I find the effects of electronic devices on society very fascinating. How things like the person computer, the calculator, and the cell phone has completely changed the way we act, our expectations, and what we consider productive in our world. Specificity the effect of the "smart phone", the current apex of portable computing. Its is the do all device, it has access to many different mediums of communications, it has any kind of function you want limited only by hardware, and it can work just as well as a PC but fits(mostly) in your pocket. Its what we hackers where dreaming of 5 years ago. But, just as with every device it has a few limitations. Its only effective when in range of the cell system, its only as capable as its OS and its operator, and its only good as long as it has battery power. This leads me to our topic, the smart phone owners constant bane,...... a dead battery!!!!
With the addition of internet access on your phone you can now stay connected 24/7, as long as your device is powered on and in range of the cell network or a WiFi AP. Every time your device talks to the network to sync email, check facebook or load a web page its transmitting. For a short period of time(the duration of the packet being transmitted) its using as much power as it would when you where on a phone call(in ratio to time duration). So of course being that the transmit duty cycle is much higher the battery lasts a shorter time. The way most people fix this is by having chargers every where they go so it only runs on battery for a short time. But what about those of us who walk, cycle or are other wise are away from a ready charger or easy power source? Well here are one of my ideas to over come this.
Charging on the go.
Most phones now charge off of a USB 5VDC power sources. So you now are finding every where 120AC to USB or car socket to USB adapters. These are great if your near a wall outlet or a car that is running. But what about on a bike, or hiking or when the car is off? My answer is solar.
Now there are several products out on the market now that are solar powered phone chargers. But my goal in this project is to build a charger that is simple, rugged and will last as long as it can with little maintenance.
Batteries vs. Supper Caps
Most products that will charge your phone from solar power use a photovoltaic panel hooked to a voltage regulator or charging circuit. This then charges a small battery which in turn charges your phone. This works as long as the battery is in good health, but over time it will fail or retain a shorter and shorter charge life and will die at some point.
The other way to do this is use a capacitor in a RC circuit. The challenge with this is to get a large enough capacitance to do this would render this device none portable. Standard capacitors with values grater than 1 farad and voltages higher than 5VDC are 3cm in diameter and at least 5cm higher, if not taller. Even though they are much lighter than a battery you would still have to carry a large pack of these things around with you. But we are in luck with the advent of double sided capacitors, or what they call supper caps. These have a value of 10 farads and a voltage of 2.5VDC and are only 3cm high and .5cm in diameter. Now with several of this our pack is looking much smaller, but how effective would they really be?
Power density
When looking at the effectiveness of a power source for a portable device one must look at power density. This has two factors to, the power(in Watt Hours) per volume and the power per weight. This will help us to determine the best balance between longevity, size, weight.
Now, most people would do research on line to out this info. But, that sounds boring! So I have put together information based on tested done in my shop to fit this required function of charging a smart phone.
What's next?
The next step in our project is to determine the power requirements of the device we will be charging. This will give us parameters to work with in when designing our power source.
Second is to test several different power sources to see which will best meet our requirements and have the highest power density.
Third, we put it all together!
In my next blog installment I'll cover the first step, determining the phones power requirements.
So stay tuned for the next installment of electrical goodness!
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