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
Saturday, May 28, 2011
Monday, May 16, 2011
Project: Make a portable phone charger (part 1)
My first encounter with this device and why I want to make my own!
My first encounter with portable handphone chargers was during my time in BMT (basic military training) in Pulau Tekong. As recruits, we were not allowed to use electricity in camp. We were told that this rule was in place so that we do not end up overloading the generators that were powering the island. I felt that it was utter bullshit because the only time we could use the electricity was at night when most electronic appliances are switched off. But still, nothing much could be done about it since we had no rights and no say.
I am someone who is really addicted to technology, so much so that I don't feel right if I leave my house with my handphone battery at half capacity even though I was only going for only a few hours. I was quite glad that I am able to bring my phone to camp although it would be next to useless if the battery went dead. Knowing this, I definitely have to do something to get around this problem. I felt that it was kind of wasteful and stupid to buy another handphone battery (about $30) when the training will only last 3 months and I'll still be left without any communication when the extra battery is used up anyway.
I needed something that is able to use disposable batteries and it seemed that a portable handphone charger was ideal choice for this situation. So, I bought this at Sim Lim Square for less than $10. It uses 4x AAA batteries and when I plugged it into my phone, it worked! The screen showed that it was charging. Wonderful.
During the first week of my 2-week confinement, I used my phone really sparingly, turning it on at night only to check for messages and to send stuff out. I even took out the battery from the phone when I did not need to use it. All these measures helped to keep me contactable but 1 week was all I can squeeze out of my battery. Still, it was longer than I expected and a boost from my newly bought portable charger should be sufficient to last me the rest of my confinement week. I even lent my battery the poor souls who did not prepare for this. Good eh?
Happily, I plugged in my portable charger and placed it inside my cupboard. After 30mins, I came back to check on it and to my greatest horror, the phone was no longer charging and my phone was showing an additional 1 bar extra. That was all it could do. Unplugging and plugging it again did nothing. The screen flashed "NOT CHARGING". Die! This time, I screwed up. I knew that my parents would be coming to pick me up from Pasir Ris for my first book-out, so I really needed my phone to tell them exactly where to meet me. There was no way a battery at 1 bar (out of 5) could last another week of sparing usage. I was even unsure if it can hold its charge even if I don't use it at all. Sure enough, when the time came, I couldn't even establish a call without the phone shutting down.
After this incident, I went to check the AAA batteries inside my portable charger. All of them were still fresh. I could power them in my trusty MP3 player and still use them for many hours before they were depleted. What was wrong with it? Why did it not continue charging even though those AAA batteries were still fresh?
These questions led me to suspect that charging may be quite a complicated process and the charger may not be capable to utilize the energy in those disposable batteries. It took me quite a few years of procrastination to finally decide that I should take a closer look at the innards of the charger I bought during my younger days.
Ah! That's the reason why I couldn't charge my phone effectively. The only component inside this charger was a diode! A diode is a component that is made such that electrical current can flow only in one direction. This prevents the backflow of electricity should the battery in the phone overpower the portable charger. Its a nice safety feature, but it doesn't make it work any better. Normal AAA batteries have 1.5V. Having 4 of these will give 6V. My phone required a minimum of 5V to charge the battery. While the portable charger charges my phone, the battery voltage drops as it depletes. The moment each battery drops to 1.25V, the phone no longer continues charging. It is quite a shame because at 1.25V, these batteries still have a lot of energy left in them.
The important question is how, if it is possible, to eliminate this voltage drop from the batteries.
My first encounter with portable handphone chargers was during my time in BMT (basic military training) in Pulau Tekong. As recruits, we were not allowed to use electricity in camp. We were told that this rule was in place so that we do not end up overloading the generators that were powering the island. I felt that it was utter bullshit because the only time we could use the electricity was at night when most electronic appliances are switched off. But still, nothing much could be done about it since we had no rights and no say.
I am someone who is really addicted to technology, so much so that I don't feel right if I leave my house with my handphone battery at half capacity even though I was only going for only a few hours. I was quite glad that I am able to bring my phone to camp although it would be next to useless if the battery went dead. Knowing this, I definitely have to do something to get around this problem. I felt that it was kind of wasteful and stupid to buy another handphone battery (about $30) when the training will only last 3 months and I'll still be left without any communication when the extra battery is used up anyway.
I needed something that is able to use disposable batteries and it seemed that a portable handphone charger was ideal choice for this situation. So, I bought this at Sim Lim Square for less than $10. It uses 4x AAA batteries and when I plugged it into my phone, it worked! The screen showed that it was charging. Wonderful.
During the first week of my 2-week confinement, I used my phone really sparingly, turning it on at night only to check for messages and to send stuff out. I even took out the battery from the phone when I did not need to use it. All these measures helped to keep me contactable but 1 week was all I can squeeze out of my battery. Still, it was longer than I expected and a boost from my newly bought portable charger should be sufficient to last me the rest of my confinement week. I even lent my battery the poor souls who did not prepare for this. Good eh?
Happily, I plugged in my portable charger and placed it inside my cupboard. After 30mins, I came back to check on it and to my greatest horror, the phone was no longer charging and my phone was showing an additional 1 bar extra. That was all it could do. Unplugging and plugging it again did nothing. The screen flashed "NOT CHARGING". Die! This time, I screwed up. I knew that my parents would be coming to pick me up from Pasir Ris for my first book-out, so I really needed my phone to tell them exactly where to meet me. There was no way a battery at 1 bar (out of 5) could last another week of sparing usage. I was even unsure if it can hold its charge even if I don't use it at all. Sure enough, when the time came, I couldn't even establish a call without the phone shutting down.
After this incident, I went to check the AAA batteries inside my portable charger. All of them were still fresh. I could power them in my trusty MP3 player and still use them for many hours before they were depleted. What was wrong with it? Why did it not continue charging even though those AAA batteries were still fresh?
These questions led me to suspect that charging may be quite a complicated process and the charger may not be capable to utilize the energy in those disposable batteries. It took me quite a few years of procrastination to finally decide that I should take a closer look at the innards of the charger I bought during my younger days.
Ah! That's the reason why I couldn't charge my phone effectively. The only component inside this charger was a diode! A diode is a component that is made such that electrical current can flow only in one direction. This prevents the backflow of electricity should the battery in the phone overpower the portable charger. Its a nice safety feature, but it doesn't make it work any better. Normal AAA batteries have 1.5V. Having 4 of these will give 6V. My phone required a minimum of 5V to charge the battery. While the portable charger charges my phone, the battery voltage drops as it depletes. The moment each battery drops to 1.25V, the phone no longer continues charging. It is quite a shame because at 1.25V, these batteries still have a lot of energy left in them.
The important question is how, if it is possible, to eliminate this voltage drop from the batteries.
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