Efficiency

When you charge and then discharge a battery cell you lose energy, the ratio of the amount of discharge to charge energy is the efficiency.

If we put 11 Wh into a battery cell when charging and recover 10 Wh when discharging the energy efficiency = 10 / 11 = 90.9%

Typical energy efficiencies:

  • Lead acid ~70%

Coulombic Efficiency

Also known as Faradaic Efficiency, this is the charge efficiency by which electrons are transferred in a battery. It is the ratio of the total charge extracted from the battery to the total charge input to the battery over a full cycle.

Coulombic efficiency values:

  • Lead acid ~85%
  • Lithium ion >99%

High coulombic efficiency usually indicates a long battery cycle life.

Voltaic Efficiency

This is the ratio of the average discharge voltage to the average charge voltage over a cycle. The charging voltage is always higher than the rated voltage to activate the chemical reaction within the battery and hence losses.

Round Trip Efficiency

A key metric for energy storage systems is the amount of energy released versus the amount of input energy. This ratio is the Round Trip Efficiency.

  • Rimac SineStack AC round trip efficiency >92%

This number can include inverter efficiency and hence should be checked.

References

  1. Vedran Bobanac, Hrvoje Basic, Hrvoje Pandzic, Determining Lithium-ion Battery One-way Energy Efficiencies: Influence of C-rate and Coulombic Losses, IEEE EUROCON-2021