EV DC Fast-Charge Time (CC-CV Taper)
Why '80% in 20 minutes' is only the first leg: DC fast charging holds constant power (the lesser of charger and vehicle acceptance) only to about 80%, then the battery tapers to protect the cells -- modeled as three bands (0-80% full power, 80-90% ~50%, 90-100% ~25%). A 60 kWh EV from 10-100% on a 150 kW charger (100 kW acceptance) does the 10-80% fast leg in 25.2 min, then the last 20% costs another 21.6 min -- essentially the same again -- for 46.8 min total. Dividing 54 kWh by 100 kW gives 32.4 min, 14 min short, because it charges the top at power the pack never accepts. This is why fast-charge etiquette is to unplug at 80%. A planning estimate; the vehicle's charging curve governs.
Formula and source
cc_power = min(charger, acceptance); bands 0-80% at 1.0, 80-90% at 0.5, 90-100% at 0.25 x cc_power; per-band time = (capacity x band_span/100) / (cc_power x fraction); total = sum over covered bands.
DC fast-charge CC-CV taper model (three constant-power bands), first-principles; the vehicle's charging curve governs the actual profile.
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