In the previous entry, I stumbled upon the rather strange requirement of how to minimize voltage drop while the batteries in the portable charger drains. I've found some youtube videos and some write-ups on instructables.com about building portable phone chargers. Being a physics undergrad who likes batteries and collects torchlights, I could see several drawbacks regarding some of these chargers.
So, discuss what these drawbacks are and what advantages do these designs actually offer.
Design 1:
9V battery + Zener Diode combination
Using a 9V battery connected to a Zener Diode, the output voltage can be effectively dropped down to the required 5V to charge most phones. According to article [1], the Zener Diode that brings the voltage closest to 5V is 1N4733. It brings the 9V down to 5.1V and has a maximum regulated current is 178mA.
Advantages
- This design is extremely simple and according to youtube videos, they do seem to trick their phones into thinking that they are plugged into an authentic charger.
Disadvantages
- 9V battery
This type of battery is not made for high current drain. Notice that applications of 9V battery include powering up remote controls, portable door chimes and fire alarms. These applications take very little current and will last many months before it needs a battery change. Phones will attempt to suck as much energy as possible during the charging process and will definitely take in more current than what the 9V battery can comfortable provide. In the first place, 9V batteries do not have much energy in them and this high drain will result in premature death of the 9V battery. According to energizer's datasheet [2], at an estimated 500mA drain (standard current that is provided by USB ports), the 9V battery provides only about 300mAh. Theoretically, that should be able to charge up a Nokia N73 to a quarter assuming that the Zener diode does not burn up and about 60% efficiency (because some of this energy heats up the phone when charging).
- Tiny current capability
If we examine the charging current provided by most wall plug-in chargers, the usual current is about 1000mA and a phone using this amount of current will take about 2-3hours to fully charge a drained battery. Assuming that the Zener diode does not go up in smoke, the current of 178mA will charge a dead phone battery in an estimated 11h-16h. That is a heck of a long time.
Design 2:
Battery + Step up/Step down Converter (Switching converter)
See http://www.ladyada.net/make/mintyboost/
This design is considerably more expensive and more complicated because it involves a switching voltage regulation microchip (not those that simply turn excess voltage into heat), some inductors, capacitors and diodes just to get it going. I won't talk about the principles of step up/down converters because it is far too complicated for me to understand. All I know is, they change the input voltage to a voltage that we desire.
Note that there is a simpler voltage regular called the linear regulator. This simply burns up excess voltage so that the output is constantly at 5V. This works only when the input is more than the desired voltage. Very inefficient and wasteful!
Advantages:
Achieves very high efficiency.
Flexibility to use batteries that are designed for high drain (AA batteries)
Circuit can be designed to handle high current comfortably.
Provides excellent charging.
Disadvantages:
Damn expensive - a well designed branded portable phone charger such as the Griffin Tunejuice costs SGD$46.90 in Singapore. [3]
Still quite expensive - a pre-packaged D-I-Y project called "Minty Boost" by Adafruit Industries provide all the necessary components for a simple step-by-step soldering job. However, when shipped from USA, the total price is about USD$30. This is only about SGD$10 cheaper than a cool-looking branded one. Not to mention, I'll still need to get a soldering iron to get all the parts together.
Design 3:
Direct charing from 4x AA batteries / CR123A batteries
(This video makes use of 2x CR123A lithium camera batteries for more juice)
This design is no design at all. It relies on the AA batteries to provide the necessary voltage needed to charge the phone battery. Unlike the portable charger I've bought (which uses smaller AAA batteries), it is possible that using AA batteries can solve the problem of voltage sagging.
Why am I still interested in this design? In a forum discussion, someone mentioned using 4x 2700mAh AA Ni-MH batteries and managed to charge his phone 5 times! Of course, that depends on the capacity of the phone battery in the first place. Meaning, my mileage may vary. Even so, this is quite a good news!
Advantages
- Less components mean less energy wasted on powering up components
Disadvantages
- Voltage uncertainty
Using 4x alkaline batteries give 6V, 4x Ni-MH rechargeables give 4.8V. Don't know if 6V will actually fry the phone or not.
- May not be able to fully utilise the batteries as it drains
Same as my original charger, this may still be a problem! Only experiments can tell us if it still plagued by this problem!
Ok, enough of theory talk and estimates. In my next entry, its time for experimentation! (If my multimeter arrives soon :( I've been waiting for almost 2 weeks already)
References:
[1] http://knol.google.com/k/electronic-circuits-design-for-beginners-chapter-7#
[2] http://data.energizer.com/PDFs/522.pdf
[3] http://www.istore.com.sg/default/tunejuice-universal-for-iphone-3g-3gs-by-griffin.html
[4] http://forums.adafruit.com/viewtopic.php?f=15&t=18360
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