Which wireless power technology can charge a phone in your pocket?

Aug. 2018

Many people consider charging a phone to be the holy grail of wireless charging . Which wireless power technology can charge a phone in your pocket? Let’s find out.

Step 1: How much power does a phone need to charge?

The battery capacity of an iPhone X, typical of other smartphones, is about 10 WH. The operating time for an iPhone X under normal conditions is about 20 hours. This means that an iPhone X consumes about 0.5 Watt every hour.

Thus, if you wanted to keep an iPhone X operating ‘forever’, you’d need to deliver at least 0.5 Watt to it. If you can deliver 0.5 Watt, the energy that you are using is going to be equal to the energy you are providing. On average, anything above 0.5W can be used to charge the phone.

Step 2: Which wireless power technology can charge a phone OUTSIDE your pocket?

When energy passes through fabric, glass or any other material, some of it is scattered, reflected or absorbed. As a result, the amount of energy coming out of the other side is less than the amount of energy entering the material.

Thus, the task of charging a phone that is outside the pocket is easier than charging a phone inside the pocket, because more energy is available on the outside than it is on the inside.

Magnetic induction (Qi pads) can charge a phone on the outside if the phone is very close to the charging pads. Qi pads can deliver high energy – some claim up to 15 Watts – but require the phone to be in immediate proximity to the pad as well as nicely aligned. If magnetic induction delivers 10 Watts, it can fully charge an iPhone X in about 1 hour.

RF wireless charging cannot charge a phone. Under reasonable assumptions of transmitter size, and while keeping within international radiation safely limits, the theoretical limit for RF power delivery to a phone-sized receiver is about 0.1 Watt. That’s the theoretical limit, so practical implementation are usually much lower. 0.1 Watt is about 5x less than what you would need to keep an iPhone X operating. If the phone is turned off and is not consuming energy, and if RF delivers energy at the theoretical limit, it would take about 100 hours (over 4 days!) to fully charge a phone.

IR wireless charging can charge a phone. For instance, Wi-Charge today has units that can deliver about 3 Watts of power at a distance. See an example here. The theoretical limit is significantly higher. In fact, IR charging can safely deliver about 100x the energy of RF charging under reasonable assumptions. If a phone receives 3 Watts and consume 0.5 Watts, then 2.5 Watts is available for charging so a phone would be fully charged in about 4 hours. This is slower than cable or Qi charging, but wireless charging is more convenient: it can happen in the background without the need to put the phone away or connect to a cable.

Step 3: Which wireless power technology can charge a phone in your pocket?

We already established that RF charging cannot charge a phone outside the pocket, so it would also not be able to charge a phone inside the pocket.

Magnetic induction will not work either because it’s not practical to put the Qi pad right outside the pocket. If you were willing to do that, you might as well put a power bank in the pocket and charge the phone using a cable.

Wi-Charge also does not claim that a phone can be charged in the pocket using our current IR power technology. While we can demonstrate power delivery through several types of fabrics and other materials, we do not claim to be charging a phone in the pocket, nor currently work on that.

Thus, the answer to the question “Which wireless power technology can charge a phone in your pocket?” is (D) today, none of the above.

The silver lining

After a decade of charging cables and charging pads, the world in on the brink of long-range wireless charging and Wi-Charge is proudly leading this exciting transition. Before attempting wireless phone charging inside the pocket, I believe it is important to master wireless phone charging outside the pocket – an easier, but still very difficult goal that few were able to achieve.

I’m optimistic that remote non-LOS wireless power will arrive someday too, but don’t hold your breath. I’m currently unaware of any technology that can become the basis for non-LOS wireless power. In my estimation, it’s likely at least a decade away.

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