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Experimenter's
Corner - [Issue 1, September 2009]
Previous Issues
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Wireless Battery Charger
By: Dave Johnson July 24, 2007 |
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| Project Description:
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. |
| Device Definition: 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. |
| Power Source: 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. |
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Exciter Coil |
| 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. |
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| Next Step -- 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. |
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