Harvesting cellphone RF energy to boost battery life

October 21, 2016 // By Nick Flaherty
A wireless charging startup is looking to boost battery life in mobile phones and tablets by harvesting the RF energy from their radio links. Nick Flaherty talks to Marc Chen, CTO of Radient Micro-Tech (Fremont, CA).

“Cellphone battery life has been a longstanding issue, so what we did was look at the cellphone and see that 90% of the energy is wasted in the air as RF,” said Marc Chen, chief technology officer of California-based Radient Micro-tech. “So we turn the back of the cellphone into an antenna to absorb the power to boost the battery life.”

The company started in 2007, coming from designing the analogue RF chips, with key patents granted in 2013. It is now looking to license its technology to a range of different companies, including wireless charging equipment makers that may already be using the Qi technology from the Wireless Power Consortium.  

“We can extend battery life by 30% and also use that extra antenna for wireless charging," said Chen."Qi charging requires you to put the phone on a platform and for me, that’s worse that using a cord. So what we are doing is video frequency RF charger [operating at around 2 GHz] that will be relatively short distance at 15 to 20 ft (5 to 7m). We want to create a product where the user can be walking around or sitting in a coffee shop and the phone is charged fully wirelessly.”

The power available is determined by the distance from the charging base and the number of devices to be charged but the power can be focussed in a particular direction. This is achieved through a data backchannel to the cellphone or tablet that uses either WiFi or Bluetooth. “In a larger environment with many users such as a coffee shop maybe the distance is only a few feet, but these things can be worked out dynamically,” said Chen.

Balancing the design and placement of the antenna for both RF energy harvesting and free space wireless charging is a challenge. “What is more important is the distance – so the closer the receiver is to the transmitter, the more efficiently the energy can be absorbed,” he said. “And because it is also used for wireless charging we also have some programmability to make it more efficient for either purpose.”