Why doesn't electricity flow immediately?
Authorisation approved. The display does not jump to full power. For a few seconds it shows Preparing…, then the power climbs: 40 kW, 120 kW, 240 kW.
Twenty minutes later it falls again, although nobody touched anything.
The questionWhat happens in those first seconds, and who decides how much power flows at each moment?
Guess firstWho sets the power level at any given second?RevealHide
Not the charger, and not the driver. The car's battery management system requests what the cells can safely take at that moment, depending on their temperature and state of charge. The charger offers what it and the grid connection can deliver. The lower of the two applies, every second of the session.
The site's grid connection was planned when the site was built. Cars charging at the same time share it, which is what load management is for.
Select each step of the energy path.
Most public charge points in Europe are AC chargers up to 22 kW, where the car's onboard charger converts the energy: about three in five stand at sites with AC only. About one in five belongs to a site with 150 kW or more. Classified by the highest rated power of each site.
What fills those seconds
Three steps run during Preparing. Only after all three does current flow.
- Safety check. Insulation measurement and connector lock. This is also why charging in the rain is safe.
- Negotiation. Car and charger exchange their voltage and current limits.
- Precharge. The charger matches its output voltage to the battery before the close.
The cells can still accept almost everything the charger can provide.
An illustrative curve for one fast-charging car. The falling number at high state of charge is the battery protecting itself, not a broken charger.
Why the power falls around minute 20
A battery fills like a stadium: quickly while the rows are empty, more slowly once seats run out. As the battery fills, the lowers the power to protect the cells, most steeply above roughly 80%, so the last 20% take disproportionately long. That is why car manufacturers state fast-charging times from 10 to 80%.
At any moment, three limits decide the power: the grid connection of the site, the converter of the station and the battery of the car. The smallest one applies.
Check your understanding
4 questions. Answer all of them to complete this stop. Each answer explains itself.
What happens during the “Preparing…” seconds?
Insulation test, connector lock, negotiation of limits and precharge: safety first, then physics.
Charging power falls from about 240 to about 110 kW by 80% state of charge. This is…
Batteries accept less power as they fill. Protecting the cells takes priority over speed.
Why does the same car charge in 20 minutes on the motorway but in 8 hours at home?
Where the converter sits, in the car or in the station, is the single fact behind the difference.
A site has four 300 kW chargers but a grid connection for 600 kW in total. Four cars arrive. What must happen?
The site's connection is a real limit. Load management shares it between the cars.
- BMS Battery management system: the car's electronics that decide how much power the battery accepts at each moment. Glossary
- Contactor An electrically operated switch for high currents; in a DC charger it connects the output to the car only after both have checked the connection and matched the voltage. Glossary
- emobility.directory: Charge point operators in Europe, from national registries of 8 countries and OpenChargeMap; Switzerland, Greece, Luxembourg and Poland left out, data as of 28 Sep 2026. https://emobility.directory/. Checked 28 Sep 2026. Charging data: Bundesnetzagentur.de (CC BY 4.0); IRVE, transport.data.gouv.fr (Licence Ouverte 2.0); NDW (CC0); DGT (CC BY); Fintraffic / digitraffic.fi (CC BY 4.0); Via Lietuva (CC BY 4.0); NOBIL by Enova (CC BY 4.0); Open Charge Map contributors (CC BY 4.0), including public sector information licensed under the Open Government Licence v2.0; combined and calculated by emobility.directory.