCV lookup highly inaccurate for LFP unless the battery is near 100% or below 10%, necessitating regular charges to 100% to calibrate the BMS.
Impedance Tracking
As batteries age, their internal resistance (impedance) increases. Impedance tracking involves measuring how much the voltage sags under a specific load. A healthy battery might drop by 0.5 volts when 100 amps are demanded. An older, degraded battery might drop by 2.0 volts under the exact same load.
Advanced diagnostic tools plug into the vehicle's OBD-II port and read the BMS's internal resistance tables to generate an independent SoH estimate, crucial for the second-hand EV market and repurposing companies assessing pack viability.
Data Extraction for Second-Life Grading
Before an EV battery can be sold for grid storage, it undergoes extensive testing. Repurposing facilities often bypass the vehicle's original BMS—since it's programmed strictly for automotive use—and run full cycle tests using specialized grid-tied inverters. The pack is fully discharged and charged in a controlled temperature environment to establish a mathematically absolute capacity reading, guaranteeing performance for the buyer.