A while back, I described on the DC forum my desire for a wireless battery charger for my wife, who often forgets to plug in her cell phone into its battery charger. Since then, I have received many inquires from various people asking me how to make such a wireless battery charger. I have also noticed that some commercial devices are now beginning to emerge. I saw one 12v powered devices for cars, so the user only had to place the phone on a small pad, to keep the battery charged. In view of all of this activity, I thought that this might be an interesting project. I think I will get started on it and walk you though the various steps I take toward an actual piece of working hardware.
One of the first things I like to do in a project like this is come up with a definition of what it is I want the device to do. Cell phones, cordless phones, MP3 players are all fairly flat and contain batteries which need periodic recharging. It is often awkward fumbling with charge adapters and connectors, to pump energy back into the portable device’s battery when needed. It sure would be nice if you could charge the battery by just plopping the device down on a rubberized pad, which could then wirelessly couple power to the device’s battery.
Battery to be Charged
The battery of choice for many of these portable devices is a 3.6v lithium ion rechargeable cell. I think I will start with the goal of charging such a cell in a few hours. The circuit could then be modified later for other types of batteries. The battery in my personal cell phone is rated at about 1 amp-hour. This is an energy storage of about 3 watt-hours. This means, to charge a dead battery in about 3 hours, I will need about 1 watt of power transferred to the battery. I think I will set that power level as my initial goal.
Power Levels Needed
I’m going to first guess that the power transfer efficiency between the power source and the portable battery will be about 25%. If it turns out that my estimates are too high or too low, I can make some adjustments later. This will be a place to start. If I collect only 25% of the power launched, then I will need to pump about 4 watts of power into the system.
I think I will make things easier by drawing power needed by the charger circuit from an off-the-shelf plug-in-the-wall adapter. I think I will first try using a very common +5v model. These are readily available and are often used to power USB adapters and many cell phones. A device rated at about one or two amps should be about right for starters.
Power Transfer Method
Based on my own experience, I already have a pretty good idea how I will perform this wireless power transfer feat. Now it is just a matter of filling in some blanks and conducting some tests.
I briefly considered using light, but some quick calculations indicate that I would need a lot of infrared LEDs. There are just too many losses in such a system. The conversion efficiency between electricity to light in an LED would only be about 10%. Then, even at the ideal wavelengths, the conversion between light to electricity at the power receiver end would be 50%. That puts the overall efficiency at about 5%. I think an RF method would work better. The light method might be an interesting approach if less power needed to be transferred.
Magnetic Coupled Technique
As illustrated below, one way to couple power to a battery without a direct connection is through a pair of wire coils. High frequency AC current pumped into one coil produces a magnetic field, which can be collected by a second coil, to transfer electrical power. The laws of physics dictate that the coupling efficiency between the coils will be much higher at frequencies higher than standard 50Hz or 60Hz, used in power lines. But, if I do use higher frequencies, I want to make sure that I pick a frequency this is less prone to producing any interference to other nearby devices. A very popular frequency, often used in RFID systems, is 125KHz. I think I will start this project using this frequency.
The concept is pretty simple. RF power is fed to a series resonant network, consisting of a coil and capacitor. The current flowing through the coil in the network induces a sizeable magnetic field around the transmitting “exciter” coil. A smaller coil of wire, with a capacitor wired in parallel, forms a parallel resonant receiver circuit. When the smaller coil is placed in the center of the larger coil, a sizable amount of power can be coupled between them. After some rectification, the received RF signal can be turned back into DC, which can then charge a battery.
For ease of use, I think the charging pad should be about the size of a computer mouse pad. I think I will try to install the exciter coil in a pocket I make in the underside of a suitable mouse pad, with a rubber backing. The mouse pad I use with my laptop computer is about 8 inches by 9 inches. This seems like a good size to start with.
In the next installment, I will go into some more detail about the two coils. I’ll walk you through the steps and show how I come up with some coil inductance values. I’ll come up with a driver circuit design. Then, it will be time to start building some actual hardware so I can run some tests.