A three-year-old diesel van's residual value is mostly a question of mileage and condition. A three-year-old EV's residual value is mostly a question nobody in the transaction can answer with confidence: what shape is the battery in?
What State of Health actually measures
State of Health, or SoH, is the battery's usable capacity today as a percentage of its capacity when new. A battery reporting 100% SoH holds a full charge the way it did on day one. One at 85% has permanently lost 15% of its range, regardless of how it's charged from here.
SoH degrades gradually with every charge cycle, but not at a fixed rate. Fast charging habits, extreme temperatures, and letting a battery sit at very high or very low charge for long periods all accelerate the decline. Two identical vehicles, driven and charged differently, can arrive at very different SoH figures on the same odometer reading.
Why it decides the resale price
A buyer valuing a used EV is really valuing the battery, because it's the single most expensive component in the vehicle and the one most expensive to replace. Without a trusted SoH figure, that buyer has to guess, and guesses skew conservative. That shows up as a lower offer, or as your vehicle sitting unsold while a comparable one with documented SoH sells first.
Today's SoH is tomorrow's resale value. A fleet that can document it at de-fleet has a stronger negotiating position than one that can only estimate it.
Monitoring it continuously, not once
A single SoH reading taken at de-fleet tells you where the battery ended up, but not why, and gives you no time to act on a vehicle degrading faster than expected. Tracking charging and driving patterns continuously against degradation from day one turns SoH from a number you discover at resale into one you can manage across the vehicle's life, adjusting how a vehicle is used, or flagging it for earlier disposal, while there's still a decision to make.