EV Charging Infrastructure Interview Questions & Answers

eMobility Academy — EV Charging Infrastructure interview questions

1. Q1.What is the difference between AC charging and DC charging?

   Fresher

   In AC charging, the charger (more correctly the EVSE) supplies grid AC to the vehicle, and the vehicle's on-board charger converts it to DC for the battery. Charging power is limited by the OBC rating, typically 3.3 kW, 7.4 kW single-phase or 11 to 22 kW three-phase. The EVSE itself is relatively simple: protection, a contactor, metering and the control pilot. In DC charging, the AC-to-DC conversion happens inside the off-board charger, which feeds DC directly into the battery through the vehicle inlet, bypassing the OBC. Power can be 15 kW to 350 kW, limited by the charger, the vehicle's battery and its charge acceptance. DC chargers are far more expensive and need digital communication with the vehicle's BMS.

2. Q2.What are Bharat AC-001 and Bharat DC-001?

   Fresher

   They are early Indian public charger specifications issued around 2017 by a committee under the Department of Heavy Industries, aimed at low-cost charging for the low-voltage vehicles common at the time. Bharat AC-001 provides three independent 3.3 kW outputs, each 230 V, 15 A, using industrial IEC 60309 sockets, about 10 kW in total, with OCPP connectivity for remote management. Bharat DC-001 is a low-voltage DC fast charger of up to 15 kW, using the GB/T connector with CAN-based communication, intended for vehicles with low-voltage packs. Both are now considered legacy: modern public charging in India uses Type 2 AC and CCS2 DC, and the BIS IS 17017 series aligns with IEC standards.

3. Q3.What connector standards are used in India?

   Fresher

   For cars: Type 2 (IEC 62196-2) for AC charging, single- or three-phase, and CCS2 (Combined Charging System, IEC 62196-3), which adds two large DC pins below the Type 2 interface, for DC fast charging. Most new electric cars in India have a CCS2 inlet. CHAdeMO was used by a few early imports and is now rare. GB/T appears in Bharat DC-001 and some older low-voltage applications. For two- and three-wheelers, many OEMs used proprietary connectors or the household 15 A plug; BIS has since standardised a combined AC and DC light EV connector, often called LECCS, to bring interoperability to small vehicles. Plain 16 A industrial and 15 A household sockets are still widely used for slow charging.

4. Q4.Explain charging modes 1 to 4 as defined in IEC 61851.

   Fresher

   Mode 1: the vehicle plugs directly into a standard socket with no control or protection in the cable; it is not allowed for cars in most markets. Mode 2: a standard socket with an in-cable control and protection device (IC-CPD) that provides a control pilot and residual current protection; this is the portable 'granny' charger. Mode 3: a dedicated AC EVSE with a control pilot, protection and a Type 2 socket or tethered cable; this is the home wallbox and most public AC chargers. Mode 4: a DC charger where conversion is off-board and the charger communicates digitally with the vehicle, as in CCS2. In India, IEC 61851 is adopted through the IS 17017 series.

5. Q5.How does the control pilot signal work in AC charging?

   Fresher

   The EVSE generates a ±12 V, 1 kHz PWM signal on the CP line through a 1 kΩ resistor. The vehicle connects a resistor and diode to ground, which pulls the positive level down, and the EVSE reads the state: State A +12 V, not connected; State B +9 V, connected but not ready; State C +6 V, ready to charge; State D +3 V, charging with ventilation required; State E 0 V, error; State F −12 V, EVSE not available. The duty cycle tells the vehicle the maximum current: for 10 to 85% duty, current is duty × 0.6 A, so 53% duty means about 32 A. A 5% duty signals that digital communication (ISO 15118) is required. The EVSE closes its contactor only in State C or D.

6. Q6.What is the proximity pilot in a Type 2 connector?

   Fresher

   The proximity pilot (PP) lets the vehicle and EVSE detect that a plug is inserted and, in Type 2, determine the current rating of the cable. A resistor between PP and protective earth in the plug encodes the cable capacity: about 1.5 kΩ for 13 A, 680 Ω for 20 A, 220 Ω for 32 A and 100 Ω for 63 A. The EVSE limits current to the lower of its own rating and the cable rating, so an undersized cable is never overloaded. The vehicle uses PP to prevent driving away while plugged in and to start the locking sequence. Many chargers report a PP fault when the cable resistor is damaged, which is a common field issue.

7. Q7.What is OCPP and why is it important?

   Fresher

   OCPP (Open Charge Point Protocol) is an open protocol from the Open Charge Alliance for communication between a charger and a central management system (CMS or CSMS). It lets an operator manage chargers from any manufacturer: authorise users, start and stop sessions remotely, read meter values, receive status and fault notifications, update firmware, change configuration and apply smart-charging limits. OCPP 1.6J (JSON over WebSocket) is the most widely deployed version in India; OCPP 2.0.1 adds better device management, security profiles, ISO 15118 support and richer transaction events. OCPP prevents vendor lock-in, which is why tenders and most Indian CPOs require it. OCPI is a different protocol, used for roaming between operators.

8. Q8.What does a CCS2 DC charging session look like from plug-in to unplug?

   Experienced

   After plug-in, the CP goes to State B and the charger sets 5% duty to request digital communication. The charger and vehicle PLC modems pair through SLAC (signal level attenuation characterisation) over HomePlug Green PHY on the CP line. Then IPv6 communication starts with SDP discovery and application protocol negotiation (DIN 70121 or ISO 15118-2/-20), followed by SessionSetup, ServiceDiscovery, payment selection and Authorization. In ChargeParameterDiscovery they exchange voltage and current limits. The charger performs CableCheck (an insulation test of the DC circuit), then PreCharge to match its output voltage to the battery before the vehicle closes its contactors. PowerDelivery starts the CurrentDemand loop, where the vehicle requests current every few hundred milliseconds. At the end come PowerDelivery stop, WeldingDetection and SessionStop.

9. Q9.What is ISO 15118 Plug & Charge?

   Experienced

   Plug & Charge lets a vehicle authenticate and authorise payment automatically when plugged in, without an app or RFID card. It is based on ISO 15118 and a public key infrastructure: the vehicle holds a contract certificate issued by a mobility operator, and the charger and vehicle use TLS with certificates rooted in a V2G root CA. The charger forwards the contract identity to the CSMS, which verifies it with the mobility operator and starts billing. ISO 15118-2 introduced it for AC and DC; ISO 15118-20 adds bidirectional power transfer, wireless charging and an improved security model. Implementation challenges include certificate provisioning across OEMs, CPOs and PKI operators, which is why adoption lags behind the standard.

10. Q10.How do you design the electrical supply for a site with four 60 kW DC chargers?

    Experienced

    Four 60 kW chargers need about 240 kW of output; at roughly 94 to 95% efficiency and a power factor close to 1, that is about 255 to 260 kVA of input, plus auxiliaries, lighting and future expansion. Options: apply for an HT connection with a dedicated transformer (for example 315 or 500 kVA, 11 kV/415 V) or use load management to cap the site below a smaller sanctioned load. Size the LT panel, cables (current, voltage drop and derating for Indian ambient temperatures), and protection with discrimination: ACB or MCCB at the incomer, MCCBs per charger, earthing to IS 3043 with low earth resistance, surge protection and metering. Check the state's EV tariff and the DISCOM's process, since timelines and costs vary widely by state. Model the business case with the charging station ROI calculator.

11. Q11.What is load management or smart charging and how is it implemented with OCPP?

    Experienced

    Load management keeps total site demand under a limit, set by the sanctioned load, transformer rating or a time-of-day tariff, by dividing available power among active chargers. Static load management sets fixed caps; dynamic load management measures the building or site load with a meter and adjusts charger limits in real time. In OCPP 1.6 the CSMS sends SetChargingProfile messages with limits and schedules at three levels: ChargePointMaxProfile (the whole charger), TxDefaultProfile (default for new sessions) and TxProfile (a specific transaction). OCPP 2.0.1 extends this with more flexible profiles and ISO 15118 integration. Common strategies are equal sharing, first-come priority, and priority by fleet or SoC.

12. Q12.What residual current protection does an AC EVSE need?

    Experienced

    Vehicle on-board chargers can produce smooth DC fault currents, which can blind a standard Type AC RCD, and even Type A RCDs, so an EVSE needs protection that detects them. The two accepted approaches are a Type B RCD, which detects AC, pulsating DC and smooth DC residual currents, or a Type A RCD combined with a 6 mA DC residual current detecting device (RDC-DD) as defined in IEC 62955, which many wallboxes integrate. Typical trip settings are 30 mA AC for personal protection. Installation must follow the wiring rules (IEC 60364-7-722 and the Indian equivalents) and the Central Electricity Authority safety regulations, with proper earthing. An electrician installing a wallbox on a plain MCB is a frequent safety issue in Indian homes.

13. Q13.A DC charging session fails at CableCheck. What do you investigate?

    Experienced

    CableCheck is the charger's insulation test of the DC output and cable before power transfer; a failure means its insulation monitoring device saw low resistance or the test voltage did not stabilise. Check whether it fails with one vehicle or all vehicles. If all, suspect the charger: moisture in the gun or cable, damaged cable insulation, a faulty IMD, DC contactor or output relay issues, or a power module not reaching test voltage. Use an insulation tester on the cable and gun pins to PE with the charger isolated. If only one vehicle model fails, suspect vehicle-side issues, such as Y-capacitance on the vehicle inlet side or its own isolation fault, or a protocol timing incompatibility. Logs from the charger and CSMS, and a PLC trace if possible, are the key evidence.

14. Q14.What causes interoperability problems between DC chargers and vehicles?

    Experienced

    Common causes: protocol implementation differences in DIN 70121 or ISO 15118 (timeouts, optional parameters, unexpected message order); SLAC pairing failures from PLC noise or crosstalk between adjacent chargers; voltage and current range mismatch, for example an 800 V vehicle on a 500 V charger, or a charger's minimum voltage above a small pack's voltage; pre-charge tolerance differences; isolation and Y-capacitance limits; and firmware bugs on either side. Indian CPOs often see a new car model failing on one charger brand until a firmware update is released. Good practice: maintain a vehicle compatibility matrix, capture PLC logs, use protocol testers, and take part in interoperability testing events with OEMs.

15. Q15.What are the key performance and reliability metrics for a charging network?

    Experienced

    Operational metrics: uptime or availability (percentage of time chargers can deliver a session, typically targeted at 97 to 99%), session success rate (successful sessions over attempts, the metric drivers feel most), mean time to repair, and fault rate by type. Commercial metrics: utilisation (hours used per day or energy delivered as a share of capacity), revenue per charger, energy cost and demand charges, and payback period. Customer metrics: average wait time, charging speed delivered against nameplate, app ratings. Utilisation is the biggest lever on profitability, and most public chargers in India have historically run at low utilisation outside highway corridors and fleet hubs, which is why site selection matters so much.

16. Q16.How do you choose a location for a public fast charger?

    Experienced

    Look at demand: EV density in the area, traffic counts, fleet operators nearby (ride-hailing, delivery, e-buses), and dwell-time behaviour, since a mall suits AC or medium DC while a highway stop needs fast DC. Look at supply: available grid capacity, distance to the transformer, the DISCOM's connection timeline and EV tariff. Look at site economics: land lease or revenue share with the host, visibility and access, 24×7 security, amenities like toilets and food on highways, and space for future chargers. The Ministry of Power guidelines have encouraged public charging roughly every 3 km in cities and every 25 km along highways. Check competition and the vehicle mix, such as whether many local cars accept only 50 kW, before choosing power ratings.

17. Q17.What maintenance does a DC fast charger need?

    Experienced

    Preventive maintenance typically includes: cleaning or replacing air filters and checking fans or the liquid cooling loop, since dust in Indian conditions is the leading cause of power-module derating and failure; inspecting guns, cables and holsters for damage, pin wear and moisture; checking contactors and relays for wear; verifying IMD and RCD operation; torque checks on power terminals and thermal imaging of connections; checking surge protection devices; verifying meter accuracy and display or payment terminal function; and confirming firmware and OCPP connectivity. Corrective maintenance relies on remote diagnostics from the CSMS so a technician arrives with the right spare module. Keeping a spare-module pool for each charger model sharply reduces downtime.

18. Q18.How does CHAdeMO differ from CCS2 and GB/T in communication?

    Experienced

    CCS2 uses power line communication (HomePlug Green PHY) superimposed on the control pilot, carrying IP-based messages defined by DIN 70121 or ISO 15118. CHAdeMO and GB/T DC both use CAN bus on dedicated communication pins between charger and vehicle, with their own message sets (GB/T DC uses GB/T 27930), plus analog signalling lines for sequence control. CHAdeMO supported bidirectional power early on, which made it popular in V2G pilots, while CCS gained bidirectional capability with ISO 15118-20. In India, CCS2 has become dominant for cars, GB/T survives in Bharat DC-001 and some legacy fleets, and CHAdeMO is largely legacy.

19. Q19.What tariff and regulatory factors affect charging business in India?

    Experienced

    Charging is treated as a service, not a sale of electricity, so CPOs do not need a distribution licence; the Ministry of Power charging guidelines set this out along with timelines for connections and a ceiling approach for tariffs at public chargers. State electricity regulators set EV-specific tariffs for public charging connections, often single-part tariffs without demand charges or with time-of-day rates, and these vary widely by state. GST applies to the charging service. Land at government sites may be offered on revenue-share models. Central schemes such as PM E-DRIVE have funded public charging. Always check the current state tariff order and guidelines, since this area changes frequently.

More free EV career resources: emobility.academy/career-resources