BMS/SoC Calibration

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SoC Drift
The SoC has an inherent drift based on inherent sources of error:
  • An integral is not generally computable from time-sampled measurements.
  • Battery current sensors seem to have calibration targeted at the high end of the range (highway speeds) and do not have a known recalibration program; likely a non-linear output.
  • Temperature measurements must rely on sensors in specific positions in the battery (with no feasible ideal position).

Steady throttle usage should experience less drift than wildly varying usage, but the inherent measurement problems still lead to some drift.

SoC Drift Correction
SoC drift is only really eliminated by the battery reaching an equilibrium state.
The BMS seems to recalibrate SoC for the 100% condition:
  • Pack voltage at 116.2V.
  • Battery state at rest via either:
    1. 0A charge or discharge current.
      with some few minutes time spent with the charger disconnected.
    2. ~0.8kW charge rate
      NOTE: observed by testing with a controllable switching supply; onboard charging can substitute.