That graph is misleading. It specifically references DST cycles
not distance. If you read the
research paper, the Dynamic Stress Test (DST) cycle is a single cycle of the battery
just to the levels specified. It is NOT a normal battery cycle of 100%-0%-100% or equivalent. So doing 4000 DST cycles of the black line (75% Depth of Discharge) would provide 7.5x the energy output of yellow line (10% DoD).
Recognizing this is critical, as we need to convert each of the respective DST cycles to an equivalent distance in miles. By assuming a pack size (78 kWh) and car efficiency (250 Wh/mi), we can make that conversion to distance, which is what we really care about. I did this a few years ago and here’s the result...
View attachment 671153
From this, a few observations:
1) Charging to 100% leads to significantly higher degradation over time
2) The degradation seen in this test is over an equivalent distance that far exceeds how long many people would own and operate a vehicle (many 100,000 miles)
3) Average SoC is potentially more important than low DoD to reduce degradation. Notice the black (0.63 Ave SOC) is consistently above the blue (0.75 Ave SOC).
4) Trends from the teal, magenta, green and yellow lines are harder to discern as the conclusions vary based on distance (battery age) as well as their DoD and average SoC assumptions.
So yes,
recharging nightly (small DoD) and hovering around mid-charge is technically better for the battery, but the reduction in degradation is likely to be hard or impossible to discern as long as you avoid routine charging to high SOC. Important caveats are that these results are dependent on Li-ion chemistry, battery temperature, storage duration at high and low SOC, thermal management algorithms, discharge/recharge rate, etc.
