In-car wireless mobile phone charging is now becoming familiar, mostly employing the industry standard Qi system in low power consuming conditions. The concept is also used with items like tablets, electric toothbrushes and audio headphones. They all use electromagnetic fields to safely transfer power from a transmitter to any suitably enabled battery powered device, but without the use of any physical connection. It is based on the principle of magnetic resonance, or Inductive Power Transfer, in which there are essentially four steps involved:
ïMains AC voltage of 50 Hertz is converted into a higher frequency (many kHz) form of AC.
ïThis radio frequency AC induces a variable magnetic field in the charger’s transmitter coil.
ïBy inductive coupling, the transmitter coil induces a similar magnetic field in a receiver coil in the user device, causing both coils to resonate at the same frequency.
ïCurrent flowing within the receiver coil is then converted into direct current (DC) by a rectifier in the receiver’s charging circuit, which then wirelessly charges the device’s battery.
Energy is thus safely transferred over a relatively small air gap, typically with Qi systems over distances up to four centimetres, and will also pass through non-metallic media, like thin wood and plastics. The power capacity of the various system components defines the rate at which energy can be transferred, but charging rates are generally only of a few watts, but quite enough for a full charge to be made in around three-to-five hours.
On a greatly increased scale, involving many kilowatt hours (kWh) of energy, as against the mere milliwatt hours of such mobile devices, wireless charging of electric vehicle batteries is one of the most hotly researched areas of wireless power transfer. Electric cars, whilst growing in range, still mostly use a process that involves access to a charging point, the use of charging cables, and relatively long charging times compared with that of conventional refuelling. With the introduction of wireless charging, the experience becomes radically simpler and, ultimately, probably much faster.
It’s early days yet in the UK for wireless EV charging, but existing units, such as the Plugless from US company Evatran, use a one inch thick 33-inch by 16-inch floor-mounted transmitter plate, with a 4-inch air gap to the vehicle under-body receiving unit. It offers charging rates of up to 7kW, giving a full charge in around four-to-six hours for cars like a Renault Zoe or Nissan Leaf. Parking spaces with such charging pads will become widely available in time, and it will be possible to charge your car whilst shopping, stopping at a service area, or in special spaces in multi-storey car parks, without the inconvenience of charging cables. At home you will drive into your garage and position your car over the charging plate with reasonable accuracy, and simply press a button to charge up. The latest BMW 5 Series is one of the first cars to be made available with such a feature, and will be an option on the executive saloon later in 2018.
In time though, you may not even need to stop to charge your electric car, if the test work being done by Highways England comes to fruition. With wireless inductive charging devices like this built into the road surface ñ obviously not on every road, but on many major roads ñ electric cars could keep their batteries charged up as they drive. Highways England say: ìThe concept of wireless power transfer equipment installed under the road surface is seen as a potential opportunity to extend the charging infrastructure for our customers.î How big a step further might it be then, for simpler, lighter, and thus significantly cheaper electric vehicles ñ cars, buses, and even HGVs ñ that could simply draw their power from roads more widely equipped with such supply systems, and require little, or even no, battery power storage? Is that too much to imagine?