intro How to make a solar iPod/iPhone charger -aka MightyMintyBoost
I wanted a charger for my iPodTouch and the MintyBoost was definitely my first choice. I wanted to take it a bit further and make it not only rechargeable but also solar powered. The other issue is that the iPhone and iPodTouch have large batteries in them and will deplete the two AA batteries in the MintyBoost rather quickly so I wanted to increase the battery power as well. What I really wanted was a MightyMintyBoost!
Apple has sold over 30 million iPodTouch/iPhone units- imagine charging all of them via solar power…. If every iPhone/iPodTouch sold was fully charged every day (averaging the battery capacity) via solar power instead of fossil fuel power we would save approximately 50.644gWh of energy, roughly equivalent to 75,965,625 lbs. of CO2 in the atmosphere per year. Granted that’s a best case scenario (assuming you can get enough sunlight per day and approximately 1.5 lbs. CO2 produced per kWh used.) Of course, that doesn’t even figure in all the other iPods, cell phones, PDAs, microcontrollers (I use it to power my Arduino projects) and other USB devices that can be powered by this charger- one little solar cell charger may not seem like it can make a difference but add all those millions of devices together and that’s a lot of energy!
There are some really nice features about this charger:
It’s solar powered!
Large battery capacity- 3.7v @2000mAh
On board charger charges via solar, USB or wall wart. Accepts input power from 3.7v to 7v.
Remove the solar cell after charging and you have a nice compact USB power supply.
Unplug the solar cell and use the Velcro to secure the MightyMintyBoost inside a backpack or messenger bag- now plug in a larger solar cell attached to your bag for even faster charging. Using a slightly larger solar cell (6v/250mAh) you can generate enough power to fully charge an iPhone in about 5.5 hours and an iPod Touch in 4 hours.
Building this is really easy and straightforward- it only took me around an hour so follow along and build one for yourself!
Safety note and general disclaimer: Be careful cutting the Altoids tin as it can have some really sharp edges- file them smooth if necessary. Assemble this at your own risk- while it is really easy to build, if you mess something up there is the potential to damage the electronic device you are trying to charge. Be careful in your assembly and soldering work and follow good safety practices. Only use a type of battery charger specifically designed for the type of battery you are using.
step 1 Tools and materials
Here’s what you’ll need to build your own MightyMintyBoost:
Pliers (or muiltitool)
Clear packing tape
Lithium Polymer battery charger
3.7v 2000mAh Lithium Polymer battery
Small solar cell
2″ x 3″ adhesive backed Velcro
Small double sided adhesive squares
The single cell Lithium Polymer charger can accept input power that ranges from 3.7 to 7v maximum. When the cell reaches full charge the charger will automatically switch to trickle charging. When charging using the mini USB port, the charging current is limited to 100mA. When charging using the barrel plug jack, the charging current is limited to 280mA.
The solar cell maxes out at approximately 5v @ 100mA in bright sunlight. If you need faster charging simply use a larger solar cell- a 6v cell @ 250mA would work very well and they are easily obtainable and inexpensive. I used the size of solar cell that I did because I wanted it to be super compact.
I could not find out from the manufacturer if the solar cell I used has a blocking diode. A blocking diode is used in many solar charging systems to prevent the solar cell from draining the battery during low light conditions. Instructables member RBecho pointed out that the charging circuit used negates the need for a blocking diode in this application. You can tell when the solar cell is producing enough power because the little red LED on the charger will come on during charging.
step 2 Build the Minty Boost kit
First build the MIntyBoost kit according to its instructions. It’s really easy to assemble- even a complete novice can do it.
Instead of connecting the battery holder in the kit, we’re going to solder a JST connector to the MintyBoost PCB. This tiny connector will then allow the MintyBoost circuit to connect to the Lithium Polymer battery charger circuit. Make sure you get the polarity correct!
Test the MintyBoost by connecting the battery pack (make sure the battery pack has a charge) and charger circuit. The MintyBoost connects to the connector marked SYS on the charger board and the lithium polymer battery connects to the connector marked GND.
Now cut a notch in the Altoids tin for the USB port and use some double sided adhesive to mount the PCB to the Altoids tin.
step 3 Add the battery and charger
Now cut a notch out of the other side of the Altoids tin to fit the charger and secure the charging circuit to the bottom of the Altoids tin with double sided adhesive. Reconnect the battery and the MintyBoost PCB to the charging circuit. Make sure nothing on the bottom of either one of the circuit boards is touching the bottom of the Altoids tin.
step 4 Add the solar cell
There are a couple of different ways to connect the solar cell. The first is by simply shortening the connector leads and plugging the barrel plug into the barrel jack on the charging circuit.
The second method is to replace the connector with another JST connector and plug it into the third connector marked 5v on the charging circuit. I didn’t have another JST connector handy so I just soldered a salvaged two pronged connector to the charging circuit where there are two open pins on the 5v line.
Using the second method certainly is a bit cleaner since you don’t have the big barrel plug sticking out of the side of the tin.
Now attach the solar cell to the top of the Altoids tin using some 2″ wide Velcro. I wrapped the battery pack with a layer of clear packing tape to help protect it. Then the battery pack is simple set down on top of the two circuit boards- it’s a near perfect fit.
Now set your MightyMintyBoost out in the bright sun and charge it up! You should see a little red LED on the charger board light up. Once it’s fully charged connect your iPod/iPhone/USB powered device and enjoy!
How to make a solar iPod/iPhone charger -aka MightyMintyBoost
step 5 FAQ and additional info
Here’s a list of frequently asked questions:
Q: Is it possible to overcharge the Lithium Polymer battery?
A: No- the charger will automatically switch to trickle charging and then shut off.
Q: Is it possible to drain the Lithium Polymer battery completely and damage it?
A: No- the battery has its own low voltage cut off circuitry that will prevent it from completely discharging- the low voltage cut off is around 2.8v
Q: Does the solar cell have a blocking diode to prevent it from draining the Lithium Polymer battery?
A: No blocking diode is necessary- the Lithium Polymer charger prevents the battery from leaking current.
Q: How long will it take to fully charge the Lithium Polymer battery and how long will it take to charge my iPod/iPhone?
A: How long it will take to fully charge depends on the amount of sunlight available but as a rough guesstimate it would take around 20hrs using the small solar cell in direct sunlight. Using a larger solar cell could easily take half if not one third the amount of time. Those same figures would apply if you were charging it over USB or using a wall wart power supply.
Charging your iPod is much faster. How fast it does it depends on your device’s battery capacity. An iPod Touch has a 1000mAh battery so it should fully charge it in around 2hrs. A 3G iPhone has a 1150mAh battery so it will take slightly longer and a 2G iPhone has a 1400mAh battery, so it will take around 3 hrs.
Q: The Lithium Polymer charger has an input voltage range of 3.7v minimum to 7v maximum- what if I want to use a higher output solar cell for faster charging?
A: To use a solar cell with a voltage output greater than 7v, you need a voltage regulator to drop the voltage to a level that the charger can handle. You could use a 7805 voltage regulator to limit the output to +5v -they only cost about $1.50 and are very simple to wire up. The 7805 will give you as fixed +5v and is usually good up to 1A current. You could also use a LM317T which is an adjustable regulator, but it would involve a bit more circuitry to use. Some people also use diodes to drop voltage, since many diodes have a voltage drop of .7v
There’s a lot more info here:
The other option would be to use a 6v/250mA solar panel. This will stay within the current input range and voltage input range of the Lithium Polymer charger. Remember that you can also connect smaller solar cells in parallel to increase the available current- two 5v/100mA solar cells connected together in parallel will give an output of 5v @200mA
Q: What if I want to use a charger with a higher input current limit?
A: Sparkfun does have a Lithium Polymer charger that maxes out at 1A:
Q: How would I connect the more powerful charger- there doesn’t appear to be a clear way to do this?
A: To use the more powerful 1A charger you would need to wire a two way switch to the battery so that in one position the battery would be connected to the charger and in the other position the battery would be connected to the MintyBoost circuit.
Q: Will this work with USB devices other than iPods and iPhones?
A: You bet! There’s a list here:
Q: Won’t the inside of the Altoids tin short out the circuit?
A: No- using double sided foam tape to mount the circuit boards keeps the bottom of the board from coming into contact with the inside bottom of the tin. If you’re really worried you can cover the inside bottom of the tin with clear packing tape.
Q: How much does this cost? Can I build it for less? Is it cost effective?
A: If you buy everything as listed it would cost $70.75 (not including the Altoids tin or shipping.) If you wanted to scratchbuild it using the MintyBoost PCB from Adafruit, building your own charging circuit and supplying your own parts from various sources you can save quite a bit. Both the charging circuit and the MintyBoost circuit are available online- just go to the web pages listed in the tools and materials section- they’re also listed at the bottom of this page.
Both Maxim and Linear Technology supply free samples (according to their websites) of their ICs so you just need to provide all the other bits (available from places like Mouser and Digikey.) Using a slightly smaller solar cell and a 2200mAh battery it is possible to build it for a lot less:
After adding up the small parts for the MintyBoost circuit, a small blank PCB for the charging circuit (you would have to etch the board yourself) and a mini USB connector, you could conceivably build this for around $21.00 (not including shipping or an Altoids tin.) It wouldn’t be exactly the same of course, but it would be functionally the same. I don’t know if the 2200mAh battery would fit into an Altoids tin either. It would be a LOT more work of course, and there could be a fair bit of troubleshooting if you’re not experienced in building these types of circuits or soldering surface mount components.
So is it cost effective? Absolutely- it just depends on the amount of work you want to do. Either way, you get a very useful and versatile solar powered charger.
Q: How did you calculate the power usage and equivalent CO2 values?
A: Here’s the math-
3.7v (LiPo rated voltage) x .1A (solar charge current)= .37W
.37W x 12.5hrs (charge time based on average battery capacity) = 4.625Wh
4.625Wh x 365 days = 1688.125Wh per year
1688.125Wh per year x 30,000,000 units sold = 50,643,750,000Wh total used per year (50.644gWh)
50.644gWh per year x 1.5 lbs CO2 produced per kWh used = 75,965,625 lbs. CO2 produced per year
Granted these are more or less maximum values but they clearly show some potential for some serious energy savings. A 12.5hr solar charge time per day isn’t realistic for the majority of the planet but if you shorten the solar charge time to approximately 4.5hrs at a 280mA current the results still remain the same.
General information about the Lithium Polymer charging circuit as well as a circuit diagram and data sheet can be found here:
A complete description and documentation of the MintyBoost circuit can be found here: