Removing the inconvenience of recharging


This subject presumes that the reader is familiar with the OCPCD, which is described elsewhere on this wiki page.

This project has evolved from an intension to make the OCPCD less or totally independent from recharging.

This intention is based on the idea of making the use of the OCPCD even more pervasive and independent from routine service etc.
The main idea is to make the device wirelessly rechargeable in its everyday environment.

In its present state the OCPCD is destined to be recharged manually every couple of days, like you would a normal cell phone. This need for recharging differs from the OpenCare ideology where such a device, ideally would be totally autonomous and free of human interaction.
Without a wireless recharger, the resources saved from the using such a device, would balance out the time spend, monitoring and then manually recharging the device. And meanwhile recharging via conventional charger (wire), the device is rendered useless.

In order to recharge the device battery, it should harvest energy from an energy source. This source could be anything as long as it adds power to the battery without additional human interaction.

There are numerous ways to generate power but their efficiency and outputs varies.
In the following table some typical energy harvesters are listed.


Table 1 - Characteristics of typical energy harvesters

The “Harvested power” column tells us the harvested power versus a geometrical area.

This column is important to us, because we have a very limited amount of physical room in/on the OCPCD.
This leads us to rule out any energy sources that takes unreasonable amount of space in our device.

Here is an example to illustrate this point, using Light (solar panels):
Determine the size of a solar panel needed to charge a 3.7V Li-Ion Battery with a charging current of 150mA, indoors:
Power/Area: 100uW/cm2 from the table above.
Power needed: 3.7V * 150mA = 0.555W
Area of solar panel: 0.555W/100uW = 5550cm2 = 0.550m2

Over half a square meter of solar panel attached to the OCPCD is not convenient. And since “Light” is the most space efficient source in the table, none of the sources above is plausible as energy sources.

To accomplice a wireless charging of the OCPCD, an inductive charger has been developed.

The physics involved in such a device, can be recognized from a transformer where two inductor coils transfers energy without a galvanic connection.
A magnetic field in one coil, inducts voltage in the other. This is the main principle of the system and is defined more precise by Faradays law of induction:


Equation 1 - Faradays law of induction.

Where tilwiki3.jpg is the electromotive voltage (the induced voltage)
tilwiki4.jpg is the change in magnetic flux
tilwiki5.jpg the change in time

To improve the transfer and making the wireless coupling between the coils more loose, the coils are tuned to the same frequency, which makes them resonate.

The concept of resonance is known from a variety of things.
· Mechanical resonance.
Resonance in various structures causes undesirable side effects such as vibration, swaying motions etc.
Neutralizing resonance is a challenging task when building skyscrapers, bridges and buildings in general.
· Acoustic resonance.
Very relevant in the making of stringed acoustic instruments such as violins etc.
Or when one tuning fork makes another resonate (vibrate), when both are tuned alike on another.

The last example has some similarities with our product and is probably the most illustrative in relation to our application, due to the fact that “energy is transferred through the air”.

The System

The charger system can be split in to two main parts, a transmitter and a receiver.
Each part consists of an inductor coil and is associated circuit.

The sketch below portrays the system (simplified) where a light bulb illustrates the load, which normally would be the OCPCD (battery).

Figure 1 - Sketch of system
The sketch depicts a wireless transfer of power via. the two resonating coils.


The purposes of the circuits are:
Transmitter circuit:
· AC-AC converting 230Volt supply from power grid to the desired supply of the actual printed transmitter circuit.
· Forming the AC signal to the exact specifications of the desired signal, regarding duty cycle, amplitude, time demands, waveform etc.
· Tuning the resonance frequency of the “transmitter coil”.
Receiver circuit:
· Tuning the resonance frequency of the “receiver coil”.
· Converting the induced AC signal to DC in order to charge battery.
The circuits are made from two different perspectives regarding their sizes. The transmitter circuit is primed to be placed in a box with few restrictions to physical dimensions.

The receiver on the other hand is limited to be placed in a small hollow space in the OCPCD.
Therefore surface mounted components are used on the receiver circuit.


The two coils are made from different specifications and do not have much in common.

The transmitter or “primary” coil are designed from numerous demands.

Inductance, measured in SI unit of “henry” (H).
Resistance, measured in SI unit of “ohm” (Ω).
And geometric parameters such as:
Wire thickness, number of windings, diameter and length of coil.

The receiver or “secondary” coil is designed of the same foundation as the primary, but do not share any similarities in any of the actual values. The only aspiration is to make sure that there is a certain relationship between the inductance of the primary and secondary coil and a lot more windings.
The secondary coil is designed to be physically as large as possible in the limited cavity of the OCPCD in which the secondary coils is placed, along with its associated printed circuit.

The more strict demands to the secondary coil, makes it that much more difficult to make.

Figure 2 - Receiver (secondary) coil
The figure above shows a late version of a receiver coil. (Diameter~45mm)

Valenzuela, Adrian. (2009). “Batteryless energy harvesting for embedded designs, In the era of 32- and 64-bit multicore behemoths, 8- and 16-bit MCUs get new respect for power management.”