C-rate, not kilowatts
Kilowatts are a rate. Rates only mean something relative to the thing being filled. The number that actually predicts how long a charge takes is C-rate: power divided by capacity.
C-rate = charging power (kW) / pack capacity (kWh)At 1C, a pack takes an hour to go from empty to full. At 3C, twenty minutes. At 0.5C, two hours. Everything else is detail.
The same kilowatts, wildly different experiences
| Vehicle | Pack | Peak power | C-rate |
|---|---|---|---|
| Tesla Model 3 LR | 75 kWh | 250 kW | 3.3C |
| Joby S4 air taxi | 150 kWh | 480 kW | 3.2C |
| Hyundai Ioniq 5 | 80 kWh | 260 kW | 3.3C |
| Chevy Silverado EV | 205 kWh | 350 kW | 1.7C |
| Tesla Semi | 850 kWh | 1200 kW | 1.4C |
| Nissan Leaf | 39 kWh | 50 kW | 1.3C |
A megawatt into the Semi is a gentler charge than 250 kW into a Model 3. That is why the Semi still needs about 34 minutes to go from 10 to 80%, despite pulling more power than twenty cars at once. It is also why the Leaf, at 50 kW, is not as slow as it sounds relative to its tiny pack, though the air-cooled pack has other problems.
Why high C-rate is hard, and rare
Pushing 3C means shoving lithium ions into the anode three times faster than a one-hour charge. Two things fight back.
Plating. As the anode fills, ions run out of places to insert. Push harder and metallic lithium plates onto the surface instead of intercalating. That is permanent capacity loss and, eventually, a dendrite that can short the cell. This is why every curve on this site tapers: the BMS walks current down as state of charge rises, because the safe C-rate falls as the anode fills.
Heat. Resistive losses scale with the square of current. A pack absorbing 3C makes roughly nine times the waste heat of one absorbing 1C, and the coolant loop has to remove it in real time or the BMS derates to protect the cells.
Aircraft sit at the far end
An eVTOL air taxi has a small pack, charges at 2 to 3C, and does it several times a day. The Joby S4 in the calculator takes about 22 minutes to gain 18 minutes of flight. That is a duty cycle no road vehicle experiences, and the consequence is that aviation packs are replaced on flight hours rather than on range loss. They are consumables.
Trucks sit at the other end: enormous packs, modest C-rates, and the grid connection rather than the cells as the binding constraint. Which is the subject of sizing a buffered station.
Next
Sizing a battery-buffered station
A battery behind the dispenser lets a modest grid connection serve 350 kW sessions. How many charges that buys, and why the answer barely depends on the inverter.