The on-board charger
The converter that turns wall AC into battery-ready DC.
- Explain the on-board charger at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive system
Overview
Every time an EV charges from a household socket or public AC point, the on-board charger (OBC) does the real conversion work — DC fast chargers, by contrast, bypass it entirely by supplying DC directly.
In depth
Two conversion stages
The OBC contains a power-factor-correction (PFC) front end that rectifies the incoming AC and shapes the input current to look resistive to the grid, followed by an isolated DC/DC stage that steps the resulting DC bus down (or up) to the battery's voltage while providing galvanic isolation.
Charging Time Calculator
How long a session takes between two states of charge, including charger efficiency.
t (h) = capacity × (target − start) ÷ 100 ÷ (power × efficiency)Only the energy between the two states of charge has to be delivered. Losses in the cable, on-board charger and cells mean the wall power is higher than the power reaching the cells, so we divide by the efficiency.
Example: A 60 kWh pack charged from 20% to 80% needs 36 kWh. At 50 kW and 92% efficiency: 36 ÷ (50 × 0.92) ≈ 0.78 h ≈ 47 minutes.
- Real DC sessions taper above ~60% SOC, so the result is optimistic for target values above 80%.
- AC charging is limited by the car's on-board charger, not by the wallbox.
- Cold packs accept far less power until preconditioning has warmed them.
Key takeaways
- The OBC converts AC input to DC suitable for the battery; DC chargers skip it entirely.
- It contains a PFC stage followed by an isolated DC/DC stage.
- Isolation protects both the vehicle and the electrical grid from cross-faults.
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