Introduction: Heavy Duty Portable Charger for Usb Devices (phones, IPad, Etc.)

Picture of Heavy Duty Portable Charger for Usb Devices (phones, IPad, Etc.)

Ever needed to charge your phone on the go? Can't find a wall socket to charge your iPod?

Whenever I'm away from home for an extended period of time, I would charge my phone and mp3 player from my laptop.  This way, I get about 3 or 4 full charges out of the laptop battery and I could leave the phone and mp3 player chargers at home.  But carrying around a laptop just to charge a phone and mp3 player seemed a little excessive.

Later on I discovered the now hugely popular Minty Boost , a small battery-powered device for charging USB devices.  However, after making my own I found that the two AA batteries used in the Minty Boost just didn't have enough juice for what I needed (a couple of 2000mAh AA cells gave me about half a charge on my phone before giving out).

So I decided to combine the capacity of a laptop battery with the portability of the Minty Boost:  A heavy duty portable charger.

The device is based around a DC/DC boost circuit, a microcontroller (I used a PIC), and a handful of 18650 lithium-ion cells.  Laptop batteries usually contain 8 of these cells (although I notice my netbook only uses 3, which explains the dismal battery life). I harvested the batteries for this device from an old Dell laptop battery, but you can buy 18650 cells cheaply on ebay, (you can see one in the top right of the picture below),.

Note: for this instructable, you will require experience with circuit building, programming and using microcontrollers.  I have included my source code for the PIC12F683, but the same circuit applies to Atmel or other microprocessors.

Note2: While I have designed the simple circuit from scratch, the general principles behind such circuits are well established, I am sure many people who have built such devices will have arrived at very similar circuits.  Infringement is not intended.

Pictured is the final device charging my phone and running a USB fan at the same time, this one contains 4 18650 cells, has two USB sockets and is built into an 8cm CD wallet, which I found was a perfect size.

Step 1: Comparison With Minty Boost-type Devices

Picture of Comparison With Minty Boost-type Devices

Those of you familiar with Minty Boost or similar devices might point out that the benefits of the AA batteries are that, being ubiquitous, you could simply go out and buy some more if you run out of power; and that this is unfortunately not the case with 18650 cells.

My argument for using the 18650 cells are: Firstly, having built and used a Minty Boost-type device for a while, I can say that I'd rather just wait until I get back to the hotel than go out and spend money buying more cells (which would give me half a charge anyway).  Secondly, lithium-ion has about 3 times the energy density compared with NiMH cells, so for the same weight in batteries, you should be able to get by until you reach a power point before having to replace them.

So both 18650 and AA have their own advantages, here's a comparison between a Minty Boost-type device and the heavy duty charger:

Battery type: 2 * AA
Approximate energy capacity*: 20kJ
- Small
- Uses readily available AA batteries

18650 heavy duty charger:
Battery type: 4 * 18650 Li-ion
Approximate energy capacity*: 128kJ
- over 6 times the energy capacity
- higher current output**

* energy capacity calculated using equation: no. batteries * battery voltage * battery Ah capacity * 3600 = energy capacity

** I've never tested maximum current output of the device, there are some issues with heat dissipation at higher output powers that limit the effective output current.

Pictured is the difference in scale between the two devices, alongside their batteries.  On the left is my own rendition of a Minty Boost-type device. built into a case that used to hold my cuff-links, running of 2 AA batteries and an LT1303 DC/DC chip (which I believe gives slightly less current out than the MAX756 of the Minty Boost).

Step 2: The Circuit: Overview

Picture of The Circuit: Overview

The heart of all these devices is the humble DC/DC boost circuit, this is essentially a transformer for DC power sources, it will step up a DC voltage (say 2.4V from two AA batteries) to a  higher DC voltage (say 5V for USB charging).

One shortfall of the Minty Boot-type devices is the use of a micropower DC/DC boost chip.  These ICs pack the transistor, sensors and oscillators needed for the DC/DC circuit into a neat 8-pin package that you can quickly build a circuit with.  Unfortunately, because they're "micropower", they're not designed to output higher power, but luckily they usually have enough to meet the 100 - 200mA required for USB.

To get around this, I've decided to build the DC/DC boost circuit myself.  The downside of not using a pre-packaged IC is that I have to build my own control circuit to control the DC/DC boost circuit.  The upside is that now I can choose components with a higher power rating so I can get more output current.

I've had to split the circuit in two halves to get them to display large enough to see the text, the next few sections will explain component selections.

Step 3: Components: Part 1

Picture of Components: Part 1

The component list and circuit is as follows, from left to right

18650 batteries (and battery clips):
I am using 4 18650 batteries in parallel, generally speaking you should probably avoid doing this since there is some overheating risk as the cells try to discharge through each other.  If you want to ensure safety, you could add a diode after each cell so that they play nice.  These lithium-ion cells have a terminal voltage of 4.2V when fully charged, dropping down to a minimum of 3V.  Any number of cells in parallel is ok.  You can also use AA cells, but you'd probably want to have them stacked in twos to create 2.4 - 3V.  Note: however you decide to arrange the batteries, do not exceed 5V, the DC/DC boost circuit can only BOOST a voltage, not reduce it.  To reduce voltage, you will need a DC/DC Buck circuit instead.
For battery clips, I cut and bent some paper clips.

a switch (SW): this is the power switch, it's optional since you could leave it switched on all the time, the circuit doesn't draw much power if it's left switched on with nothing plugged in.

smoothing electrolytic capacitor (C1): I use 220uF capacitors, this is for smoothing, so anything in the 100s of uF will do, Minty Boost uses 100uF.

a voltage reference diode (D2) + resistor (R1): you will need something like a low power 2.7V zener diode to give you the correct voltage reference for the microcontroller.  If you don't have any zener diodes, you could use a normal diode like I did (if you use a normal diode, you need to connect it the other way round to the zener diode in the diagram).  I (and also the supplied code) actually used a 1N4001 general purpose diode and a 10kOhm resistor.  You can also use a voltage reference IC

an LED (D3) + resistor (R2): any LED and suitable resistor is fine

a microcontroller: I used a PIC12F683, these are great little chips for simple circuits because they can run off any voltage between 2V and 5.5V

Step 4: Components: Part 2

Picture of Components: Part 2

an inductor (L): this is required for the boost circuit, the higher the inductance value, the less current ripple there is.  I use an inductor I wound myself, it probably has an inductance in the range of mH, and I know it's grossly oversized.  Here is a useful online calculator that'll work out inductance calculations.  I would recommend going for a minimum of 200uF

an NPN transistor and base resistor (R3): this is the main switching device in the circuit plus the base resistor (or gate resistor), make sure your transistor is designed to handle high currents (preferably more than 1A) at high switching speeds (20kHz or more), and hFE of at least 50.  I have had success with the general purpose BC337, but be careful about overheating.  A MOSFET works too (and will probably give you higher efficiency). I am using a D2012 because I had one lying around (ripped it out of a defunct CD drive)

a diode (D1): this is required for the boost circuit, a fast-switching diode is preferable, so I use a Schottky diode (1N5817), although a general purpose diode like the 1N4001 would work too

two resistors (R4, R5): the 5V output is higher than the supply voltage (ADC reference voltage) of the microcontroller, so this output needs to be stepped down, two resistors of equal value (I use 22kOhm resistors) will form a potential divider to allow the 5V output to be halved.

another smoothing electrolytic capacitor (C2): again, I use 220uF capacitors

USB sockets: I've used 2 sockets

Step 5: Components: Part 3

Picture of Components: Part 3

you will also need:

some wire

some prototyping board: strip-board or breadboard, or anything else you want to build your circuit on.

A battery charger: you need something to actually charge your batteries!  An 18650 charger can be purchased cheaply on eBay

some sort of enclosure: something to keep your circuit in.  Minty Boost used Altoids tins, you will need something bigger... I used an 8cm CD case (for those 8cm mini-CDs)

a computer and programmer for your microcontroller: obviously, you need a computer programmer to load the code onto the microcontroller.

Pictured below:   A cheap 18650 charger from eBay next to some 18650 cells. I got my 18650 from some old laptop batteries, but you can also buy them new.   There is no noticeable difference between the two, other than it's likely the old laptop batteries will have a lower capacity due to ageing

Step 6: Build!

Picture of Build!

Unfortunately, I didn't take many pictures during the build process, and I'm going to assume that you have enough experience with circuit building to be able to read the circuit diagrams and build one for yourself.  If not, there are some great instructables on building circuits out there, so explore!

Below is a picture of the almost finished board.

Step 7: Code!

The code that needs to be loaded onto the microcontroller needs to do a few things:
1. Detect the reference voltage:
This is done using the voltage reference diode, (hopefully) the this should produce a fixed voltage regardless of the battery input voltage.  If you used a 2.7V zener, then the output voltage should be 2.7V (if you sized your resistor according to the datasheet.  If like me, you used a 1N4001 general purpose diode and a 10kOhm resistor, the voltage should be around 0.5V - 0.525V

2. Set the output voltage:
The microcontroller outputs a PWM signal to control the transistor and DC/DC circuit, the higher the duty ratio, the higher the boost ratio.  To ensure the output is at 5V, the microcontroller must adjust this PWM signal so that the output stays at the target voltage.
This is done using sensing and feedback; the output voltage is sensed (through the potential divider), and the PWM signal is adjusted if this sensed voltage is different to the target voltage.
The target voltage is calculated as a multiple of the reference.
In my case, with a 0.5V - 0.525V reference, the PIC tries to keep the sensed voltage about 4.85 times the reference voltage.

3. Check the battery voltage:
This is also done using the reference voltage, if the battery voltage drops below 3V, then the indicator LED will flash. (Discharging Lithium-ion batteries too much can cause bad things to happen, I would actually advise adding an extra transistor to disconnect the output if the voltage drops too low)

Attached is some C code which will compile for the PIC using MPLAB and HI-TECH's PIC C compiler.  Hopefully it should be self-explanatory. I've used some rudimentary averaging routines, which are probably not necessary.

Step 8: Finished Product

Picture of Finished Product

And if everything goes well, you should have a functioning charger.  Enjoy!

Additional info: Some devices, in particular things like the iPhone, or Motorola phones, require some resistors to be attached to the USB's data lines (that aren't connected in this circuit), if your device doesn't charge.  There is some information on the Minty Boost website about getting the iPhone 3Gs to charge.

Disclaimer: Lithium-ion batteries may explode if handled improperly, please do not attempt this instructable if you are unfamiliar with building circuits or using lithium-ion batteries.  The author takes no responsibility for any injury or damage to self or property through the attempt to recreate things in this instructable.  The information contained within are guidelines only, and should only be followed with the necessary knowledge of building electrical circuits and electrical safety.


SuperTech-IT (author)2013-08-08

This would be an awesome project if combined with my project here:

thienthien8x (author)2012-10-11

will you please send me the hex file?
because i can't compile code, thanks

navarroraymond13 (author)2012-03-04

i wonder what is the actual effective output current when charging a mobile phone, how long does it takes for a mobile to be full charge from fully discharge? Any idea from the author?

meseta (author)navarroraymond132012-03-05

Hey,'s been almost two years since I posted this, seems like forever ago.

As I understand it, it's the mobile phone charge circuitry that decides on the current. Most modern smartphones these days will charge at around 200mA when plugged into a USB port, and some will switch to a higher charge current when it detects a mains plug or compatible USB port (the mains charger has some extra resistors connected to the USB data lines on the charger, or the phone negotiates with the computer to determine if the USB port can support the current), maybe around 500mA. The iPad for example (though not a phone) I think is designed to charge at up to 1A when plugged into a compatible source.

In short if you take a typical phone battery of 2200mAh, and a typical charge current of 500mAh, then the time it takes is very approximately 2200/500 = 4.4 hours.

These are approximates, you'll want to look at your phone's battery for the battery capacity in mAh, and look at the output rating on your phone's mains charger for the input current. This output rating is the maximum rating, and is often higher than the actual current used, so pick a reasonable value lower than the maximum (mine for example is rated 0.7A, so I'm fairly sure it's charging no more than 500mA). Divide one by the other to get hours.

With home-made chargers like the one above, you can wire some resistors to the USB port to trick the phone into charging at a higher current.

dasimpson1981 (author)2011-08-03

just use a large metal plate for the heat disipation

elinventor21 (author)2010-07-04

ok i am definitely a noob at this code stuff how would i put a code from a computer into one of this ic's?

lee321987 (author)elinventor212011-04-18

The cheapest one made by the company that makes PICs is called the "Pickit". There are clones you can buy or make, for cheaper (though they may not work for as many chips as the official one).
To build your own will require a PIC programmer, so you might as well buy one.

imakethings (author)2010-06-06

i have an inductor of 200mH will it work? i am getting a output of only a 100 ma from 2 AA batteries . should i put more batteries in parallel to get more current output??

meseta (author)imakethings2010-06-06

yes 200mH will probably work. It depends on what is limiting your current. If it's the batteries, then putting more batteries in parallel will get you more current output (but ensure that all the cells are the same type, age, capacity, etc. to avoid unbalance). If it's the rest of the circuit, you should check the switching frequency, and switching device ratings.

imakethings (author)meseta2010-06-07

its 200 Milli Henry .but the max756 datasheet suggests to use upto 22 micro henry is it still ok??

meseta (author)imakethings2010-06-07

200mH is much bigger than 22uH 200mH (milli-henries)is 0.2 Henries 22uH (micro-henries) is 0.000022 Henries. Larger inductances are generally ok for these purposes, as far as I can recall, the only disadvantages is that a 200mH inductor is much larger than a 22uH inductor, and also would have a higher resistance.

imakethings (author)meseta2010-06-09

i am using 1N5819 diode is it ok?

meseta (author)imakethings2010-06-09

yes, in that case I'm not sure why your current draw is low. Try measuring the cell voltage when you draw current. If the cell voltage is a lot lower than it is when you're not drawing current, it might be an indication that your cells aren't supply enough current. In which case you may try using AA cells in parallel. Or you could try using NiCad cells.

imakethings (author)meseta2010-06-10

i have a 10 Milli Henry inductor which looks more or less like a resistor. is there and difference between a power inductor and a normal one? also the inductor i have gets heated up very fast...will using a power inductor help?

meseta (author)imakethings2010-06-10

Oh yes, smaller inductors don't have the same power rating, they will heat up, but inductors are simple devices, as long as they're not getting hot, you're ok. Also, a word of warning with small sized inductors is that you may be approaching the magnetic saturation point of the material, leading to much reduced efficiency (and therefore low current out), the datasheet will contain a "saturation current" value, this is the maximum current you can run without saturation. A power inductor will help, they are designed for higher currents (and saturation), and are larger in size than normal inductors.

imakethings (author)meseta2010-06-10

ok thank you very much..

imakethings (author)meseta2010-06-09

what should i do to get a greater current output? i am using the max756 circuit. i am getting a current out put of 100 ma.. how much current do we generally get from a minty boost ?

meseta (author)imakethings2010-06-09

If you are using a monolithic micro-power boost IC, then there's little you can do to get a higher current than its rating. However, the datasheet suggests a maximum output of 300mA from a 2.4V input (2 AA cells in series). I would recommend using a higher input voltage, i.e. 3 AA cells in series. Are you using a Schottky diode?

imakethings (author)2010-06-06

what is the value of your inductor?

meseta (author)imakethings2010-06-06

I'm not sure, wound it myself, and didn't have a meter that could measure inductance on hand. The last inductor I wound using approximately the same number of turns and core came out in the range of 10mH. I should mention that this value is much larger than it needs to be, I'd say approximately 100uH would have been enough.

teknojo (author)2010-05-22

I love your design. I am looking for something like this that will handle a draw up to 300 to 500 mA. Yours will only take a 200 mA draw correct?

meseta (author)teknojo2010-05-22


No, the whole point of this project is to get a higher current draw than you can from a cheap micro-power converter.

The maximum theoretical current that can be drawn from this is something around 2.5A. (Given the transistor is rated 5.5A, and the 4 cells themselves can supply something in the range of 6A, and we're talking about a boost from around 3V minimum up to 5V at a minimum efficiency of 80%)

Now practically, the current is probably most limited by heat dissipation, but I'm sure you can do 500mA with no problem.

imakethings (author)2010-05-02

good one..does the pic ic replaces the max756 of the minty boost in this circuit? can you suggest modifications in the minty boost circuit itself?

meseta (author)imakethings2010-05-02

Yes, actually the MAX756 is more than that, it also has a the transistor and voltage reference.  So in reality, the PIC, transistor and voltage reference diode replaces the MAX756.
(also note that the MAX756 probably has some analog circuitry to perform the task, whereas the PIC is a microcontroller)

Hmm... for the minty boost circuit, you could do one of the following:
1. use 18650 cells.  But you'd still be limited to the current output of the MAX756

2. add an output disconnect transistor: the minty boost circuit won't allow you to add more than 3 AA batteries, their website says " In fact, with 4 batteries, the output voltage is 6V which is too high and could theoretically damage your device!" .  The truth is that 6V is probably fine, since the maximum of the MAX756 is 7V, the only problem with using a 6V is that the MAX756 is a BOOST circuit, and therefore unable to reduce the voltage to produce the needed 5V output.

The way around this is to use an output disconnect transistor, I found this little circuit on the LT1303 datasheet (LT1303 is another DC/DC Boost IC), scroll down to the ffirst figure on page 10.

The addition of a PNP transistor and resistor will allow you to use a higher input voltage and still be able to produce a regulated 5V out.  You can now use 4 AA alkaline batteries to the minty boost (max voltage 6V), or 5 AA NiMH batteries (also 6V).  OR if you used an LT1303 instead, which has a higher input maximum, you could go up to 5 to 6 AA alkalines, or 7 to 8 AA NiMH

3.  there's one thing you could theoretically do with a MAX756 circuit to increase its power output: you should theoretically be able to connect the LX pin of the MAX756 to a high power PNP transistor.  Because it's the internal transistor that is limiting your output current (hence why I opted to go for a transistor of my own selection), by connecting a transistor externally, this is like turning the circuit into a darlington pair.  I may try this at some point, I'll post an instructable if I do.

hope some of this helps.

imakethings (author)meseta2010-05-07

thank you very much.waiting for your next instructable with MAX 756!

Tetrafluoroethane (author)2010-05-03

I was curious as to what drove your decision to use a PIC vs a dedicated switch mode controller chip.  I know you mentioned the micro-power chips were under-powered in your opinion, but there are plenty of regulators which allow you to supply your own external switching element and simply do the PWM, thus allowing you to scale to a wide range of powers.

meseta (author)Tetrafluoroethane2010-05-03

The main reasons was that I was making something with the material I had, I have PICs, but no dedicated DC/DC chips.  Although I did think about ordering some LT1070 ICs, but those things are comparatively expensive.

You're right in that if I wanted to make this into a polished product, I would probably use something like that.  But this was more of a "hack together what you have" kind of project.

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