Step 5: FAQ and additional info
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: http://en.wikipedia.org/wiki/Linear_regulator
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: http://www.ladyada.net/make/mintyboost/
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: