A Leslie Wong Blog

I built a pocket size USB charger using a MintyBoost 3.0 USB charger, a 3.7 v, 2600 mAh LiIon battery and a built-in LiIon battery charger in an Altoids Wintergreen Mints tin. It can be used to recharge a cell phone, iPod or other small USB rechargeable device.

I’ve had adafruit.com‘s MintyBoost 3.0 kit and USB LiIon/LiPoly charger (this is v1.1) for a while but I never put them together. I liked the size of the Altoids gum tin of the previous MintyBoost versions I have built but I wanted a little more charging capacity than 2 AA batteries could provide. I wanted a suitable battery and enclosure that didn’t compromise charging capacity and size.

Using ladyada’s calculations for battery power, I decided to use a 3.7v 2600 mAh LiIon battery.

The battery’s capacity:
MintyBoost mWh = 3.7V * 2600 mAh = 9620 mWh input

The amount of current it can provide:
Output mAh @ 5V = 9620 mWh / 5 * 80% = 1539 mAh output (80% is the conversion efficiency)

Number of iPhone 4S recharges = 1539 mAh / 1430 mAh (iPhone battery capacity) ~ 1.1

The MintyBoost kit requires the soldering of a few components to a circuit board. I also made two mods to the internal battery charger. With this version (1.1) of the adafruit LiIon battery charger, removing the resistor R4 and replacing it with a 1K ohm resistor allows the internal battery to be charged at 1000 mA.

The internal battery charger has connections for external status LEDs. I connected current limiting resistors between the board and LEDs, then mounted the LEDs in holes in the Altoids tin. The green LED indicates that the internal battery is charging and the yellow LED indicates a fully charged battery. (LiIon/LiPoly charging tutorial at Adafruit Learning System)

The battery and circuit boards are mounted in the Altoids tin with double sided foam tape. I made a miscalculation in the height of the LiIon charger taped to battery – the cover won’t close when the charging cable for the internal battery is attached.

To charge the internal battery, a USB Mini B plug supplies the power to the internal battery charger as in the photo above. To charge a device, a USB Standard A connector is used. There’s also enough room in the Altoids tin for storing an Apple 30-pin to USB Cable.

Below are charging test results with an iPhone 4S starting with battery at 49%:

Time (minutes)      Charge (%)
0:00                           49%
0:15                           58%
0:30                           66%
0:45                           75%
1:00                           83%
1:15                           89%
1:30                           93%
1:45                           96%
2:00                           97%

If you are interested in the MintyBoost design process: http://www.ladyada.net/make/mintyboost/process.html

I was reading Robert Scoble’s post about the AT&T-T-Mobile deal (One bad company buying another: AT&T buys TMobile) and he mentioned, “Devil’s Slide (cell service) is non-existent for AT&T and TMobile, but works the entire way on Verizon for me.”

Devil’s Slide is an area of steep cliffs along the Pacific Ocean about five miles south of San Francisco, California. This stretch of Highway 1 can induce acrophobia if you start thinking about driving your car off the road. This is a good piture of it showing the road slicing right across Devil’s Slide. This part of the highway is “remote,” in that the extreme terrain doesn’t allow for much habitability for a few miles on either side. Maybe that’s the reason AT&T and T-Mobile didn’t care about a dead spot for a few miles, though I wouldn’t recommend checking your phone’s signal while driving this stretch of road, especially if you are driving in a sporting manner.

The California Department of Transportation (Caltrans) started building a tunnel in 2005 to bypass the area prone to rock slides so Highway 1 won’t keep getting closed. The tunnel is 4200 feet (1,280 m) long and will be completed in 2011. On October 1, 2010, they punched through one of the tunnels. I just wrote all this because the video reminded me of Total Recall.

I bought a Samsung Nexus S the day it was released, last December 16. Android 2.3 (Gingerbread) gives the Nexus S considerably more functionality than the last phone I bought, though it costs about 26 times more.

The only other thing I wanted for the phone was a silicone case. I ordered the Amzer Silicone Skin Jelly Case on December 17, 2010 and it finally shipped January 6, 2011.

The case covers the side and the back of the Nexus S with clean cutouts on the back for the camera, flash and speaker. The bottom of the case has cutouts for the headphone jack and charging port. On the side of the case, there are raised bumps for the volume and power switches that work OK.

There is a slightly raised edge around the front of the case, which allows for putting the phone face down on a flat surface without concern about scratching the front. The Amzer case will provide scratch protection for the back and sides of the phone and a certain degree of impact protection, though I don’t really feel like testing that capability.

The case is available in eleven colors: black, gray, transparent white, hot pink, purple, maroon red, blue, orange, green and baby pink…

Update July 20, 2011: I have since switched to the Incipio Feather case because I found that the Amzer silicone case made it difficult to pull the phone from my pants pocket. The Incipio case is a hard shell but with a soft touch coating that won’t scratch the phone and is much easier to put in and take out of my pocket. I have even dropped the Nexus S in the Incipio case from 4 feet onto concrete with no damage.

I don’t really talk to anyone, but when your newest car is 30 years old, it’s good to have a cell phone, in case on-the-road-repairs aren’t enough.

I bought an unlocked Motorola MOTOFONE F3 for about $20 at Amazon.com. With a prepaid plan, there isn’t a contract or monthly charge, just refills every now and then.

The GSM (850/1900 MHz) MOTOFONE F3 is a basic phone that seems to be targeted at the third world market. It’s good for calling someone; don’t expect to do too much beyond that and you won’t be disappointed. I wouldn’t even use it for text messaging, though it has that capability. (Maybe if you just text, “Hi.”) It’s easier just to call the other person.

The phone has an E Ink display (Electrophoretic Display – EPD) – the same type as used in the Amazon Kindle. With only two lines and six characters per line, the F3 doesn’t even display a seven digit phone number on a single line. The display is very readable in bright sunlight and it has a backlight for the keypad and screen in low light.

The F3 also has voice prompts, an address book, 500 minutes of continuous talk time and up to 300 hours of stand-by time. For $20, the thin and rugged MOTOFONE F3 is a great deal. When you don’t want to risk trashing your Android or iPhone, take the SIM out and put it in your F3.

Wikipedia has a list of undocumented codes.

We bought a Samsung USB data cable from Verizon – the rep at the store said it was difficult to get. I thought we could charge our Verizon SCH-i730 with our MintyBoost USB charger. It’s wasn’t that simple.

For some reason, Samsung decided to add another plug in the middle of the cable – so you could plug in your charger! If you wanted to charge your phone while it was plugged into a USB port, you needed the charger. I thought the USB cable was supposed to obviate the need of a charger but Samsung must have a different idea.

I called Verizon tech support and they said that the Verizon stores didn’t even sell the cable. They didn’t tell me much.

After opening up the box in the middle of the cable, I traced the +5v from one end to the other. It was obvious that soldering a jumper would enable +5v to charge the phone.

Once the jumper cable was in, I reassembled the connector in the middle of the cable and plugged it into the phone. The phone icon changed to the charging icon meaning that it was being charged by the Minty Boost.

I looked on eBay, and found there were cables that didn’t need this mod for about 75% less than what we paid for this supposedly difficult to get (according to the Verizon rep at the store) cable.

At least it works.

To get your own MintyBoost charger, that works with iPods, Sony PSP, Nintendo DS/GameBoy Advance, Blackberry and a lot of other devices, go to Adafruit Industries. For a technical overview of the charger, go to ladyada.net.