While searching for methods to evaluate HV Battery fitness, I've found this thread on the Hybrid Synergy Forum:
State of charge is primarily calculated using coulomb counting and is dynamically
corrected using SOC drift points.
Coulomb counting is a method that keeps track of current
going into and out of the battery pack. Coulomb counting generally works quite well as long as
the capacity of the battery is known and the current sensor is accurate enough.
coulomb counting system can be perfectly accurate, errors will eventually build up.
those errors, dynamic state-of-charge drift is used to compensate.
It seems that from time to time, SOC may vary by several % points with no significant currents applied.
Those variations may indicate a less-than-ideal battery condition.
Drifting detection has been added to Torque Log Analyzer: whenever such unexpected variations are detected, they'll be reported in the Analysis sheet.
If drifting is reported, a full HV Battery Fitness check is advised.