How to make a solar iPod/iPhone charger -aka MightyMintyBoost

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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:

2200mAh battery
solar cell
MintyBoost PCB

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:

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R-A2 months ago

Is it possible to charge the Lipo battery directly off the solar panel, instead of using a charging circiut - which adds cost to the project?

Honus (author)  R-A2 months ago
No- LiPo cells are very particular regarding charging rates so you must use a proper charging circuit.
fsato4133 years ago
Hello Honus, thank you for being generous and answering all the posted questions!

I just bought the v3.0 kit and I bought the new USB LiPoly Charger-single cell at sparkfun electronics as suggested by the Instructable website. The older charger circuit board seem to have connections for the battery, the mintyboost PCB and for the solar panel. I'd like to connect the solar panel to circuit board without the barrel plug ( or is the barrel plug more efficient? ) but don't know where to connect the solar panel wire to...And lastly I'd like to know if the LED for the charger turns off when the battery is fully charged.

Thanks again Honus!
jemor1433 years ago
Could you plug a pair of USB power heated slippers to the MightyMinty boost? (like the ones use here: ).
I'm a little worry about the current limit of the Minty boost (400ma?). I'm a newbie at this; any thought or advices would be much appreciated.
Thank you!
Honus (author)  jemor1433 years ago
If those insole warmers are designed to be powered over USB then yes it should work as the USB standard is 5V @500mA. Having said that there are some USB devices that can exceed that so the only way to know for sure is to try it out and see what happens.
cavelry3 years ago
This is a great project, and I'm glad to finally get my hands dirty with some soldering and circuit construction.

I know a big question regards charging apple products. I looked at some of the sites regarding soldering together the middle two prongsof the USB adaptor on the MintyBoost board, or some sort of voltage divider. But I tried charging my 64 Gb Itouch with and without the center prongs being soldered, and the Itouch still said it didn't recognize it as a charger. I did have it charging my Samsung Vibrant Droid though, so I know it works. Any thoughts on getting through to the Itouch?
Thank you
Honus (author)  cavelry3 years ago
Thanks- glad you like it! What generation iPod touch is it and what version Minty Boost circuit? Mine is the Touch 2G and charges just fine using 100K resistors on the USB data lines (using Minty Boost v2.0 circuit.) I haven't yet built the v3.0 circuit so I can't say how it works with a Touch 2G but it's supposed to work just fine with a Touch 3G.
ipodz3 years ago
I have a comment about charging rates. Lithium batteries are charged in three different stages. The charge controller you're using takes care of this, so you dont have to worry. I believe some of your calculations as far as charging rates are incorrect. Normally, a lithium battery charged at 1C can be charged fully charged in 2.5-3 hours. That means the 2000mAh battery can be charged by a 2A constant Stage 1 current in 2.5-3 hours. If you cut that stage 1 current to 1A, then the battery can be fully charged in around 4 hours. This is merely information from the datasheet of the battery. You can see that charging these batteries is not linear with respect to current. Time does not = (mAh)/mA in the case of lithium based batteries. I was wondering if anyone could help me calculate an exact time for charging a 1200 mAh lithium ion battery at 350 mA. Since it is a little bit more than 1/4C, I have calculated that it will fully charge in around 5.5 hours.

This is why iPods take 4 hours to completely charge. This will also be the case through the MintyBoost circuit. I hope someone can provide some insight into this issue.
Honus (author)  ipodz3 years ago
Hmmm... interesting.

The standard rate for charging a LiPo is 1C (2000mAh cell @2A, 1800mAh cell @ 1.8A, etc. but some newer fast chargers and LiPo cells can easily exceed this) and the charger will charge at that rate (constant current) until the cell reaches 4.2V and then it switches to constant voltage charging. This charges the battery at a gradually decreasing amperage holding the battery at its maximum rated voltage (4.2 for a single cell) usually until the amperage drops to 10% of the initial charge rate. The LiPo will usually reach 4.2V before it switches to constant voltage charging so you are probably adding only around 5% capacity by waiting for the full constant voltage charge cycle.

Charging a fully discharged LiPo at 1C rate should take about an hour (actually about 1.4 hours to reach 100% capacity if it's fully drained), no matter what capacity the battery is. I don't see how it can take 3 hours to charge a 2000mAh cell @1C rate and I don't personally know anyone that has had that experience. It's also not a good idea to completely discharge a LiPo cell anyway- most everything I've read says to charge cells when they reach 80% discharge. My iPod Touch has never taken 4 hours to fully charge, but I never run it until it is completely dead.

I calculate that with 1200mAh @ 350mA rate it should take you about 4.8 hours to charge it 100% but I'll bet you reach 95% charge @4.2 V in about 3.5 hours. That CV cycle adds a lot of time. It'll be interesting to see what you come up with!
delta19983 years ago
If you plug your iphone in the usb and the solar cell in at the same time, will the mightymintyboost charge the iphone first and then the battery or somehow do both at the same time.
Honus (author)  delta19983 years ago
The MintyBoost circuit will charge the iPhone as long as the LiPo battery has a charge- it doesn't matter if the solar cell is plugged in or not. The solar cell charges the battery and the MintyBoost circuit uses that to charge your iPhone.
delta1998 Honus3 years ago
nerdstrap4 years ago
My lithium battery seems to have some kind of drain on it. I see in Q&A #3 you say the Lithium Polymer charger prevents current leak. Are there other possible points for drain on the battery?
Honus (author)  nerdstrap4 years ago
Not unless you leave a device connected to it. All batteries will have some loss over time but the LiPos are pretty good in this regard- it should lose less than 8% per month.
if you are using a bigger solar panel that's voltage exceeds the maximum voltage of the charge controller, you could buy a DC-DC converter that will bring 12v down to 5v at a very efficient level. Have a look at these : (10-14V step down to programmable voltage through resistor)
onitreb4 years ago
Is it possible to hook up in parallel THREE 5v 100ma solar panels without any modifications to the mightymintyboost circuit?  I was thinking of making a 3-panel fold-out.

The tools and materials page says the barrel plug connection can only handle 280mA max - but the LiPoly Charger's MAX1555 "internally limits this to 300mA" on the barrel plug according to the Sparkfun product page.

If THREE 5v 100ma panels is too much for the circuit, what modifications would need to be made/added to make it work?

I'm a noob, but just assembled the kit (w/ 1 5v 100mA panel) and it seems to be working great!

dirtysteak4 years ago
Thanks Honus for the awesome instructable.

I guess I made the mega mighty minty boost?  I used your instructions and the FAQ here to charge the 6ah LiPo battery ( with the same charging circuit, and hooked it up to the 2.5 W large solar panel (

I took your advice and used a 7805 voltage regulator, and hard spliced it in to the same input that you used on the charging board.  Worked like a champ on the first go-around!  For those that are interested, pin 1 on the 7805 is the input (solder to red wire from solar panel), pin 2 is ground (solder black wire from solar panel and solder into GND port that Honus used on the charging circuit for his panel), and pin 3 is the +5V out (solder to the 5V port Honus used)

I am mounting it inside a pelican case (waterproof) with a clear lid, so it can be left outdoors at camp all day long without fear of rain ruining it.  There is room for a couple of phones and a camera in the case, so you can store most of your gadgets in it for safe, waterproof storage while in the outdoors. 

I still figure about 20 hrs to fully charge the battery, but I think we should get 4 to 5 charges from it easy once its charged.

Thanks again for the great instructions!
npmaier4 years ago
Hi there.
     Thank you so much for the great project.  I am trying to get my working solar charger to charge my HTC google phone.  The wall charger outputs 5 V at 1A.  I know that the Solar Charger does a little less than this. 
     So far, I have charged the LI battery to full (the light goes off) and then plugged the phone in at the USB jack.  The charge light on my Google phone lights for about 15 minutes (have tried it twice) then shuts off.  Then I can't turn on my phone.  After plugging it into the wall, it still charges and turns on.  It starts at zero power.
     I'm thinking there's some kind of short.  Is it possible that it is short circuiting with the Altoids tin?   I'm trying it tonight with new electrical tape lining the tin.
Thanks again,
On a Journey to Solar power,
Honus (author)  npmaier4 years ago

It doesn't really sound like a short to me. If that was the case I don't believe you would be able to charge the LiPo or get your phone to light up at all. I don't know anything about the HTC Google phone, but there could be an incompatibility issue with the MintyBoost circuit. Have you tried different resistor values or checked the Adafruit forums?
Well mighty minty boost has been working great. had it sitting outside my window charging it via solar panel. plugged my phone into it and nothign happen. thought it was odd so i check the voltage on the battery and have like 2.4V guess the thing died some how.  i plugged it into the wall wart to charge and the charging light doesnt even come on. unpluged it the battery from the lipo charger a few times, moved around some wires, no im avg between 2mV-2V randomly. i think im about to trash this thing since it gives me nothign but a headache.
Honus (author)  derrick.daniels4 years ago
It sounds to me like you have a bad battery. The battery should never drop below 2.8v and it has its own built in protective circuitry specifically to prevent that problem. I would contact Sparkfun to see if they will exchange your battery (assuming that is where you purchased it.)
It works great now. thanks for the help
ok the mighty minty boost was working until i tried to install it in the case. i think that the pos. and neg. touched the tin can. no the status light is out and i do not believe it is charging like it should anymore.  anyone know what happen?
Honus (author)  derrick.daniels4 years ago
It sounds like you might have created a short circuit. Unplug everything and try testing the MIntyBoost circuit by itself and then test the charging circuit and see what happens.
my battery has been charging for a day now and is still show 3.7V. is that normal
Honus (author)  derrick.daniels4 years ago
Individual LiPo cells are rated at 3.7v so that seems about normal. Did the cell ever drop below 2.8v?
no it didnt. it just stayed at 3.7V. after doing some reading i found out that i need to do a resistor bridge with a 39k and 59k resistor....not sure though how to do the bridge though. i dont completely understand the instructions.
So i got everythign working and it charged my phone using the normal alkaline batteries.  so i know the mintyboost part works.  i am having trouble with the lithium battery now.  i have been charging the lithium battery via solar, usb, and wall adapter for past few days and i am still reading 3.7V coming out of the battery.  the minty boost will no longer charge anything. and ideas of what i should do
(removed by author or community request)
Honus (author)  stevenpherbert4 years ago
I'm confused. You say when you take away the power source it won't charge the phone? So are you charging the LiPo battery completely without the phone connected?

I charged the battery overnight this time via USB, and I was able to unplug the power into the device and still charge my phone via the battery. I guess I didn't give the battery enough charge yesterday before doing it. Sorry for the dumb question. I'm glad it is working. My friends think I'm hilarious for wanting to build something like this myself, but it was just the thing I needed to motivate me to try some other projects. This is going to be really handy. Thanks!

Honus (author)  stevenpherbert4 years ago
Cool- glad it all worked out! There are lots of things that my friends think I'm crazy  for trying to build but I usually learn an awful lot in the process and I have a lot of fun.
bfulton4 years ago
 I have built this with the exception of using the 6Ah battery instead. My problem is that after fully charging the 6Ah battery, I still only get around 2 full charges on my iphone 3g, any ideas as to why I'm not getting 5 like I expected?
Honus (author)  bfulton4 years ago
No idea. You should really get about 4 charges on it since the Minty Boost circuit is about 82% efficient.  Are you unplugging the iPhone immediately after it's charged? If you leave it plugged in it will still draw power even if it's fully charged- it basically thinks it's plugged into a wall outlet.
bfulton Honus4 years ago
 No, I have charged it overnight, so I don't unplug it till I wake up. I am recharging the pack via the wall wart again now (it should be full tomorrow at noon) then I will try charging while I'm awake so I can unplug when it's full. Thanks for the advice, I'll let you know how it turns out.
wingrider784 years ago
What could my problem be that the charge led is not blinking for trickle charge...I haven't ever seen it blink yet.  I had been charging via usb to get the lipo up to full charge...woke up this morning and the led of off.  I measure the batt voltage and it was 4.18v.  I assumed this was fine since I remember reading that the max was around 4.2v.  I left the usb plugged in and tested it an hour later and it was it appears to still be charging, but the led is off.  I then unplugged the usb, and an hour later the batt tested at that normal or is something going on that I need to address??  I am charging with my switch turned off, so there is definately nothing drawing power off the middle (SYS) jst connection...

Also...when using the JST conncetion with the solar panel, I understand that the panel itself is limited to 100ma...BUT what is the JST connection limited to...the usb input is limited to 100ma, the barrel plug is 280ma, what about the jst???
Honus (author)  wingrider784 years ago
It doesn't blink very long before shutting off. I think everything is functioning normally. The JST input into the charger is tied to the same line as the barrel plug so the current is limited to 280mA.
I recorded the MMB charging yesterday just to find out if the led did in fact blink and it does not.  The led just goes off, it never blinked once...but if everything else is normal, I guess I just won't worry about it...

Okay, just ran some tests last night with charging my blackberry.  The battery in my blackberry is 1100ma and was at 45% when I started charging, the lipo battery was full...

Time -  lipo volts - BB% charge
5:25p - 4.21v - 45%
5:45p - 3.75v - 80%
6:00p - 3.73v - 85%
6:30p - 3.75v - 95%
6:50p - 3.74v - 100% and charge indicator on BB was off

This morning the volts on the lipo read 3.79...Just trying to find out if these are normal/good numbers to be getting
Honus (author)  wingrider784 years ago
I don't have a Blackberry for comparison but that sounds about right to me.
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