Battery knowledge · Capacity, energy, power and ratings
Voltage efficiency
Average discharge voltage divided by average charge voltage over a cycle. The gap comes from resistance, overpotential and hysteresis, so voltage efficiency falls as current rises. Multiplied by coulombic efficiency it gives energy efficiency; a lithium-ion cell at moderate rate reaches about 95 %, a vanadium flow battery often only 80–90 %.
- Abbreviation
- VE
- Also called
- voltaic efficiency
- Unit
- %
- Relationship
- VE = V̄_discharge / V̄_charge; EE = CE × VE
Example
Combine charge and voltage efficiency
If average discharge voltage is 3.5 V and average charge voltage is 3.8 V, voltage efficiency is about 92.1%. With a matched-cycle coulombic efficiency of 99%, energy efficiency is about 91.2%.
Explore the graph
Returning most of the charge does not return the same fraction of energy.
99% × 92% = 91.08% energy efficiency.
η_energy = η_charge × V̄_discharge / V̄_charge
Illustrative model. Matched cycle and measurement boundaries; auxiliary loads omitted.
Use in Lemonergy
Separate nameplate ratings from test results.
This is a reference concept. No executable Lemonergy block has been verified for this term.
References & further reading
Further reading on this topic
These sources provide broader context; they are not presented as citations for every statement in this entry.
- The International System of Units (SI), 9th edition
Bureau International des Poids et Mesures · 2026
Topic background · Definitions of SI units and coherent derived units; 2026 revision.
- Battery Test Manual For Electric Vehicles, Revision 3
Jon P. Christopherson · Idaho National Laboratory · 2015
Topic background · Performance characterization, pulse resistance/power, life testing and reference performance tests; historical 2015 edition.