Download The purpose of this project was to develop a cell phone charger that

Off The Grid Charger
Anthony Bird
George Brooks
Jennifer Dorsch
Chad Floria
Final Report
4/29/2010
Executive Summary
The purpose of this project was to develop a cell phone charger that is self-charging
(mechanically charged). The need for this type of device would be to eliminate the limitation of having
to charge cell phones by the use of electrical outlets. Some of the limitations of having to charge
through electrical outlets would be that outlets are at a fixed location, which would be a hindrance to a
person that is busy, and people have the tendency to forget to plug-in and charge the cell phones that
are instrumental to daily life. This device would harness the mechanical energy of a person and convert
it to an induced electromotive force. This is done by the use of Faraday’s Law of Induction, in which
magnets are passed through copper coils to create magnetic flux which creates an induced
electromotive force (EMF), according to Faraday’s Law. The device would rely on the movements of a
person to move the magnets within a small track through a series of copper coils in order to create EMF
or otherwise known as Volts. The device would then store the electromotive force in a capacitor so that
it can then be utilized at a later time to charge a cell phone. The charge stored within the device will
need to meet with the specifications of the cell phone’s battery, specifically the amount of Volts needed
to charge the battery. Due to this the capacitor needed will have to be able to build up and hold a
charge for significant period of time. The device would be “phone specific” meaning that the device
would have hardware that would be specific to certain type of cell phone or cell phone provider. The
way in which the device will capitalize on the motion of a person would be by having the device made to
be a cylinder. The overall shape of the device would be quite small. The device would be designed so
that it could be an attachment that could be placed on a phone or, at least, small enough so that it could
fit comfortably within a person’s pocket. In order to achieve a small size, the device would need to have
a strong magnet and a very conductive wire. The device works to an extent. The device has the ability to
charge a cell phone, but it may not be as effective as desired.
Table of Contents
Introduction and Problem Definition............................................................................................................ 4
Conceptual Design Descriptions ................................................................................................................... 6
Implemented Design Solution ...................................................................................................................... 8
Technical Description and Approach ............................................................................................................ 8
Results ......................................................................................................................................................... 13
Conclusions ................................................................................................................................................. 14
References .................................................................................................................................................. 15
Appendix ..................................................................................................................................................... 16
Introduction and Problem Definition
Cell phones have revolutionized the way people communicate in the modern world. As long as
the cell phone remains charged any person has the capacity to communicate with millions of people.
This has huge benefits for people in general for a variety of reasons. Cell phones provide people with
security knowing that the help is just a call away and cell phones also allow people to have the ability to
continue their business from anywhere. Cell phones remaining charged are the key for using the
benefits of these devices. This necessity of the cell phone has an inherent problem that is getting worse
as people rely more on technology and the cell phone. People tend to be away from home more often
now as the lifestyles of people get busier. Due to this, it is much more difficult to keep a cell phone
charged. The easiest way to charge a cell phone is to use a cell phone charger at home. This is not a
convenient method of charging a cell phone for people, because the majority of a person’s day may not
be spent at home and if anybody uses a cell phone extensively, the cell phone will lose its charge. Cell
phone chargers that can be plugged into vehicles are another option that does not meet the needs of
the people, completely. People use cars and other modes of transportation for commuting from one
spot to another. Other than that people generally do not spend too much time within a vehicle. So this
method of charging a cell phone is not all that viable to a majority of people. People need a device that
has the ability to charge a cell phone while away from a source of energy. To do this the device would
need to be able to generate charges by itself with minimal input from the owner of the device.
The main principle behind the development of this particular device is a law in nature known as
Faraday’s Law of Induction. Faraday’s Law of Induction is a simple law of electromagnetism that states
that electromotive force or EMF can be generated from the change of flux created by the movement of
a magnet through a coil made of a material like copper. This law of nature is the main premise in the
addressing of the problem with cell phone chargers today. The majority of cell phone charges require to
be plugged in to some sort of energy source like a power outlet or car battery in order to perform its
desired function. This severely affects the portability and usefulness of a cell phone charger. Generally,
cell phone chargers do not match the degree of portability that is characteristic of today’s cell phones. In
order to match the portability of a cell phone and its ability to function most anywhere, the proposed
device will need to be small and have the ability to generate a charge and maintain it so that a cell
phone can be charged by the device at a later time. This makes this new device much more desirable
than the current cell phone chargers. Current cell phone chargers have major drawbacks in that either a
person has to be home in order to charge the person’s respective phone, which defeats the purpose of a
mobile phone. Also cell phone chargers that employ the energy of car batteries tend to not be included
in the purchase of a cell phone and buying the unit separately is quite costly. Another drawback to these
chargers is that pedestrians will have no use for a cell phone charger that requires the access to a car.
There are some constraints that need to be addressed with this design. To make a cell phone
charger that has a greater appeal to consumers, the new cell phone charger has to be small and be very
efficient at generating electromotive force. This poses a challenge to the overall design. To be efficient
at developing a charge the device would either need a large area for the magnet to move through or a
very strong magnet. A strong magnet is the more logical choice in this matter due to the fact that the
overall size of the device is a major constraint. Another constraint would be the duration in which the
device can hold a charge. In order to assure a sufficient enough time for the device, super capacitors
would need to be used. The final major constraint of this device would be the overall cost of production
and how competitively priced this product would be in the market. Fortunately, due to the relatively
small size of the device and the inexpensive nature of the components the cost of production of this
device is not overwhelming. This allows the device to be competitively priced in the market. Cell phone
chargers are priced anywhere between twenty and thirty dollars, normally. This means that this design
would need to be priced within the same range in order to entice consumers to purchase the product.
By taking all of these constraints into consideration it is possible to define the end product. The device
would be small in size and would make us of cheap materials like plastic for its covering and external
components. However, the device would make use of some very sophisticated materials. The device
would use components like super capacitors and Neodymium magnets, which are the strongest
permanent magnets known, to insure that the device would perform at high level for a long time.
Conceptual Design Descriptions
The premise of the initial design was based off the idea of leveraging normal motion of a walking
body to generate a storable charge; thence to be utilized by a cell phone in need of charging. Therefore,
it is clear maximizing the extent of the aforementioned leverage will induce the most charge to be
stored. Based off the design of the device, to induce the most current (charge per unit time) one must
seek to maximize loop area, change in magnetic flux per unit time, number of loops, and size of the wire
in which the induce current travels. All of these factors came into consideration during the process of
conceptual design – coupled with the intent of maximizing of leverage and propagation of instability.
Design #1: The Torus (Figure 1)
Figure 1 - The Torus
The original idea was to build a ring/donut shaped shell or encasing that the magnet traveled in.
The magnet was to be spherical and the coil would be wrapped around the coil. In this design, when the
device is moved the magnet travels within the ring inducing (when in the correct orientation) a current;
the direction of which is consistent with Lenz’s Law. However, the issue that arises from going with this
design is the orientation of the magnet flux, with respect to the coil wrapped around the torus, is
constantly less than optimal. The reason for this is due to the construction of the magnet itself and the
fact that as the spherical magnet rolls, its polarity rolls with it. This design does create a very good
instance of an unstable equilibrium, but it is also very difficult to construct.
Design #2: Two Half, Elliptical Torus (Figure 2)
Figure 2 – Elliptical Torus
This design is was a permutation of the previous one. The basic idea was to break the previous
torus design in half and incase the halves - to allow for two magnets to move through it simultaneously.
Thereafter, the two halves were to be placed back together – looking like a torus, or donut, once again
but this time with divisions. This construction creates an even more unstable equilibrium if the magnets
are oriented such that their poles are opposite and constantly repel each other as the device moves
about. The second part of this construction was to use custom fitted magnets that resemble a curved
cylinder. The reason for this is to create an optimal orientation for induced current, with respect to the
coil. The ultimate reason for not choosing this design was its difficultly to construct.
Implemented Design Solution
The decision to go with the design put forth in this report was one of the most difficult tasks of
this project. The decision came down to a few important qualities that were deemed necessary in
making the phone charger. The first focus was on optimization. From the beginning it became apparent
that in order to obtain the results that were needed to charge a cell phone, this device would have to be
outputting at its maximum, while maintaining a convenient size and an affordable price. To shine a light
onto the path, an excel file (Table 1) was made that showed a theoretical EMF output based on certain
criteria using some basic introductory physics equations. The criteria involved every aspect of the charge
generating part of the device: magnet size, speed of each pass, coil area, and number of coil loops. This
optimization program proved valuable as different aspects of the design could be manipulated to
determine in which direction to proceed. This program, along with the theory behind the equations used
in this program, was the intellectual basis on which most project decisions were made.
Table 1 - Theoretical Calculations tailored to our device specifications (one shake of charger)
Induced Vemf Calculations
Symbol
Description
Unit
Value
Br
T
R
Residual Induction
Thickness
Radius
tesla
meters
meters
1.480
0.0127
0.00635
x1
Initial distance between
magnet and coil
meters
0.05
x2
t1
t2
A
N
B1
B2
dB
dt
dB/dt
Vemf
Final distance between
magnet and coil
Initial Time
Final Time
Area of Coil
Number of Loops
Initial Magnetic Flux
Final Magnetic Flux
Magnetic Flux Change
Time Change
emf Voltage Induced
meters
seconds
seconds
meters squared
winds
tesla
tesla
tesla
seconds
tesla per second
volts
0.00
0.00
0.03
0.00012668
200
0.002130
0.661876
0.659746
0.03
21.99152
-0.5572
The first part of the project that seemed to be most important after messing with the
theoretical calculations was determining which type of magnet to use. The first and simplest idea was
that of a cylindrical magnet. Although other magnet shapes were considered, most seemed too
complication to build a coil for the magnet to run through. After a cylindrical shape was chosen the size
of the magnet was the next question. The magnet had to be large enough due to the fact that induction
is influenced by the area of the coil the magnet travels through while also relying on a short distance in
between magnet and coil. Originally a 1 inch diameter by 1 inch tall magnet was going to be chosen but
due to its strong pull force the idea was ditched due to fear of the magnet being too dangerous.
Ultimately a ½ inch diameter by ½ inch tall magnet was chosen. The magnets used were made of
neodymium rare earth materials and received the highest grade at N52.
The next major step was deciding in what container to hold this magnet so that it would reduce
friction, harness everyday motion, and be simple to produce. The first prototype was built using a
curved PVC tube which seemed to be too thick to produce any profound results. The next container was
a clear plastic tube of minimal thickness that was manufactured with the magnets specifications in mind.
The clear plastic tubes fit the magnets perfectly and caused little friction on the moving magnets. The
length of 6 inches for the plastic tubing was chosen to increase the number of turns of the coil while
maintaining a practical size.
As with most engineering problems, there were a lot of ideas on how specifically to build this
device without the proper means of manufacturing the envisioned technology. This was a large reason
why the dual straight tube design (Figure 3), and the torus designs were scrapped. The torus designs
seemed to make sense theoretically in areas but overall seemed impossible to build any kind of
prototype that could provide insight into further development of this product. With the dual straight
tube design, a prototype was able to be manufactured that could show some results in the means of
electronic readings of voltage output. The dual design, as shown below, was chosen because one tube
was not outputting enough voltage so another tube was added to essentially double the effect.
Figure 3 - Left to Right, Exploded image of device, interior workings of device, and exterior design of device
The most important part of this whole apparatus was the circuit board. One thing was for sure
with this device, it did produce current. The major hurdle that arose during this project was figuring out
a way to harness this current efficiently. The first breakthrough on the circuit end came about with the
discovery of a full wave rectifier. This rectifier, or diode bridge, essentially manipulates a current that
alternates direction and creates a one directional flow of current. The rest of the circuit then became
quite a challenge. Without sufficient expertise it was difficult to decide which circuit layout would do the
job that needed to be done in terms of harnessing electricity, storing it, and efficiently charging a cell
phone with the electricity. Eventually a protoboard was used as a temporary circuit board while
different circuit configurations were being tested. The challenge if this project were to continue past this
semester would be to figure out how arrange a proper circuit to fit the expectations of this device.
Technical Description and Approach
The initial design of charge generation from movement came with the intention of convenience
and practicality. One of the easiest and most feasible design ideas (i.e. it is within the scope of this
project) is taught in the second level physics classes here at Michigan State. It is Faraday’s law of
induction. This law states when that a changing magnetic field through a loop of wire (not specific)
induces an electromotive force, or more precisely a current. In our case we will utilize this current to
charge a cell phone.
To utilize this induced current one must first understand the basic elements of an integrated
circuit. The basic layout of the circuit is as follows: Our aforementioned current generating device will
produce a charge difference, with respect to ground, by inducing a voltage in a capacitor – adhering
ideally to Ohm’s Law and Kirchhoff’s Voltage Laws. This capacitor will then discharge, with respect to
ground (most likely through a switch), into the phone battery, when at a sufficient enough charge
and/or voltage (in our case a voltage ratio of 5.5 Volts to 3.2 Volts); at a rate which is consistent with the
ideal charging of the battery itself (i.e. it doesn’t damage the battery by introducing too high a voltage).
One of the most prized elements in this design is the capacitor. This element stores charge by
holding a charge difference from what can be seen as two metal plates; for a certain period of time
depending on its capacitance value. This value is dictated by the dielectric constant of the material
between the two plates. In our case we will be utilizing a 1 milliFarad electrolytic capacitor (or
1microFarad – depending on design and time requirements). This value was chosen for an
approximation of ideal discharge rate into the phone battery and convenience of parts available. The
phone battery produces 950 mAh of current – time, or in terms of power, 3.48 watt -hours. Once the
capacitor is sufficiently charged, the circuit will be completed through the phone battery (by plugging it
in). The capacitor also allows the voltage to have a 0Hz frequency (direct current) – which is required in
this case to charge a battery. The amount of time it takes to charge the battery remains to be seen and
will be determined by the area of the loop in which the current is being induced, the capacitance value
mentioned above (1 milliFarad), and the amount dissipated (with respect to resistance) through the
parts of the circuit not associated with the battery.
There is a problem with this design however. Faraday’s law does not specify in which direction
the current is induced – this is done by Lenz’s Law which states it being in accord with the “right hand
rule.” To manage this problem we will introduce into our circuit what is called a full-wave rectifier diode
bridge (FWR). The FWR will be used to effectively turn the chaotic, back and forth nature of the induced
current into a single (sufficed it to say in terms of circuit elements) current source. One possible issue
with FWR’s is that they are simply a compilation of diodes. Diodes require a minimum .7 Volt charge
difference to become active – generally not an issue with most devices. However, this .7 Volt drop is
avoided by one) the use of a forcing current source (our device) and two) the current induced is so small
and the activity requirement of a diode are such that this allows for current to come through while still
maintaining the effect of the diode.
There is another potential risk which falls more in the realm of optimization than actual
requirement. This problem is the fact that the current produced by the discharging capacitor could be so
large that it damages the battery. This problem can easily be solved by implementing what is called a
current limiter (a permutation of a voltage limiter in terms of circuits). However, due to this not being
required for actual proof of concept and the discussion of the device being far out of the scope of this
report, we can save this discussion for another time.
Results
The latest development in the prototype was the cell phone beeping to indicate the charge
entering the cell phone battery for storage. Charging the battery of off one tube was very time
consuming and seemed unobtainable. The final prototype contained two coiled tubes with magnets in
them; the two tubes enabled a shorter time to charge the capacitor. After shaking the prototype for an
extended amount of time, depending on the person approximately ten to twenty minutes, the capacitor
reached 5 volts, indicated by a multimeter and instantly sent the charge to the cell phone battery. This
very simple design taking advantage of Faradays law enabled a Motorola cell phone to charge by the use
of kinetic energy. Additional experimentation was started, in that there was an extra coil introduced in
parallel, on the outside of one of the original tube. This addition leads the prototype to charge faster in
some instances. However, in some other instances the two parallel coils caused the system to not
charge at all or to even dissipate without charging the phone. This experimentation was very exciting,
yet will was not included in the final prototype because it was not yet perfected, with the way the
parallel coil is wrapped around the first coil it seems that some problem occurs with the charge in the
capacitor. It is possible that there should be a second diode bridge rectifier introduced in order to use
this technique, however, further research will be needed before this new technique can be utilized.
Conclusions
A comparable unit the Power Monkey Explorer Solar Charger, charges cell phones by use of
solar panels. It has a price tag of $149.99 and is limited by its requirement to have direct sun light, thus
charger must be folded open and in a stable position to protect solar panel from damage.
Mass production of the kinetic phone charger drives unit cost down dramatically. When supplies for
assembly are bought in bulk the unit cost drops.
References
Standard Specifications for Permanent Magnet Materials. Publication
no. 0100-00. Magnetic Materials Producers Association. Web. 27 Feb.
2010. <http://www.intl-magnetics.org/pdfs/0100-00.pdf>.
"Graphs of Magnetic Flux Density - Online Calculator from Australian
Magnetic Solutions." Australian Magnetic Solutions - Your Best
Source of Magnetic Products in Australia and the World - The
Attraction Is Quite Simple Web. 27 Feb. 2010.
<http://www.magneticsolutions.com.au/magnet-formula.html>.
"Electromagnetic Induction." Wikipedia, the Free Encyclopedia.
Web. 27 Feb. 2010.
<http://en.wikipedia.org/wiki/Electromagnetic_induction>.
"Faraday's Law of Induction." Wikipedia, the Free Encyclopedia.
Web. 27 Feb. 2010.
<http://en.wikipedia.org/wiki/Faraday%27s_law_of_induction>.
Appendix