Equivalent full cycles (EFC)

Equivalent full cycles express how much energy has passed through a battery, counted in units of its usable capacity. They make different charging patterns comparable, but EFC alone does not predict degradation.

A simple definition

One equivalent full cycle is the amount of energy that corresponds to discharging the battery's full usable capacity once. EFC is calculated as the total discharge energy throughput divided by the usable capacity. It does not matter whether that energy came from one deep cycle or from many small ones.

A charging session from 40 to 90 % adds about half of the usable capacity, so it is roughly 0.5 EFC worth of charge. Two such sessions together equal about one EFC, the same as one discharge from 100 to 0 %.

40 → 90 %: about 0.5 EFC
20 → 80 %: about 0.6 EFC
10 → 100 %: about 0.9 EFC

Why EFC is useful

Lab studies usually report cycle life as a number of cycles, and cycle-life models for LFP express capacity loss as a function of throughput, temperature and C-rate. Converting your driving into EFC makes it possible to relate real-world use to such studies. It also lets you compare a driver who charges a little every day with one who charges a lot once a week.

Your annual EFC is roughly your yearly driving energy divided by usable capacity. A driver with a large battery and modest mileage accumulates far fewer EFC per year than a driver with a small battery and high mileage.

What EFC does not tell you

Two drivers with the same EFC count can see very different degradation. Studies on NMC and LFP cells show that wear per cycle depends on depth of discharge, the SOC window used, current and temperature. Cycling around mid-SOC tends to cause less wear per EFC than cycling through the extremes.

EFC also says nothing about calendar ageing, which continues while the car is parked. Models that combine calendar and cycle components and were validated on realistic profiles show that both need to be considered together.

How Voltgevity uses EFC

Voltgevity calculates EFC from your annual distance, energy consumption and usable capacity, and uses it as one input to the cycle ageing estimate. That estimate is then adjusted for cycle depth, SOC window, charging power and temperature, and combined with calendar ageing.

Key takeaways

EFC equals discharge energy throughput divided by usable capacity.
A 40 to 90 % session is roughly 0.5 EFC worth of charge.
EFC makes usage patterns comparable but is not a degradation prediction on its own.
Depth, SOC window, current, temperature and calendar time all change what a given EFC count means for ageing.