Relays Race to Help Bridge the Gap in EV Charging Capacity

­Data from the European Automobile Manufacturers’ Association (ACEA) highlights a huge asymmetry between demand for electric vehicles and the provision of reliable charging facilities. To meet CO2 targets, sales of electric vehicles will need to pick up massively in all EU countries. A recent study shows that up to 6.8m public charging points would be required by 2030 to reach the proposed 55% CO2 reduction for cars – equating to twenty-two times growth in less than 10 years.

Conventional Mode 4 chargers are designed to deliver a much faster charge, allowing more vehicles to be serviced by each unit per day. Increasingly, public and privately owned car parks will need to offer users the facility to charge their vehicle quickly where they park.

In addition, vehicle-to-home (V2H) and vehicle-to-grid (V2G) chargers allow the nations electric car fleets to offer new freedom and flexibility to become batteries on wheels. Electricity stored in them can be drawn into the home or the grid when the need arises, providing convenience and cost efficiency. Fast charging ensures that the EV battery can be readily replenished when the opportunity presents.

All three styles of chargers require compact, energy-efficient DC power relays for the main relay safety shutdown facility and in other areas of the design. 

Requirements for EV charging

Also called ‘DC fast charging,’ Mode 4 charging uses an off-board charger (a stationary, permanently wired DC wall box and pedestal charger system) with a DC output (Figure 1). It is the only mode in which the on-board charger in the EV is bypassed, and the DC current is delivered directly to the battery. This system offers highly efficient charging for conventional, V2H and V2G applications. The specifications include a mandatory requirement for a digital communication between the electric vehicle and the EV supply equipment as described in IEC 61851-24.

Charging of the electric vehicle can be both faster and more efficient than in AC Modes 1, 2 or 3. This mode can provide 600V DC with a maximum current of 400A. The high-power delivered in this mode mandates stricter safety features, including a disconnect facility.

EV Charging Modes

    • Mode 1 charging means connecting the EV directly to a standard household socket.

    • Mode 2 charging involves the use of the charging cable provided with the EV

    • Mode 3 charging involves a dedicated charging station or a home mounted wall box

    • Mode 4 uses an off-board charger with a DC output and bypasses the charger in the EV

Mode 4 conventional chargers

In conventional Mode 4 chargers, current flow takes place in one direction only, from the charger to the vehicle. The main function of the relay is to reliably provide charge current and to make and break the supply circuit under IEC regulated signal protocol - normally switching at low or zero load. While charging is being carried out, the relay should maintain a very low, stable contact resistance whilst providing a fast-acting emergency disconnect if required. A monostable SPST-NO configuration in the positive rail is generally used in combination with additional STST-NO in the negative rail to provide additional disconnect security in the event of one of the contacts becomes welded. This is particularly important in bi-directional applications. In any case the EV-Charger manufacturer must be able to continuously detect the relays operational condition and disconnect to stop current flow. A pre-charge circuit consisting of SPST-NO relay usually in series with a resistor can be optionally included depending on level of DC working voltage and number of operate times under high load turn on conditions. This arrangement provides DC inrush protection further helping to extend the contact life of the main relay. The charger manufacturer determines durability specification to follow including safety isolation and any approval certification.

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Figure 2: OMRON G7L-2A-X 12 DC used for the main relay disconnect in this conventional Mode 4 charger design

Some chargers use circuits that enable the switching of the output voltage to charge both high-voltage battery (1,000V) vehicles i.e., electric trucks and buses and low-voltage battery (500V) electric passenger vehicles. In this application, relays are used in the switching circuit to provide complete physical insulation between the outputs for safety.

V2H and V2G chargers

Mode 4 V2H systems allow the EV battery to supply power as part of the household’s electricity system. In V2G systems, the EV battery is generally connected via an inverter and meter to export energy to the grid instead of using it for home consumption. In both cases, the essential difference is that current flow is bi-directional, which has implications for the choice of relay. 

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Figure 3: G9KB relay offers bi-directional functionality

Relays for Mode 4 chargers

OMRON supplies compact, energy-efficient relays for the main relay safety shutdown in conventional V2H and V2G chargers, as well as solutions for the pre-charge circuit providing inrush current protection.

For the main disconnection relay, as an alternative to G7L-X (polarized DPST-NO dual break configuration) the SPST-NO (non-polarized, dual break) G9KB is a new addition to OMRON’s line-up of high-power DC relay switching products. Rated for DC600V / 50A the device is designed for single or bidirectional high-power DC switching. It saves space and simplifies the design by enabling the charge and discharge function to be switched by the same relay providing safety related function is confirmed as detailed earlier. Features include low initial contact resistance for lower heat dissipation and an exceptionally low coil power consumption of just 570mW.

For DC pre-charge, options include the highly compact G2RG-X, a durable 500VDC/ 10A power relay The G2RG-X features a low coil power consumption of just 0.8W and has just one coil as opposed to two, which are more generally available in the market. An alternative is the also highly compact G5PZ-X. A single G5PZ-X relay, at just W15.2 x L26.4mm, covers 200VDC/20A, but two relays connected in series will switch up to 400VDC. Both include an arc control feature in common to G7L-X and new G9GB to maximise life.

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Figure 4: the G2RG-X is a highly compact solution for pre-charge and limiting inrush current

Conclusion

Addressing the asymmetry in demand identified at the start of this article means providing not only a greater number of chargers, but also increasing charging speeds. As so often, electromechanical relays, thought of as unglamorous components, are as critical to addressing this issue as sexier component solutions. The widespread provision of safe, convenient Mode 4 units will transform the availability of public charging facilities. In addition, an increase in the use of V2H and V2G charging points will give EV owners with a private charging facility a whole new level of flexibility and freedom. 

OMRON Electronic Components Europe