Charging electric cars is not without its challenges

February 15th, 2019, Published in Articles: EE Publishers, Articles: EngineerIT

For most people, the biggest challenge of an electric car is to keep the battery charged for long distance travel. However, for others it is more about how to fast-charge the car battery without destroying their own and immediate neighbours’ radio and TV reception and interfering with the wireless local area network (LAN) and the internet.

The challenge of travelling long distance and keeping the car driving will be resolved when more electric cars are on the road and it becomes more viable for filling stations to complement their fuel services with fast electric charging stations. The uptake of electric vehicles in South Africa is slow. Recent media reports estimate that South Africa will have 145 000 electric vehicles (EV) within the next six years and that the number will grow annually by 43 000.

In partnership with the electric vehicle charging company, GridCars, Jaguar has invested R30-million in 82 public EV charging stations across South Africa. Nissan and BMW have added charging facilities at some of their dealerships and are assessing installations on major routes. But looking at the current spread of EV charging stations, a driver travelling from Johannesburg to Cape Town would have to carefully calculate how to transit across the Karoo.

There are an estimated 5000 fuel filling stations in South Africa. EVs could challenge that market because EV charging stations can be installed at any venue where there is an adequate main power supply, such as a shopping mall. This could have a negative impact on traditional filling stations. The option of powering these charging stations by solar-powered systems is another option already widely implemented by in the US by Elon Musk. From a cost perspective, to charge an EV with a 450 km range would cost between R300 and R350.

There are different methods of charging EVs, either by connecting the car to the mains or a wireless transfer system which is fitted in the owner’s garage floor which automatically charges the car when the car is parked for the night.

Charging times vary. Different car models are fitted with different sized batteries from 24 to 90 kWh and different chargers require different charging times. Charging typically takes eight hours overnight, while charging time at a public fast-charging station may vary from 30 – 72 minutes for an 80% charge. Smaller vehicles with 27 kWh batteries have a driving distance of 195 km while models with 90 kWh batteries have a typical driving range of 420 km. All EV manufacturers have a qualifier that the driving distance is dependent on the style of driving.

There is no doubt that EVs are environmentally friendly but there is very little discussion around the unintended consequences that EVs are responsible for. For instance, in South Africa most power that will be used to charge the EV batteries is generated by burning fossil fuels and, on a more technical level, there is the possibility of chargers and in particular wireless chargers (wireless power transfer or WPT) generating excessive radio frequency noise that may interference with communication systems in the vicinity.

WPT poses the biggest threat

Fig. 1: Typical illustration of WPT. At the receiver end the current is rectified. To conserve battery life, it is recommended that EV batteries are charged to 80% of capacity. Chargers include circuits that will shut off charging when that point is reached.

WPT dates back to 1890 when inventor Nikola Tesla experimented with transmitting power by inductive and capacitive coupling using spark-excited radio frequency resonant transformers, now called Tesla Coils, which generated high AC voltages. Early in his career he attempted to develop a wireless lighting system based on near-field inductive and capacitive coupling and conducted a series of public demonstrations where he lit Geissler tubes and even incandescent light bulbs from across a stage. He found he could increase the distance at which he could light a lamp by using a receiving LC circuit tuned to resonance with the transmitter’s LC circuit using resonant inductive coupling. Tesla failed to make a commercial product out of his findings but his resonant inductive coupling method is now widely used in electronics and is currently being applied to short-range wireless power systems.

The concerns that such systems may be creating unacceptable interference to radio communication are as a result of harmonics created across the entire high frequency (HF) and even very HF (VHF) spectrum. To achieve fast charging high currents in the order of up to 22 kW are required to create strong magnetic fields. Currently there are no agreed limits that these systems have to operate within. During discussion at International Special Committee on Radio Interference (CISPR) meetings, the EV industry put forward that EV chargers should comply to the same limits as short-range devices in the home environment that use low current for short durations. EV chargers do not fit this profile. Electromagnetic interference (EMI) standards for short-range devices were developed on a wholly different set of assumptions about duty cycle, location and whether the victim and emitter share the same frequency. These EMI limits are not appropriate for WPT(EV). WPT(EV) has a high duty cycle, located in residential areas and its harmonics are likely to be spread across a band of frequencies. A similar situation existed with power line telecommunications and this was resolved by a 40 dB improvement in the emission mask for all amateur bands and similar provisions for the SW broadcast bands. Radio amateurs in Europe seek appropriate limits to the levels of WPT(EV) spurious emissions to ensure that radio communications services can operate as intended.

Discussions are continuing at various standards bodies. Governments in countries that are the major producers of EVs are under pressure from the manufacturers not to support the call for stricter EMI limits. While the problem is currently not of great importance to South Africa at this point in time, it can come to bite us if South Africa is not actively participating in the CSIPR discussions. It is up to TC73 of the SABS to ensure that the best possible EMI limits are agreed and for ICASA to regulate these standards and enforce compliance.

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