Wireless Chargers (Inductive charging)


Wireless battery charging or wireless inductive charging as it is also called, is a method for transferring electrical energy from a charger to a device without the need for a physical wire connection.
Wireless battery charging has many advantages in terms of convenience because users simply need to place the device requiring power onto a mat or other surface to allow the wireless charging to take place.
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What Is Wireless Charging?


Situations often occur in which it is inconvenient to bring along a regular battery charger for many popular electronic items, such as cell phones, laptop computers, and portable music devices. Solving this issue is what the concept of wireless charging strives to do. As many might guess from the very name, this type of technology allows myriad electronics to charge without having wires attached. Another aspect of the idea that is often convenient for many people is the fact that most wireless chargers are able to charge nearly any device, not just a specific kind. This means that only one charger usually is needed to charge a cell phone, MP3 player, laptop, or other small mechanism that runs on electricity.
Though wireless charging is likely convenient for many, the majority of people do not understand the concept, which usually involves inductive charging. The main power behind this kind of device is electromagnetic induction, which involves making a magnetic field that does not leave the charger like most wired products do. Instead, the field flows parallel to the surface of the charger, spreading magnetic force across the entire device. Due to this action, a thin receiver coil is created within the charger, so that there are two coils instead of the usual one. The small gap between the two coils makes it possible for an electrical transformer to be created, so it does not need an outlet to obtain power.

How does wireless charging work?

To appreciate the practical difficulties in transmitting power without wires, it helps to know a little about how electricity works. When an electrical current flows down a conductor, it generates a magnetic field, orientated at right angles to the conductor.

By creating a coil, the magnetic field is amplified and if a second coil is placed within the magnetic field of the first, then an electric current will be generated in the second coil, a process known as induction.

However, because the size of the magnetic field is proportional to the energy of the current running through the coil, and the fact that inductance over distance is a fairly inefficient transfer method, the two coils have to be placed in close proximity.

In an electric toothbrush, for instance, the two coils are less than 10mm apart. In order to increase the distance between the coils, both the size of the coils and the amount of current flowing through them, has to be significantly increased, although because the magnetic fields radiate in all directions, efficiency decreases.

Is increased resonance the answer?

One way to increase the efficiency and distance over which induction can occur, is to use resonance. Every object has a frequency at which it will naturally vibrate, called its resonant frequency. Researches at MIT discovered that if you enable the coils and fields to resonate at the same frequency, it increases the efficiency of the induction and were able to demonstrate this principle by using resonating coils to power a light bulb, over a distance of two meters.

With this sort of distance, the idea of being able to walk into a room and whatever gadgets you are carrying are immediately able to receive power from a transmitter buried in the wall or ceiling starts to gain some traction. Unfortunately, even though MIT demonstrated the principle nearly six years ago, the technology is still very much in the development stage.

Intel has also demonstrated resonant power transmission, but as can be seen the coil size needed for a light bulb is huge
Intel has also demonstrated resonant power transmission, but as can be seen the coil size needed for a light bulb is huge

Using larger induction coils is one way in which to increase transmission distance. In the MIT experiment, for instance, the coils were 60cm in diameter, but only about 45 per cent of the power was transmitted at two meters. With portable electronics, their size and the amount of free space within the casing is a major limiting factor.

An electric toothbrush is only used for a few minutes a day and spends the rest of the time being charged, so can have quite small coils. However, a smartphone has a very high capacity battery and using a standard charger, needs to achieve full charge in one or two hours.

Charging up vehicles

One area where the size of the coil doesn’t really matter is in vehicles. Using specially built inductive roadways, trials have been run which enable an electric car or bus to receive power as it travels along the road. Wireless charging points built into bus stops and parking bays have also been successfully used to recharge on-board batteries, but it’s still less efficient than physically plugging a cable in.

WiTricity is one company that markets wireless charging solutions for the automotive market. The company has also demonstrated its inductive resonance technology wirelessly powering a television as well as a number of mobile phones, is supplying its technology to OEMs and believes the first products should be on the market this year.

WiTricity's system enables electric vehicles to be charged without wires, delivering 3.3kW of power
WiTricity’s system enables electric vehicles to be charged without wires, delivering 3.3kW of power

There already some products on the market, such as Duracell’s Powermat, which doesn’t use the resonance technique, so is much shorter range. In addition, devices such as mobile phones don’t yet have induction coils built in and so have to be fitted with special cases containing the necessary circuitry.

However, if there’s one sign that a technology is becoming more mainstream, it’s when car manufacturers start to adopt it. Chrysler has announced that in 2013, its Dodge Dart car will have the option for a wireless charging bin. As devices to be charged will require special sleeve or cases, it’s not clear if this is a bespoke Powermat solution, or something else, but the option to do away with a wired cigarette lighter adapter is certainly a welcome move. The $200 price tag may be a bit much to swallow though!

Chrysler will be adding an optional wireless charging system to the Dodge Dart in 2013
Chrysler will be adding an optional wireless charging system to the Dodge Dart in 2013

It seems that the technology still has quite a way to go, before it becomes an attractive proposition. Duracell has been refining the Powermat technology and has a vision where tables in bars or cafes have embedded wireless charging points. A lot of people leave their phone on the table when they are out socialising, so why not top up the battery while you’re at it? However, so long as you need to add a case of sleeve to your device, the appeal of wireless charging is limited.

Ironically, some phones such as the Samsung S3, already contain part of the technology needed for wireless charging and it’s even built into the battery. RFID, or Near Field Communication (NFC) uses very similar principles, in that a coil in the phone’s battery, induces a current in the chip that you are trying to read, which then has enough power to transmit back the required information.

The history of wireless charging

Wireless charging isn’t a new concept. In fact, it’s been around since the 19th century when physicist Nicolas Tesla came up with the idea of wireless power transfer. It was first demoed by Intel back in 2007, with the idea being that if you can do it safely and efficiently, it would work for the majority of the devices we use every day. The idea of a laptop that people could just keep using and never, ever run out of juice, that could go directly off of wireless power or charge wirelessly? Sounds like something straight out of a sci-fi movie, but if the goal is to eventually have a completely wireless experience, these are just some of the scenarios we could possibly be looking at.
Wireless charging is seen by many as one of the biggest possible advancements we could have for personal computing in this century. Cables – messy, unwieldy, and with a predilection to getting lost at the most inconvenient times – could be a thing of the past in just the next couple of years. In addition to computers, wireless charging could make its way to the automotive industry with electric vehicles, making the charging process virtually automatic.

Two kinds of wireless charging

There are two kinds of wireless charging technologies (WCT): magnetic induction and resonance charging. Basically, the difference is distance: magnetic induction requires that the receiver be in direct contact with the transmitter, or charging device; resonance charging requires that the receiver merely be placed near the transmitter for charging.
Eventually,  the technology is aiming towards an Ultrabook coming pre-built with WCT detection software, enabling users to merely place their smartphones or tablets in the vicinity of their Ultrabooks and charge away (near field communication, or NFC). This would be in a “BE-BY” configuration, whereby two different devices don’t necessarily have to be touching in order to exchange energy, as opposed to a “BE-ON” configuration.
As debuted at IDF 2012, the Ultrabook transmitter recharging configuration will actually take up very little space (21 cm ², 7 cm x 3 cm x 5 mm) within the form factor. On the receiver side, we’re talking even smaller 5.6 cm ²), so definitely no chance of our smartphones, tablets, or mice getting bigger all of a sudden.
Intel and IDT
Integrated Device Technology (IDT) will be developing and delivering integrated transmitter and receiver chipsets for Intel’s Wireless Charging Technology based on resonance charging technology, targeted for deployment within Ultrabooks, PCs, smartphones, and the plethora of other standalone devices (like Smart Watches) out there on the market.
Now, this isn’t necessarily limited to inductive charging or smartphones/tablets on a charging mat usage; Intel is working with IDT, vendors (smartphones, printers, cameras, and much more), OEMs, and other partners to make WCT a completely non-touch-based reality for the devices we use every single day.  Intel is definitely putting its money on wireless charging, and plans to build the technology into Ultrabooks by 2013, implementing transmitters into these machines with receivers built within a range of devices using Intel’s own chips.
Ultrabooks and WCT
As detailed by Intel execs this past week at IDF 2012, the battery life of Ultrabooks will be greatly increased with Intel’s upcoming Haswell processors.   Battery life will be essentially doubled, with battery life of up to ten hours for Ultrabooks, even more (12 hours or more) in the case of convertible Ultrabooks. Ultrabooks with Haswell configurations will also feature wireless charging and NFC capabilities, making that move to no cords even more of a reality.

Wireless battery charging basics

Wireless battery charging uses an inductive or magnetic field between two objects which are typically coils to transfer the energy from one to another. The energy is transferred from the energy source to the receiver where it is typically used to charge the battery in the device.
This makes wireless charging or inductive charging ideal for use with many portable devices such as mobile phones and other wireless applications. However they have also found widespread use in products such as electric toothbrushes where cordless operation is needed and where connections would be very unwise and short-lived.
The system is essentially a flat form of transformer – flat because this makes it easier to fit into the equipment in which it is to be used. Many wireless battery charging systems are used in consumer items where small form factors are essential.
Wireless battery charging concept
Wireless battery charging concept
The primary side of the transformer is connected to the energy supply that will typically be a mains power source, and the secondary side will be within the equipment where the charge is required.
In many applications the wireless battery charging system will consist of two flat coils. The power source is often contained within a pad or mat on which the appliance to be charged is placed.


Wireless battery charging advantages / disadvantages

As with any system, there are both advantages and disadvantages to wireless battery charging systems.
  • Convenience – it simply requires the appliance needing charging to be placed onto a charging area.
  • Reduced wear of plugs and sockets – as there is no physical connection, there are no issues with connector wear, etc. Physically the system is more robust than one using connectors.
  • Resilience from dirt – some applications operate in highly contaminated environments. As there are no connectors, the system is considerably more resilient to contamination
  • Application in medical environments – using wireless charging no connectors are required that may harbour bacteria, etc.. This makes this solution far more applicable for medical instruments that may require to be battery powered.
  • Added complexity – the system requires a more complicated system to transfer the power across a wire-less interface
  • Added cost – as the system is more complicated than a traditional wired system, a wireless battery charger will be more expensive
  • Reduced efficiency – there are losses on the wireless battery charging system – resistive losses on the coil, stray coupling, etc. However typical efficiency levels of between 85 – 90% are normally achieved.


Wireless charging has now become a mainstream technology. Initially it was a novelty, but with its applications and advantages becoming recognised, it has now become a mainstream application. It is anticipated that wireless battery charging will become very widespread, if not the most common method.
With standardised interfaces and techniques, only a single wireless battery charger will be required to charge a variety of items. No longer will a whole myriad of chargers be required. Also reliability and convenience will be improved as it is far easier to place the item to be charged on the charging mat, rather than having to use a small connector.
Although the efficiency of wireless battery charging is less than that using direct connections, the added intelligence could reduce the end of charge current, thereby reducing the overall power consumption as many normal chargers are left connected even when they are not charging

The obvious advantage of wireless charging is the ability to place electronics on a wireless charger device, rather than take a cell phone charger, laptop charger, or other type of charger everywhere in case the batteries run out of charge. Another less known benefit of wirelesscharging is that such chargers can be placed near water when necessary. This is because all the parts are enclosed, with no wires sticking out, so some electric razors or toothbrushes come with wireless chargers for the sake of safety. Additionally, the majority of wireless chargers can sense how much power each type of electronic device needs, so batteries are not typically overcharged.

One disadvantage of the ability to charge electronics wirelessly is the typically higher cost when compared to wired chargers. To get the most efficient wireless charging devices, it is often necessary to spend a lot of money, which usually results in the latest charger. Otherwise, older wireless chargers are frequently found to be slower at charging. They also often generate more heat than wired chargers, which can be considered a danger despite the somewhat smaller chances of electric shock when it comes to wireless charging devices.