EV Charger Design for Two-Wheelers: Battery Voltage, Packaging and Thermal Constraints

Quick Answer

Electric two-wheeler onboard chargers typically operate at 48 V–96 V battery voltages with 3.3 kW power ratings. Design priorities include compact packaging, air-cooling, AIS-138/BIS compliance, cost optimization, and compatibility with single-phase 230V residential grids. eDrift offers 48 V–96 V, 3.3 kW SiC on-board chargers specifically for 2W EV platforms with proven thermal and packaging optimization.

What You Will Learn

This guide covers 2W EV charger design considerations: battery voltage ranges, power rating selection, packaging constraints, thermal management, EMI/EMC compliance, AIS-138 certification, and cost optimization. You'll learn how to specify an OBC that balances performance, size, cost, and regulatory requirements for electric two-wheelers.

1. Battery Voltage Architectures in 2W EVs

1.1 Common Voltage Ranges

Key point: OBC must support the full battery voltage range, not just nominal voltage. A "72V OBC" should handle 63V–84V throughout the charge cycle.

1.2 Charging Profile

Lithium-ion battery charging follows CC-CV (constant current, constant voltage):

Constant Current (CC): Charge at fixed current until battery reaches max voltage

Constant Voltage (CV): Hold max voltage, current tapers as battery fills

Termination: Charge ends when current drops below threshold (e.g., 5% of rated current)

OBC requirement: Must implement CC-CV algorithm with accurate voltage and current regulation (±1%–±2%).

2. Power Rating Selection for 2W EVs

2.1 Charge Time vs Battery Capacity

Using the charge time formula:

t_charge ≈ (E_battery × DoD) / (P_OBC × η)

Examples (80% DoD, 94% efficiency):

Industry standard: 3.3 kW is the sweet spot for 2W EVs:

Acceptable charge time (1–3 hours for most batteries)

Compatible with single-phase 230V, 16A residential grids

Cost-effective for high-volume production

2.2 Grid Compatibility

India residential grid:

Voltage: 230V AC single-phase

Current: 16A standard (some 10A connections)

Power: 230V × 16A = 3.68 kW maximum

Implication: 3.3 kW OBC is the maximum practical power for standard 16A residential connections without upgrading wiring.

3. Packaging Constraints in 2W EVs

3.1 Space Limitations

2W EVs have severe packaging constraints:

OBC location: Under seat, near battery, or integrated into frame

Dimensions: Typically <200 mm × 150 mm × 60 mm

Weight: <1.5 kg preferred

Design strategies:

High-frequency switching (100 kHz–200 kHz) → smaller magnetics

Planar transformers → reduced height

Compact EMI filters

Integrated connectors and mounting

3.2 Environmental Protection

IP rating: IP65 or IP67 recommended (dust-tight, water-resistant)

Operating temperature: -20°C to +60°C ambient

Vibration: Must withstand road shocks and vibrations

Mounting considerations:

Secure mechanical attachment

Vibration damping

Connector strain relief

4. Thermal Management for 2W OBCs

4.1 Power Loss Calculation

For 3.3 kW OBC at 94% efficiency:

P_loss = 3300 × (1/0.94 - 1) ≈ 213 W

This 213 W must be dissipated in a compact enclosure, often with natural convection or small fan.

4.2 Cooling Methods

Design practices:

Maximize heatsink surface area

Position OBC for airflow (near vents or moving air)

Use thermal interface materials (TIM) between components and heatsink

Avoid enclosing OBC in sealed compartments without ventilation

4.3 Thermal Derating

At high ambient temperatures (>45°C), OBC may need to derate power to stay within safe operating limits. Example:

3.3 kW at 25°C ambient

2.5 kW at 50°C ambient

Communication: OBC should report thermal status to vehicle BMS via CAN or analog signals.

5. EMI/EMC and AIS-138 Compliance

5.1 AIS-138 Requirements

AIS-138 is the Indian automotive standard for EV supply equipment and onboard chargers. Key requirements:

Electrical safety: Isolation, creepage/clearance, ground fault detection

EMI/EMC: CISPR 25 emissions, ISO 11452 immunity

Environmental: Thermal cycling, vibration, humidity testing

Communication: CAN bus or analog signals for charging status

Certification: BIS (Bureau of Indian Standards) certification is mandatory for EV chargers sold in India.

5.2 EMI Design Considerations

2W OBCs face EMI challenges:

Compact layout → components close together

High-frequency switching → more EMI

Cost constraints → limited filtering

Design practices:

Input EMI filter (common-mode + differential-mode)

Shielded magnetics (planar transformers)

Proper grounding and PCB layout

Enclosure shielding (metal enclosure or conductive coating)

6. Cost Optimization for High-Volume 2W Production

6.1 BOM Cost Drivers

6.2 Design for Manufacturing (DFM)

Component availability: Avoid obscure or single-source parts

Testability: Include test points for production testing

Assembly: Minimize manual assembly steps

Connectors: Standard, readily available connectors

Target BOM cost: For high-volume 2W OBCs (10,000+ units/year), BOM cost should be ₹8,000–₹15,000 ($100–$180) depending on features and compliance.

7. Communication and Vehicle Integration

7.1 Communication Interfaces

CAN bus: Preferred for modern 2W EVs (charging status, fault codes, enable/disable)

UART/RS485: Simpler alternative for cost-sensitive designs

Analog signals: Enable, power-good, fault indicators (lowest cost)

7.2 Charging Indicators

LED indicators: Charging, charged, fault

Mobile app integration: Via vehicle controller (not direct OBC connection)

BMS communication: SOC, battery voltage, current, temperature

8. Common Mistakes to Avoid

Undersizing thermal management: 213 W loss in compact enclosure requires careful thermal design

Ignoring AIS-138 early: Retrofitting for compliance is costly and delays certification

Overlooking input voltage range: Must handle 180V–270V AC for unstable grids

Skipping DFM review: Manufacturing issues discovered too late increase cost

Inadequate connector rating: High-current DC output requires properly rated connectors

9. Decision Framework for 2W OBC Selection

10. Application Examples

10.1 48V Electric Scooter (2 kWh Battery)

OBC: 48V, 3.3 kW

Charge time: ~0.5 hours (30 minutes) to 80%

Packaging: Under-seat mounting, air-cooled

Compliance: AIS-138, BIS

10.2 72V Electric Motorcycle (7 kWh Battery)

OBC: 72V, 3.3 kW

Charge time: ~1.8 hours to 80%

Packaging: Frame-integrated, forced air

Communication: CAN bus with BMS

10.3 96V High-Performance 2W (10 kWh Battery)

OBC: 96V, 3.3 kW

Charge time: ~2.6 hours to 80%

Packaging: Dedicated enclosure, liquid cooling optional

Features: Mobile app integration via vehicle controller

FAQs

What is the typical onboard charger power for electric two-wheelers?

3.3 kW is the industry standard for 2W EVs. It provides 1–3 hour charging for 3 kWh–10 kWh batteries and is compatible with single-phase 230V, 16A residential grids in India.

What battery voltage ranges do 2W EVs use?

Common ranges are:

48V systems: 42V–58V (1 kWh–3 kWh)

60V systems: 52V–72V (2 kWh–5 kWh)

72V systems: 63V–84V (3 kWh–7 kWh)

96V systems: 84V–112V (5 kWh–10 kWh)

OBC must support the full operating range, not just nominal voltage.

Is AIS-138 certification mandatory for 2W chargers in India?

Yes. AIS-138 compliance and BIS certification are mandatory for EV chargers sold and used in India. Non-compliant chargers cannot be legally deployed.

How do I manage thermal constraints in a compact 2W OBC?

Use high-frequency switching (100 kHz–200 kHz) for smaller magnetics, planar transformers for reduced height, optimized heatsinks with forced air (small fan), and position the OBC for airflow.

Can I use the same OBC for 48V, 72V, and 96V batteries?

No. OBC output voltage range must match the battery architecture. However, eDrift offers a 48 V–96 V OBC family with different models covering 48V, 60V, 72V, and 96V systems, all at 3.3 kW power.

Conclusion

Designing an EV charger for two-wheelers requires balancing battery voltage range, 3.3 kW power rating, compact packaging, thermal management, AIS-138/BIS compliance, and cost optimization for high-volume production. By following this framework, 2W OEMs can specify OBCs that meet performance, regulatory, and cost targets.

eDrift Electric offers 48 V–96 V, 3.3 kW SiC on-board chargers specifically for 2W EV platforms, with proven thermal optimization, compact packaging, and AIS-138/BIS compliance.