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
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
- 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
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
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
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
you will also need:
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!
Below is a picture of the almost finished board.
Step 7: Code!
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
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.