Coulombic or Faradic efficiency

Coulombic (or Faradic) efficiency is defined as the ratio of cumulative discharging current to charging current. This ratio is typically high, approximately 97% for lead-acid batteries.

For Li-ion technologies, we lack explicit data. We assume a value of 96% to approximate the overall energy efficiency of 90-95% reported by some manufacturers.

This parameter results from electrochemical conversion current efficiency.

The coulombic efficiency should not be confused with the battery energy efficiency, which includes also other losses, mainly the resistive losses due to the internal resistance, which increases the charging voltage and decreases the discharging voltage.

NB: In lead-acid batteries, a "gassing" phenomenon occurs due to water molecule dissociation when the battery is overcharged. This consumes additional current, which is accounted separately (not in the coulombic efficiency) in the PVsyst model.