EV charging: AC, DC fast charging and Indian standards

Why it matters

How fast and how conveniently an EV can be charged decides whether people buy one. In India most charging happens at home or work on single-phase AC, while highway and fleet operators rely on DC fast chargers, and two- and three-wheelers add battery swapping and light-EV connectors. Engineers working on vehicles or chargers must know the charging modes, connectors, the role of the on-board charger and BMS, and how to estimate charging time and grid demand.

Key ideas

AC versus DC charging.

• AC charging: the charge point (EVSE) supplies grid AC; the vehicle's on-board charger (OBC) rectifies it, corrects power factor and converts it to the battery's DC voltage through an isolated DC-DC stage. Power is limited by the smaller of the supply rating and the OBC rating — typically 3.3 kW or 7.4 kW single-phase (230 V) and up to 11–22 kW three-phase (400–415 V) in India.
• DC charging: the AC-to-DC conversion happens in a large off-board charger; it supplies DC straight to the battery through the vehicle's DC contactors, bypassing the OBC. Powers range from about 15 kW (light vehicles) to 50–150 kW and more for cars and buses.
• In both cases the vehicle's BMS is in control: it tells the charger (directly over a communication link for DC, or by limiting the OBC for AC) what current and voltage it will accept. A charger never "pushes" more power than the vehicle requests.

Charging modes (IEC 61851, adopted in India through the IS 17017 series).

• Mode 1: ordinary socket, no protection or communication (not permitted for cars in most standards).
• Mode 2: household socket with a portable cable that has an in-cable control and protection device (the "portable charger" supplied with many cars and scooters).
• Mode 3: dedicated AC charge point (wall-box) with a control pilot signal that tells the car the maximum current available and confirms safe connection before energising.
• Mode 4: DC charging from an off-board charger with digital communication.

Connectors and Indian standards. India has adopted Type 2 for AC and CCS2 (Combined Charging System, Type 2 plus two DC pins) as the main car standard, with CHAdeMO also seen on older installations, and a light-EV combined connector (LECCS) for two- and three-wheelers. Early low-power government specifications were Bharat AC-001 (three 3.3 kW single-phase outputs) and Bharat DC-001 (up to about 15 kW at low voltage). BIS standards evolve, so check the current list rather than memorising sub-part numbers.

Charging profile. Lithium-ion cells are charged constant current (CC) until the cell reaches its maximum voltage, then constant voltage (CV) while current tapers. On DC fast chargers, the BMS also limits current for temperature and SoC, so power is high from low SoC to roughly 80 % and then falls sharply — which is why fast-charge times are quoted "10–80 %". Fast charging in the cold is restricted to avoid lithium plating, so batteries are pre-heated before fast charging.

Control pilot. In Mode 3, the EVSE sends a 1 kHz PWM signal on the control-pilot wire; its duty cycle tells the car the maximum current it may draw. The car signals its state (connected, ready to charge) by changing the pilot voltage with resistors. A separate proximity pin tells the car the cable's current rating and that a plug is inserted (so it cannot drive away).

Other approaches. Battery swapping for two- and three-wheelers (supported by Indian policy), inductive (wireless) charging, and bidirectional charging (vehicle-to-load, vehicle-to-grid).

Grid and site issues. Many fast chargers at one site need a transformer and distribution capacity; harmonics and power factor are controlled by the charger's front end; time-of-day tariffs and load management spread demand.

Formulas

P = V × I × cos φ (single-phase AC); P = √3 × V_L × I_L × cos φ (three-phase)

• P: power (W), V: phase voltage (V), V_L: line voltage (V), I: current (A), cos φ: power factor (≈ 1 with PFC).

P = V × I (DC)

ΔE = E_batt × (SoC₂ − SoC₁)

• ΔE: energy that must go into the battery (kWh), E_batt: usable battery energy (kWh), SoC as fractions.

t = ΔE / (P × η)

• t: charging time (h) at constant power P (kW) measured at the supply, with charger-to-battery efficiency η. Valid in the CC region; the CV taper must be treated separately.

E_grid = ΔE / η

I_max = 0.6 × D (control pilot, for duty cycle D between 10 % and 85 %)

• I_max: maximum current the car may draw (A), D: pilot duty cycle in percent (e.g. 50 → 30 A). Other ranges use a different rule in the standard.

Worked examples

Example 1 (standard). A car with a 30 kWh usable battery is charged from 20 % to 80 % on a 230 V, 32 A single-phase wall-box (unity power factor). The OBC efficiency is 90 % and electricity costs ₹8 per kWh. Find the supply power, energy into the battery, energy from the grid, time and cost.

1. P = 230 × 32 = 7360 W = 7.36 kW.
2. ΔE = 30 × (0.80 − 0.20) = 18 kWh.
3. E_grid = 18 / 0.9 = 20 kWh.
4. t = 20 / 7.36 = 2.72 h (about 2 h 43 min).
5. Cost = 20 × 8 = ₹160.

Answer: 7.36 kW, 18 kWh, 20 kWh, about 2.7 h, ₹160.

Example 2 (GATE level). A 50 kWh (usable) car has an average pack voltage of 350 V during charging. A 50 kW DC charger is limited to 125 A. Charger-to-battery efficiency is 95 %. (a) Find the time from 10 % to 80 % assuming constant current. (b) Above 80 % the BMS tapers power to an average of 15 kW at the connector. How long does 80–100 % take? (c) A Mode 3 AC charge point sends a pilot duty cycle of 25 %. What current may the car draw?

1. Connector power = 350 × 125 = 43,750 W (current-limited, below the 50 kW rating).
2. Battery power = 0.95 × 43.75 = 41.56 kW; ΔE = 50 × 0.70 = 35 kWh; t = 35 / 41.56 = 0.842 h = 50.5 min.
3. 80–100 %: ΔE = 10 kWh; t = 10 / (15 × 0.95) = 0.702 h = 42.1 min.
4. I_max = 0.6 × 25 = 15 A.

Answer: about 50.5 min for 10–80 %, a further 42 min for the last 20 %; 15 A.

Common mistakes

• Assuming a 22 kW AC charge point charges every car at 22 kW; the OBC (often 7.4 kW single-phase) sets the limit.
• Multiplying the charger's nameplate power by time when the vehicle or the current limit allows less.
• Ignoring the CV taper — the last 20 % can take as long as the first 70 %.
• Forgetting charger and OBC losses, so grid energy and cost are underestimated.
• Using US figures (120 V "Level 1", 240 V "Level 2") for Indian supplies of 230 V single-phase and 400–415 V three-phase.
• Forgetting √3 in three-phase power, or using phase instead of line voltage with it.
• Thinking the charger decides how much current to push; the vehicle's BMS sets what it will accept.

For GATE ME

Charging questions are power–energy–time problems: single- and three-phase power, energy from SoC change, efficiency, and time at constant power, sometimes with a current limit. Practise unit discipline (W, kW, kWh, minutes) and recognising which limit (supply, OBC, charger current, battery) applies.

Quick check

1. Which component converts AC to DC during AC charging?
2. What is Mode 3 charging?
3. A three-phase 415 V, 16 A supply at unity power factor gives what power?
4. Why are fast-charging times usually quoted for 10–80 %?
5. A pilot duty cycle of 50 % allows what current?

Answers: 1. The vehicle's on-board charger. 2. AC charging from a dedicated charge point with a control-pilot signal. 3. √3 × 415 × 16 = 11.5 kW. 4. Above about 80 % the CV taper and BMS limits slow charging sharply. 5. 30 A.