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A battery that can convert your body heat into electricity could lead to mobile phones capable of charging themselves while in your pocket.

Researchers at the Massachusetts Institute of Technology have developed a button sized self-charging battery that can scavenge energy from low temperature sources of heat.

The device can charge itself at temperatures between 20°C(68°F) and 60°C (140°F), far lower than other heat-harvesting technologies.

The button sized battery designed by Dr Gang Chen and his team at the Massachusetts Institute of Technology is a prototype for a new way of charging mobile devices from surrounding heat

Dr Gang Chen, head of the mechanical engineering department at MIT, who led the work, said the technology could lead to new mobile phone batteries that can be charged without needing to be plugged in.

He said: 'It is a self-powered device and may be attractive for places without electric grid.

'To generate enough power for practical applications (such as in mobile phones), better material and system designs are needed, and we are working on it.

'With further development, there is potential to use such batteries to convert low grade heat into electricity at an attractive cost for broader applications. It uses inexpensive materials.

'It may have potential application for harvesting thermal energy from the environment, especially in remote areas.' 

The two centimetre wide battery works by exploiting the relationship between temperature and voltage known as the thermally regenerative electrochemical cycle.

This cycle means that a battery charged at high temperatures can deliver more electricity at lower temperatures than has been used to charge it in the first place because of the energy absorbed as heat.

Dr Chen and his colleagues found they use this to create a 'heat engine' to generate electricity purely from the heat surrounding the battery.

Previously the team were able to show they could harvest heat to generate electricity at temperatures of between 60 degrees C (140F) and 100 degrees C (212F). These early batteries needed relatively high differences in temperature - of around 50 degrees C.

However, by tuning the battery's electrodes, which were made from lead and ionic iron, Dr Chen and his colleagues were able to produce a device that could achieve this at low temperatures.

This they believe, would allow a phone to be charged by harvesting energy from body heat and then cooling down when it is removed from a pocket.

However the charging would become less efficient in warm environments when the room temperature matched body temperature.

Dr Chen believes it should be possible to convert heat to electricity with around 12 per cent efficiency at temperatures between 20 degrees C and 60 degrees C.

Mobile phones may never need to be plugged into the wall again as the new battery technology would allow devices to be charged by putting them in a pocket close to the body and then letting them cool down again

The battery exploits the thermally regenerative electrochemical cycle to charge up the battery using low levels of heat that could also find applications cars and industrial processes where excess heat is wasted

However, their prototype, details of which were published in the Proceedings of the National Academy of Sciences, was only able to achieve a heat to electricity conversion efficiency of up to two per cent.

Dr Chen believes this can be improved by improving the materials and tweaking the design further.

He said: 'Tremendous low-grade heat is stored in industrial processes and the environment. Efficient and low-cost utilization of the low-grade heat is critical to imminent energy and environmental challenges.

'The upper limit of efficiency for harvesting heat is proportional to the temperature difference between hot source and cold source - room temperature.

'For low temperature heat sources, this temperature difference is small, so that upper limit of efficiency is already low.

'It takes some temperature difference to transfer heat into the heat engines, which further reduces the temperature range the heat engine works.'