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IN my last article I explained how plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs) operate.
This week we delve into their charging technologies.
When charging the PHEV or BEV from a grid-tied power source, the high-voltage system must convert alternating current (AC) power provided to direct current (DC), which the battery pack can accept. When charging commences, the vehicle must be ready to receive the power.
This requires the cells to be within a specified temperature band.
To provide optimal high-voltage battery charging, there are numerous vehicle subsystems that operate as one integrated system.
When the PHEV or BEV is plugged into the charger, the onboard high-voltage battery charger will control the voltage, charging current and charging time of the battery pack.
This will require communication with an off-board charging station.
The charging strategy has a significant effect on the service life of the battery pack.
Therefore, the onboard charger will always charge the high-voltage battery pack within the confines of an optimal charging control strategy.
Most onboard high-voltage battery chargers are liquid-cooled to minimise the heat that is created as the electric power is converted from AC to DC.
Maintaining onboard charger temperature is critical to maintaining optimal delivery of electrical current to the high-voltage battery pack.
Elevated temperature can decrease the onboard charger operation and cause premature failure if not controlled.
Elevated onboard charger temperature can cause the charger to operate at a reduced output state, affecting it by increasing the charging time of the high-voltage battery pack.
If the onboard charger temperature rises to a point that output power is reduced, a diagnostic trouble code (DTC) will be logged and the malfunction indicator lamp (MIL) illuminated to visually alert the driver and technician that a fault has occurred.
DC fast charging of the high-voltage battery pack may not be available due to battery pack, mechanical or electrical constraints.
Fast charging a high-voltage battery pack does not use the onboard vehicle battery charger.
It involves direct electrical power transfer between the charging station and the high-voltage battery pack. The vehicle battery pack controller communicates with the fast charger controller.
This is meant to control charging of the high-voltage battery pack.
The one which comes with PHEV or BEV that plugs into the home utility AC supply is not called a charger, but rather electric vehicle supply equipment (EVSE).
As the name itself suggests, it supplies AC and the onboard charger will convert this to DC to charge the high-voltage battery pack, which can only store direct current.
The latest charging technology is wireless.
Wireless charging on an electric vehicle (EV) allows the operator to easily drive over a charging mat to allow for charging.
The power is transferred through a magnetic resonance field between the transmitting pad and the receiving pad, which is attached to the vehicle.
Like any small consumer device, once the vehicle is positioned over the charging pad attached to the ground, the vehicle communicates with the charging system and then charging commences.
This ability to park the car and walk away gives the consumer peace of mind that they will have a charged vehicle when they return.
The Society of Automotive Engineers (SAE) has developed the J2954 standard to govern the wireless charging system operation. This standard sets the specification for three levels of charging power.
Wireless power transfer (WPT1) (3,7 kilowatts), WPT2 (7 kilowatts) and WPT3 (11 kilowatts) are the three levels created by the standard.
The specification on the vehicle height from the ground assembly has generated 94 percent charging efficiency at the 25,4-centimetre clearance mark.
Along with the wireless charging standard, the wireless communication control for WPT, J2847/6, sets the requirement of communication between the vehicle and the wireless infrastructure.
Before the standard was enacted, each manufacturer had utilised the proprietary communication protocol for its own application.
The shape, amperage and other requirements of the charging connector are controlled by SAE’s standard J1772.
SAE J1772 is one of many standards. It standardises the shape of the electrically conductive charge coupler.
In the case of a PHEV or BEV, the original manufacturer equipment (OEM) will utilise this standard to design the vehicle charge port and charging station connector, so the owner of the vehicle will have the ability to charge the vehicle at a variety of charging locations.
This standardisation is similar to the on-board diagnostic (OBD II) standard J1979 that became a requirement throughout the United States in 1996, standardising the accessible parameter ID (PID) data on all vehicles manufactured for sale in the US.
The world has since adopted this standard.
When charging a PHEV or BEV, the vehicle battery controller must be able to communicate with the charging station to control voltage, amperage and charging time of the high-voltage battery pack.
Charging a cold or hot battery pack is not ideal, and may cause cell failure.
Charging strategies take into account the state of charge (SOC) percentage, temperature, state of health and C-rate of the battery cells.
The C-rate reflects the time for a battery to charge and discharge; for example, a 1C-rate battery’s discharge lasts for one hour.
Thermal control of the battery pack is already a part of the pack’s operational design.
The battery pack controller will continually monitor the battery pack cells during charging to mitigate the possibility of thermal runaway.
*Taurayi Raymond Sewera is ASE and AutoCate Association-certified World Class Master Technician with 39ASEs, ASE Advanced Level Specialist L1, L2, L3 and L4, AMI-Accredited Master Electric Vehicles and Master Automotive Manager, and ACDC-certified Master Hybrid and Electric Vehicles Technician. He is the founder and CEO of TauRay Automotive. He can be contacted on: +263772341193, +263772357296 or [email protected]
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