Recycling cellphone RF energy to boost battery life

Recycling cellphone RF energy to boost battery life

Interviews |
By Rich Pell

“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 2GHz] 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.”

The solution is largely software based. “We don’t need a dedicated IC for our technology to be implemented as there is already an RF IC module and power management in the devices so all we need to add is an RF to DC converter which is just a few transistors and everything is in the device already so the additional cost is negligible.”

The advantage for the phone maker is reducing the size of the battery to give more space in the design or increasing the manufacturing tolerances to reduce costs. “If you can reduce the battery size by 30% that’s a cost saving and gives more space to do things,” he said.

However, having an extra antenna on the back of a phone can change the performance of the device, and he is well aware of this. “When the cellphone is transmitting only about 10% of the energy gets to the cell tower – the direction of the cell tower is generally not known but there are things we can do to determine whether our receiver is effecting the link,” he said. “One of the things we are doing is working with the algorithms in the transmitter to determine whether it is affecting the transmission.

This is a challenge for all phone makers. “Even the Qi charger or even the camera will affect the transmitter and have to be placed very carefully so we place the antenna close but not too close, so maybe we are collecting 20 to 25% of the energy and even that is enough to make a difference.”

The company is looking to license the technology to phone makers but also to wireless charger makers and even phone case suppliers, although this is not an immediate route to market

“Even if it in an external case it has to make use of the signals from the internal signal processor otherwise you could actually jam the signal – we could have rushed out with the external case but we didn’t as we know this has to be designed into the cellphone,” he said.

“We are working with potential partners such as RF wireless charger companies as many have been working on this for a number of years – our expertise is working within the cellphone and working with a partner we can get to market much faster.”

However, as a small company he is not a big fan of the standards groups that are dominated by the large industry players.

“Generally it’s not good to be part of a standard but as the technology is patented we are comfortable. Being part of a standard without patent protection is a problem as without patents the other partners can use the technology and it just becomes a price war. As we have patented technology we can offer a player in a consortium a competitive advantage,” he said.

With the need to design into cellphones and develop a wider ecosystem of wireless charging, the technology will not come to market quickly. “A recent IDC report analysed the wireless charger space and by 2020 over half will be truly wireless and not Qi based and that’s where we see our technology rolling out,” said Chen.

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