A use case for voltage efficiency to quantify cell losses
Here's real data from a cell that starts performing poorly at around 400 cycles. But aside from the reduction in capacity, how else can the losses be quantified? One option is to examine the voltage difference between the equilibrium voltage and the voltage during charging or discharging. This voltage difference, determined by the cell's internal resistance, provides insight into the cell's ohmic and polarization overpotentials.
Voltage Efficiency, or the ratio of average discharge voltage to average charge voltage, is a quick and easy way to quantify this insight into a single number.
From these two plots, it is apparent that Voltage Efficiency is steadily decreasing decreasing for the first 400 cycles. This is because average charge voltage is increasing, while the average discharge voltage is decreasing. This effect accelerates after 400 cycles.
It may be desirable to plot Voltage Efficiency vs Average Voltage for both charge and discharge. This removes the noise associated with typical cycling tests. This plot is useful for mapping the Voltage Efficiency associated at specific voltages. This is useful for developing process thresholds, BMS algorithm limits, or end-of-life criteria.
Try it yourself:
1. Select 'Mean Charge Potential' and 'Mean Discharge Potential' on the x-axis.
2. Select 'Voltage Efficiency' on the y-axis.
3. In formatting, select 'Element Formatting', and under 'Trace Style and Color,' choose 'Marker Only'.
