You pull into a DC fast charging station, plug in, and within 20 minutes your battery is at 80%. But what's actually happening inside that cabinet — and is any electricity stored locally, or does it all come from the grid in real time?

AC vs DC: The Fundamental Difference

The grid delivers alternating current (AC). EV batteries store and release direct current (DC). Every kilowatt-hour that goes into your battery must be converted from AC to DC somewhere. The question is: where?

In AC charging (Level 1 & 2), this conversion happens inside the car. Your vehicle has an onboard charger (OBC) — a piece of hardware that converts AC from the wall into DC for the battery. The OBC is the bottleneck: if it's rated at 11 kW, you'll never charge faster than 11 kW regardless of what the station offers.

In DC fast charging, the conversion happens in the charging station itself. The station contains large, powerful rectifiers that convert grid AC to the high-voltage DC that feeds directly into your battery pack. This bypasses your car's OBC entirely, which is why DC charging can deliver 150–350 kW — far beyond what any OBC can handle.

What's Inside a DC Fast Charger?

A DC fast charging cabinet contains several key components:

• Power electronics / rectifiers: Convert grid AC (typically 3-phase at 480V) to high-voltage DC (up to 1,000V). This is the most expensive component and determines the maximum power output.
• Battery management communication: The charger and vehicle negotiate via CCS, CHAdeMO, or NACS protocols. The car tells the charger its current battery voltage and the maximum current it will accept — the charger adjusts in real time.
• Cooling systems: High-power electronics generate significant heat. Modern 350 kW units use liquid-cooled cables and active cooling circuits.
• Grid connection equipment: Transformers, fuses, and metering hardware connecting to the utility grid.

Is Energy Stored On-Site?

Most DC fast chargers draw directly from the grid with no local storage — they're essentially very powerful, sophisticated extension cords. However, this is changing rapidly in 2025–2026.

Battery-buffered charging stations are becoming increasingly common at high-demand sites. These stations include a local battery pack (typically 100–500 kWh of lithium cells) that charges slowly from the grid and discharges rapidly when multiple EVs need charging simultaneously. Benefits include:

• Avoiding expensive grid demand charges and capacity upgrades
• Enabling ultra-fast charging (350 kW+) at sites with limited grid connections
• Grid stability services — the station can absorb excess renewable energy
• Backup power during grid outages (though most stations still go down)

Tesla's V4 Superchargers at high-demand locations, Electrify America's latest sites, and many new European hub charging stations use this battery-buffer architecture. As EV adoption grows and peak demand on charging networks rises, battery buffering will become the norm rather than the exception.

Why 800V Charges So Much Faster Than 400V

Power = Voltage × Current. To deliver 350 kW to a 400V battery, you'd need 875 amps — far beyond what any cable can safely handle. An 800V battery can receive the same 350 kW at only 437 amps. This is why 800V architecture (Hyundai E-GMP, Porsche J1, Mercedes MMA) enables ultra-fast charging while keeping cables and connectors practical.

The Charging Curve: Why It Slows After 80%

DC fast charging isn't constant — it follows a curve. From 10–80% SoC, the battery accepts near-maximum power. Above 80%, the battery management system reduces current to prevent lithium plating and heat damage. The last 20% (80–100%) often takes as long as the first 70% (10–80%). This is why 10→80% is the industry standard metric for DC fast charging speed.