A driver plugs into a 150 kW charger, sees 140 kW on the screen, and twenty minutes later finds the car charging at 40 kW. Nothing is broken. This is the charging curve: the way charging power changes over the course of a session as the battery fills, warms up and protects itself.
Understanding the curve helps drivers plan stops and helps operators set realistic expectations, design pricing and choose hardware.
- DC charging power is highest at low state of charge and falls as the battery fills, especially above about 70–80%.
- Temperature has a large effect: cold or very hot batteries accept less power.
- Peak power is a marketing number; average power over a 10–80% session matters more.
- For operators, the curve affects throughput, idle time, power sharing and pricing fairness.
What is a charging curve?
A charging curve shows charging power (kW) on one axis and battery state of charge (%) or time on the other. Each car model has its own curve, defined by the battery chemistry, cell design, cooling system and the manufacturer's software. Two cars on the same 150 kW charger can therefore behave very differently.
Why power falls as the battery fills
Lithium-ion batteries are typically charged in two main phases:
- Constant current (CC): the charger pushes a high current while the battery voltage rises. Power is high and roughly stable or rising.
- Constant voltage (CV): once the cells approach their maximum voltage, the current must be reduced to avoid overcharging. Power falls steadily until the battery is full.
Modern cars use more complex, stepped strategies, but the principle remains: the fuller the battery, the less current it can safely accept. That is why the last 20% often takes as long as the first 60%.
Factors that shape the curve
| Factor | Effect |
|---|---|
| State of charge | Highest power at low charge; tapering above roughly 70–80% |
| Battery temperature | Cold batteries are limited; very hot batteries are throttled to protect them |
| Preconditioning | Warming the battery before arrival at a fast charger raises power |
| Battery architecture | 800-volt systems can sustain high power at lower current |
| Battery age and health | Older batteries may be limited more conservatively |
| Charger limits | Maximum current, power sharing or site load management cap the curve |
Peak power vs average power
Manufacturers often advertise peak charging power. For drivers and operators, the more useful figure is the average power over a typical 10–80% session. A car with a high peak that drops quickly may charge slower overall than a car with a lower but flatter curve. Our guide on how long it takes to charge an electric car shows typical session times.
AC charging: a flatter curve
On AC, power is limited by the car's on-board charger, typically 7.4 or 11 kW, which is far below what the battery could accept. The curve is therefore flat for most of the session and only tapers near 100%. This is one reason AC charging is gentle on batteries and well suited to long stays.
What the curve means for operators
Throughput
Drivers who charge to 100% on a fast charger occupy it for a long time at low power. Encouraging sessions that end around 80%, through information, app notifications or pricing, increases the number of drivers a charger can serve.
Idle and blocking fees
Because the end of a session runs at low power, distinguishing slow charging from idle occupation matters. Fair idle fees start only when charging has really finished, not merely when power drops.
Power sharing
Many DC chargers with two outlets share power between them. As one car tapers, the other can receive more. The charging curve is what makes power sharing efficient: two cars rarely need peak power at the same time.
Pricing fairness
Per-kWh pricing charges the same for every kWh regardless of how fast it arrived, which is fair to drivers whose cars charge slowly. Time-based pricing during charging penalises cars with flatter or lower curves, which is one reason per-kWh pricing is preferred and, in the EU, required for ad hoc charging at 50 kW and above.
Seeing the curve in your data
OCPP chargers report power, energy and often state of charge through MeterValues at regular intervals. Plotting power against state of charge for each session shows the curve for each car, and comparing sessions reveals chargers that deliver less than expected. Our guide to OCPP MeterValues explains which measurands to configure, and our article on kW vs kWh explains the units.
Frequently asked questions
Why does my car never reach the charger's maximum power?
The car's battery and software decide how much power it accepts. If the car's peak is lower than the charger's rating, or the battery is cold or already well charged, power stays below the maximum.
Is it better to charge to 80% on a fast charger?
For most trips, yes. Power drops sharply above 80%, so stopping there saves time and frees the charger for others. For long gaps between chargers, charging further may still make sense.
Does cold weather slow down charging?
Yes. Cold batteries are limited to protect the cells. Preconditioning the battery while driving to the charger can significantly improve speeds in winter.
Is the charging curve the same on every charger?
The car's curve is mostly the same, but chargers with lower power, current limits or shared power can cut it off earlier.
Analyse power, energy and state of charge per session from any OCPP charger, and spot sites that underperform. Explore Üreticy OCPP software or contact us.