The difference between AC and DC charging is not just about speed — it is about where the AC-to-DC conversion happens, and that location determines the charging rate ceiling for every EV on the market.
- AC charging converts power inside the vehicle's onboard charger (OBC), limiting speed to the OBC rating (typically 3.3–11 kW for Indian 4W EVs).
- DC fast charging bypasses the OBC — the station converts AC to DC and delivers it directly to the battery, enabling 25–150 kW in India's current infrastructure.
- The IEC 61851 control pilot protocol governs Mode 3 AC charging: the EVSE communicates its current rating to the EV via PWM duty cycle, and the EV self-limits accordingly.
- CCS2 is India's mandatory DC fast charging standard; Bharat AC-001 and Bharat DC-001 serve the lower-cost 2W/3W market.
- The charging curve tapers after ~80% SOC — the practical advice to stop fast charging at 80% is rooted in real cell physics, not arbitrary rules.
Charging an EV looks simple from the outside: plug in, wait, unplug. The engineering underneath is a layered system of power conversion, communication protocols, and safety interlocks that spans from the electricity grid to the individual cells in your battery. Understanding the four charging modes — and specifically why DC fast charging is so much faster than AC charging — explains most of the charging experience questions that EV owners encounter.
The Fundamental Difference: Where Conversion Happens
Battery cells store and deliver direct current (DC). The grid delivers alternating current (AC). To charge an EV, you need to convert AC to DC. The location of this conversion is what defines AC vs DC charging.
AC charging (Modes 1–3): The AC-to-DC conversion happens inside the vehicle in the onboard charger (OBC). The EVSE provides controlled AC power to the vehicle inlet. The vehicle's OBC converts it to the correct DC voltage and current to charge the cells.
DC charging (Mode 4): The AC-to-DC conversion happens in the charging station. The station delivers DC power directly to the battery at high current, bypassing the OBC entirely. This is why DC charging rates can be 50–350 kW — the station can contain large, powerful conversion equipment that would be impossible to fit in a vehicle.
For AC charging, the maximum speed is limited by the OBC inside the vehicle — not by the EVSE. A 22 kW EVSE connected to an EV with a 7.4 kW OBC will charge at 7.4 kW, not 22 kW. For DC charging, the battery's maximum accepted current (communicated to the charger via the BMS) is the limiting factor, and this is typically much higher than the AC charging limit.
The Four IEC 61851 Charging Modes
| Mode | Connection | Power Level | Conversion Location | Typical Use |
|---|---|---|---|---|
| Mode 1 | Standard socket, no control | 1.2–2.3 kW | Onboard | Not recommended for EVs — no safety controls |
| Mode 2 | Socket + ICCB (portable EVSE with control box) | 2.3–3.7 kW | Onboard | Emergency or travel charging — slow but controlled |
| Mode 3 | Dedicated EVSE with control pilot | 3.7–22 kW (AC) | Onboard | Home and public AC charging — most common in India |
| Mode 4 | DC fast charger with digital communication | 25–350 kW | Off-board (in station) | Highway fast charging, fleet depot |
For AC charging (Modes 1–3), the maximum speed is determined by the onboard charger (OBC) inside the vehicle, not by the EVSE's rating. A 22 kW EVSE connected to a vehicle with a 7.4 kW OBC will charge at 7.4 kW — the excess EVSE capacity is simply unused. The EVSE communicates its current limit to the vehicle via PWM control pilot duty cycle; the vehicle draws only what its OBC can process. For DC fast charging, the limiting factor shifts to the battery's accepted current as communicated by the BMS — which is typically much higher than the AC limit.
Mode 3: How the Control Pilot Works
When you plug into a Mode 3 EVSE (the standard home or public AC charger), the EVSE does not simply switch on power. It first establishes a communication signal called the control pilot.
The control pilot is a 1 kHz square wave on a dedicated pin in the connector:
- 9V DC (pilot unconnected): Charger standby, no power
- 9V PWM square wave (cable connected): EVSE ready, vehicle not ready
- 6V PWM: Vehicle present, requesting charge, EVSE closes contactor
- 3V PWM: Vehicle requesting charge with ventilation (rare for sealed cars)
The duty cycle of the PWM signal (the percentage of time the signal is high) tells the EV what maximum current the EVSE can provide. 16% duty cycle = 10A maximum; 25% duty cycle = 16A; 50% duty cycle = 32A. The EV reads this and limits its OBC to draw no more than the rated current.
This safety handshake means the EV adapts to whatever EVSE it is connected to — a 32A home charger and a 10A portable EVSE both use the same protocol, and the EV adjusts automatically.
DC Fast Charging: The Communication Layer
DC fast charging requires a more complex communication protocol because the charging station must send the exact voltage and current that the battery can accept at every moment — and this changes continuously as the battery charges.
For CCS2 (the Indian standard for DC fast charging), the communication protocol is ISO 15118 or DIN 70121 over the power line communication (PLC) channel on the control pilot pin. The charging session includes:
- Identification: EV identifies itself to the charger (vehicle contract certificate or payment)
- Capability negotiation: EV communicates its maximum voltage, maximum current, and target SOC
- Dynamic control: Every 50–100 ms, the BMS sends the current actual requested voltage and current. The charger adjusts its output accordingly.
- Termination: EV signals charge complete or fault
The Hyundai Ioniq 5 and Kia EV6 use 800V battery architecture, which allows 350A of charge current to achieve 233 kW peak charging without requiring physically thicker cables (power = voltage × current; doubling voltage halves the current for the same power). For Indian highway infrastructure with 50–150 kW CCS2 chargers, 800V architecture EVs achieve the same charging speed as 400V EVs, because the limiting factor is the charger's power output, not the vehicle's capability.
Using CCS2 in India, the charger and vehicle communicate over ISO 15118 or DIN 70121 protocol via power line communication (PLC) on the control pilot pin. At session start, the BMS sends its maximum voltage, maximum current, and target SOC. Every 50–100 ms during charging, the BMS sends a dynamically updated requested voltage and current — reflecting the battery's current state of charge, temperature, and health. The charger adjusts its output to match this request. This continuous closed-loop communication is what enables the charging rate to taper smoothly above 80% SOC rather than cutting off abruptly.
India's Charging Infrastructure Reality
| Connector Type | Standard | Max Power | Deployed Count India (2025 est.) | Suitable For |
|---|---|---|---|---|
| Bharat AC-001 (3-pin industrial) | IS 17017 | 3.3 kW | ~15,000+ | 2W, 3W, small 4W |
| Type 2 AC | IEC 62196 / IS 17017 | 7.4–22 kW | ~8,000+ | 4W passenger EVs |
| Bharat DC-001 (CHAdeMO compatible) | IS 17017 | 15 kW | ~5,000 (legacy) | Older 2W/3W, being phased out |
| CCS2 DC | IEC 62196-3 / IS 17017 | 30–150 kW | ~7,000+ | Passenger 4W, some 3W |
| GB/T (Chinese standard) | GB/T 20234 | 30–250 kW | Minimal | Import vehicles only |
India's public charging network is heavily weighted toward low-power AC chargers. The Bharat AC-001 and Bharat DC-001 standards were introduced for cost accessibility but impose performance constraints that frustrate 4W EV drivers accustomed to European or Chinese charging speeds. The government's FAME-III push toward higher-power CCS2 deployment is addressing this, but the installed base will remain mixed for years.
For 4W EV drivers planning inter-city trips in India, verifying charger power ratings before departure is essential — not just charger existence. A 15 kW Bharat DC-001 charger adds only 15 kWh per hour to a 72 kWh pack (the standard Tata Nexon EV battery). A 50 kW CCS2 charger adds 50 kWh/hour — but you will never find one at 50 kW throughput because charging rate tapers above 80% SOC. Real-world plan: 30–40 minutes at a 50 kW charger adds 20–25 kWh (100–125 km of range in city driving).
India's 2W and 3W EV segments — which make up the majority of EV sales volume — have battery packs of 1–5 kWh and charge at 3.3–15 kW. CCS2 connectors and charger infrastructure are overspecified and more expensive for these vehicles. Bharat AC-001 (3.3 kW, based on a 3-pin industrial socket) and Bharat DC-001 (15 kW, CHAdeMO-compatible) were introduced as cost-accessible standards for the mass-market segment. CCS2 was mandated for 4W passenger EVs and above in 2021. The two-tier standard reflects the reality of India's EV market structure.
The Charging Curve: Why 20–80% Is the Sweet Spot
EV fast charging is not linear. The typical charging curve:
- 0–20% SOC: High constant current (CC phase) — maximum rate
- 20–80% SOC: Constant current, full rate sustained in good weather
- 80–100% SOC: Constant voltage (CV phase) — current tapers to protect cells
For a 60 kWh pack on a 50 kW charger: 20–80% (36 kWh) takes approximately 45 minutes. The additional 80–100% (12 kWh) can take another 30–45 minutes. This is why the standard advice is to charge to 80% at fast chargers and leave — the marginal time for the last 20% is disproportionate to the range gained.
Key Takeaways
- AC charging uses the vehicle's onboard charger (OBC) for AC-to-DC conversion, limiting speed to the OBC rating (typically 3.3–11 kW for Indian 4W EVs). DC fast charging bypasses the OBC, enabling 25–150 kW from the charger directly to the battery.
- The IEC 61851 control pilot protocol governs Mode 3 AC charging safety — the EVSE communicates its current rating to the vehicle via PWM duty cycle, and the vehicle self-limits accordingly.
- CCS2 is India's mandatory DC fast charging standard. Bharat AC-001 and Bharat DC-001 standards serve the lower-cost 2W/3W market at 3.3–15 kW.
- The charging curve is not linear — current tapers after ~80% SOC to protect cell longevity. The practical advice to stop at 80% during fast charging is based on real charging curve physics, not arbitrary rules.
- For inter-city trip planning in India, verify charger power ratings (kW) and connector type, not just existence. A 15 kW Bharat DC-001 charger adds only 15 km of range per 10 minutes — inadequate for highway range anxiety management.