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