On-Board Charger

What the OBC does

Mains electricity is alternating current (AC). EV batteries only accept direct current (DC). The on-board charger is the power electronics unit - physically mounted inside the vehicle - that bridges that gap during AC charging.

When you plug into a home wallbox or public AC charger, AC power flows in through the cable and into the OBC, which converts it to DC and passes it to the battery management system at the appropriate voltage and current. The OBC also handles communication with the charger, negotiating the available power and managing the charging profile.

The OBC as a speed ceiling

This is the part that catches people out. Installing a faster wallbox at home - going from 7.4 kW to 11 kW, say - only improves your charging speed if your car’s OBC is rated for 11 kW or higher. If the OBC is 7.4 kW, the car will charge at 7.4 kW from any AC source, whether it’s a 7 kW charger or a 22 kW charger.

Common OBC ratings in vehicles currently available in Australia:

• 3.7 kW: Some budget or older models
• 7.4 kW single-phase: Most mainstream EVs - Tesla Model Y, BYD Atto 3, MG ZS EV, GWM Ora
• 11 kW three-phase: Tesla Model 3 Highland, IONIQ 6, BYD Seal, Volvo EX40
• 22 kW three-phase: Renault Zoe, some Mercedes EQ models, Porsche Taycan

For Australian homeowners on single-phase power, anything above 7.4 kW AC is essentially unusable at home regardless of the car’s capability. A 7.4 kW wallbox is the practical maximum for the vast majority of residential installations.

DC fast charging bypasses the OBC

When plugged into a DC fast charger via CCS2 or CHAdeMO, the OBC is bypassed entirely. The charger’s internal electronics handle the conversion and deliver DC directly to the battery. This is why DC charging speeds are an order of magnitude faster - the OBC’s capacity is no longer the limiting factor.

OBC placement and heat

In most EVs, the OBC is packaged alongside the motor controller and power electronics in the front of the car (or in the inverter unit on some rear-motor vehicles). It generates heat under load, particularly during rapid AC charging. Extended high-power AC charging in hot ambient conditions can cause the OBC to reduce its charge rate temporarily - the same thermal management logic that applies to DC fast charging.