How it works: Kinetic phone charger

What is it?

As mobile phones get loaded with more functions and higher speeds, they tend to get used for longer periods than if they were used only as a phone. These functions use up battery energy and it is not uncommon to find that a mobile phone needs to be plugged in for a recharge at least once a day. If the phone user forgets to recharge in the night or he does not have access to a power outlet, the mobile phone battery could run out when he is using it. Many attempts are being made worldwide to find a solution to recharging a battery "on-the-go".

Kinetic phone charger

The simplest means first adopted were to carry a spare charged battery that could be swapped with the drained battery. Other devices invented included a wind-up charger (derived from the mechanism used for wind-up toys) and an external AA or AAA battery pack that could recharge the mobile phone. Researchers have always been fascinated with the idea of using body motions of the phone user to recharge mobile phones. An early use of this idea was in the 1980s, when Citizen, Japan, made a wrist- watch that was powered entirely by the hand movements of the user. The concept was widely celebrated but the watch, however, was not a commercial success.

Kinetic chargers are devices that convert body movements into electric charge through the use of piezoelectric crystals. This charge is either directly used to charge the mobile phone battery or is stored in another battery which then externally connects to the mobile phone. In the current state of development, kinetic chargers do not generate enough energy to fully recharge a drained mobile phone battery. Instead, kinetic chargers are used for "top-up" of charge between phone usage times so that the battery life gets extended.

Consumer electronics products including mobile phones, tablet PCs and music players are estimated to use some 600 tera-watthours of power annually and growing 10 percent per year. Any means to draw a part of this power from the untapped human motion source would be welcome.

Materials Used

Kinetic chargers are built around the piezoelectric crystal. These were initially made of materials such as quartz (SiO2) or Barium Titanate (Ba TiO3) but are now largely made of ceramic. When the piezoelectric crystal is subjected to mechanical forces (compression or tension or torsion) they generate a high voltage electric charge across their surface. Two common applications illustrate the wide range of possibilities with piezo crystals. The first is the gas lighter, used to light up an LPG stove. In this, a spring hammer strikes the crystal, producing the charge that causes an electric arc to strike across an air gap to light the gas. The second familiar application is the microphone, where the air pressure variation due to the different frequencies in speech or singing, causes the voltage induced in the piezo crystal to vary and gets reproduced as a voice signal through amplifiers.

This charge produced by the piezo crystal is momentary, when the force is applied. This charge is led off by an electronic circuit and stored in a capacitor. Each time the piezoelectric crystal is subjected to a force, a burst of energy gets generated. It is this energy that serves to recharge the mobile phone battery.

How does it work?

Various approaches have been used for kinetic recharging of the phone battery. One is of course to build the piezoelectric transducer into the phone itself so that when the user carrying the phone walks or jogs, the movement of the phone in his pocket or purse generates the charge. Such a phone can also be mounted on a key-ring and twirled around a finger to produce a quick charge. With built-in transducers, it is also possible to strap the phone on to an arm or leg when walking or jogging that would recharge the battery.

Since the need for kinetic recharging is not for every phone, the building-in of piezo crystals and circuitry would add to the cost of all phones. Some others instead conceived of a kinetic charger device separate from the phone. One such example is the Viber Burst which is a small computer mouse like package that houses the piezo crystals and a battery that stores the energy generated from movement of the device. When the mobile phone battery runs down it gets a quick recharge from the Viber Burst. Once the concept of the kinetic charger being separate from the phone is adopted, many alternative packaging and charging methods become possible. Thus we see kinetic chargers designed for powering from a bicycle wheel, from a foot pump like the one used to inflate a bicycle tire and indeed from a wind-up mechanism of the type used in toys.

In addition to a number of independent designers and small companies producing different designs of kinetic chargers, Nokia, one of the leaders of the mobile phone industry, filed for a patent in 2008 for a kinetic charger. The major impediment to wider use of kinetic chargers is the unfortunate lack of standardization in the batteries used in mobile phones and the voltages and plug connection points for recharging. An attempt has been made in 2010 by a team of mobile phone manufacturers to standardize on the micro-USB port for recharging and to adopt a common voltage specification. The adoption of such a standard could give a fillip to the deployment of kinetic chargers. The introduction of these for mobile phones would, logically, extend to the other portable consumer electronics products giving not only additional freedom to the user but also save some of the energy drawn from utilities.