AC vs DC bidirectional charging: what is the difference?

AC vs DC bidirectional charging: the short answer

The main difference between AC and DC bidirectional charging is where power conversion happens. An EV battery stores DC electricity, while homes, buildings and the public grid use AC. In an AC system, the vehicle normally converts power through a bidirectional onboard charger or inverter. In a DC system, that conversion sits in the off-board charging equipment.

Neither label guarantees a working V2G, V2H or V2B service. The vehicle, charger, firmware, communication, energy-management software, metering, electrical protection and grid connection must all support the intended use.

How AC bidirectional charging works

An AC wallbox supplies alternating current to the vehicle. For energy export, the vehicle needs onboard power electronics that can work in reverse, converting battery DC into grid-compatible AC.

This can suit lower-power charging at homes, workplaces and apartment buildings because much of the conversion hardware remains in the vehicle. Compatibility is closely tied to the vehicle, however. An AC charger cannot make a one-way onboard charger export electricity.

How DC bidirectional charging works

DC charging bypasses the vehicle’s onboard AC charger during power transfer. A bidirectional DC charger exchanges DC with the battery and uses an external inverter to convert exported energy into AC for the site or grid.

Off-board conversion can give the charging-equipment designer more control and can support higher power than a typical AC wallbox. It also tends to require larger equipment, more site space and more detailed planning for protection, cooling and grid connection. Cost and efficiency still depend on the product and installation, not only on whether it is AC or DC.

CharIN’s DC bidirectional interoperability guide covers communication using ISO 15118-20. It also makes clear that communication guidance does not replace the electrical safety and product requirements for a production system.

Is AC or DC bidirectional charging better?

Neither route is universally better. AC can fit distributed, lower-power installations when compatible vehicles provide bidirectional onboard conversion. DC may suit projects that need off-board conversion, different vehicle interfaces or higher site power.

Before choosing, check:

• which vehicle models, connectors and firmware are supported;
• maximum charging and export power, including phase requirements;
• where the inverter, heat and maintenance burden sit;
• metering, anti-islanding protection and export limits; and
• which standards, backend functions and grid approvals are active.

The UK V2X Innovation Programme includes both approaches: V2VNY is demonstrating AC V2G, V2B and V2V, while Papilio3 uses a 30 kW DC V2X microgrid. AC and DC are therefore system architectures, not different names for the end use.

Standards and software still matter

ISO 15118-20 defines communication between the EV and charging equipment for bidirectional power transfer. It does not choose a tariff, control a building or grant permission to export. OCPP covers the separate connection between the charger and its CPMS. Energy and grid functions may sit with other specialist systems.

How amina C2 and M2 fit

amina C2 and amina M2 are AC charging platforms with hardware prepared for ISO 15118-20, Plug & Charge and bidirectional charging. A live AC export service still needs a vehicle with compatible bidirectional onboard power electronics and firmware, together with suitable control software and an energy partner.

amina M2 also provides MID-certified metering, Ethernet, Modbus RTU and local load balancing. These support commercial integration and local energy control, but do not by themselves make a vehicle or site bidirectional.