How to Select an On-Board Charger: A 7-Step Checklist for OEM and Tier-1 Buyers

Choosing an on-board charger looks like a power rating decision. In practice it is five decisions tied together — battery voltage, charging power, cooling, communication and the standards of the market you sell into — and getting one wrong forces a redesign later. This is the sequence we walk through with OEM and Tier-1 engineering teams before a single part number is discussed.

Step 1: Fix the battery platform

400 V or 800 V. This is the first question because it sets the isolation barrier, the output stage and the creepage distances inside the unit. Grapcharger OBC platforms are specified to a voltage window rather than a fixed value, so a mid-life move to a higher-voltage pack does not automatically force a new charger — but it must be declared up front.

Why the window matters

A charger quoted at “400 V” with no range is unusable on a pack that charges to 470 V at the top of its curve. Always ask for the output voltage window, not the nominal figure.

Step 2: Match charging power to the market

3.3 kW suits two-wheelers and small city cars. 6.6 kW is the European passenger-car default for single-phase. 11 kW is the three-phase expectation in much of Europe. 22 kW appears on premium platforms and in markets with three-phase domestic supply. Over-specifying costs mass, cost and cooling capacity that the vehicle may not have.

Step 3: Decide the cooling concept

Forced air is simpler and cheaper and takes the coolant loop out of the vehicle entirely. Liquid cooling gives higher power density and suits a packed engine bay, but adds a pump, a radiator connection and a leak-test step. The honest answer is that cooling is decided by the vehicle architecture, not by the charger supplier. Our thermal management note covers the trade-off in detail.

Step 4: Pin down the interface

CAN 2.0B is standard; an enable line is available where a platform has no charger node. The important part is not the bus, it is the signal list. Confirm which messages the vehicle will implement — charge enable, current limit, status, fault — and get the DBC before integration starts, not during it.

Step 5 and 6: Protection, environment, compliance

Protections are standard across the range: output overvoltage, overcurrent, short circuit and reverse polarity, input under- and overvoltage, and over-temperature. Environment is -40 °C to +85 °C operating with IP67 as standard and IP6K9K on request. Compliance is where programmes slip: UNECE R10 for EMC, and ISO 26262 where a safety goal is allocated to the charger. Both need to be in the plan at concept stage, not at validation.

Step 7: Decide on bi-directional now

V2L, V2V and V2G reuse the primary power stage for the reverse path. Including the capability at design time is far cheaper than adding a dedicated inverter later, which brings its own housing, cooling and harness. See bi-directional charging and V2X.

The buyer checklist this suggests

• Ask for the output voltage window, not the nominal pack voltage.
• Ask what the power rating is measured at — full load, rated input, and at what ambient. Efficiency quoted at 25 °C is not the number you integrate with.
• Ask for the derating curve, not a single current figure.
• Ask which CAN signals are mandatory and get the DBC before integration.
• Ask what compliance evidence exists — test reports, not statements of intent.
• Ask about bi-directional capability, even if you do not need it this programme.

Conclusion

An on-board charger is not a commodity part you buy on power rating. Seven questions — platform, power, cooling, interface, protection, environment and compliance — determine whether the unit drops into your architecture or forces it to change. Ask all seven in writing before the concept is frozen.

Related questions → Grapcharger FAQ