Relay switching solution for electric car battery charging systems

The electric vehicle has long been muted as the ideal road transport, zero emission at the place of use,  quiet, efficient and eminently suitable for a wide range of transport designs.  However recharging at regular intervals outside the vehicle home is a requirement since batteries have a very limited travel distance when compared with other forms of vehicle power systems.   As such an extensive and available infrastructure for recharging electric vehicle batteries is required for electro mobility to really succeed. Based on the current German plans in the UK alone it has been envisaged that this should mean that, by 2020, at least 7,000 high power charging points and 28,000 normal power charging points should be installed.

At this time cable charging systems are enjoying the most attention and therefore are most readily available. Standard IEC 61851-1 makes a distinction between charging modes on the basis of their installation and system components. What they all have in common is that they require a switching device to connect and disconnect the electricity supply system to and from the vehicle. In the normal charging mode, this occurs in the power up stage between the charging station and the vehicle or in the event of a fault mode trigger.  Such a fault mode may be present if a leakage current is detected by means of the RCD function and triggering of the isolation monitoring device

Where the control and protective function is integrated into the cable the essential functions of the control and protection box can be summarized as switching, RCD detection, and pilot communication between the vehicle and the control box. For this purpose, the vehicle manufacturers equip their cars with an appropriate charging cable up to 32A. This is carried in the vehicle and can be connected to a regular household outlet or CEE-socket to recharge the battery.

In effect, a power relay is an ideal way to implement the isolation switching function and its advantages can be summarized as follows:

• High power density resulting from high switching capacity in a compact size
• Best current-carrying characteristics with the aid of contact resistances below 3mΩ
• Efficiency - lowest holding power - 170mW to 300mW
• Insulation capacity - clearance and creepage distances >8 mm between coil and contact sets
• Galvanic isolation of the load circuit and between the control circuit and load circuit itself

The ability of an electromechanical relay to withstand high voltage spikes between the control circuit and load circuit of over 10kV along with inrush current capacity against current spikes <250A caused by the filter system in the car charger is also of great value in battery charging circuit design.

The HE product family (with its unique HE-S relay with feedback contact) is designed for use in charging stations in the public and most importantly the private sector, in so-called wall boxes with charging capacities of up to 22kW and 43kW, on the basis of the technical characteristics mentioned above. All the relays handle the described requirements, especially for carrying the high charging currents for several hours at ambient temperatures of +85°. Even for direct sun light exposure when used outdoor the charging current doesn´t need to be de-rated.

The double-pole Panasonic HE-S relay - each contact with a 35A rating.   The HE-S relay is not only small in size at a mere 36x30x40 (L x W x H) but it is also the first relay with an integrated mirror feedback contact mechanism without external components. The armature system not only operates the two load contacts with contact gaps exceeding 3mm but it also operates a separate n/c signal contact. With welded main contacts the signal contact retains a contact opening of at least 0.5mm. This configuration is certified for mirror contacts by the VDE in accordance with IEC 60947-4-1.The current carrying capacity is raised to 48A and 90A in the HE-PV relay the 90A version also having contacts with a 3mm gap.   The HE-PV can operate with a holding coil voltage of just 40% of the nominal coil operating voltage.  This allows an operating power of approx. 300mW in use.  The range of high current carrying contacts in a very small space with highly efficient coils offers the designer the maximum flexibility alongside the highest reliability.

Electric drive is revolutionizing the automotive market and is featuring some particularly interesting systems with regard to electric propulsion, battery systems and their associated charging systems. Expectations are high that the existing systems and charging technologies will continue to be developed, especially with regard to global projects up to 2030 and 2050 for further reducing the CO2 emissions of the transport sector.  This will significantly reduce the proportion of combustion engines.  Panasonic is already making its position clear with advanced relay designs supporting the design of the highest quality battery charging systems.