My DIY USB external battery pack

[raspabalsa]

There is no fan-out problem, as I'll use 3 op-amps, and each op-amp will drive one LED.

Check the schematic of the SOC circuit below. The whole circuit is powered directly by the battery (Vin). This way I make sure that the voltage powering the op-amps is always higher than the voltage in the USB ports (Vout).

S1 is the switch that has to be pressed for the indicator to work.

The zener is a 5V, 0.3W, with a series resistor to ensure at least 15mA, enough to keep it working properly.

The voltage divider has two fixed resistors and two pots. The two pots (top and bottom) are used to fine tune the divider. The bottom pot sets the lower threshold (3.6V). The top one sets the higher threshold (4.2V). The two central resistors divide the range (4.2V - 3.6V = 0.6V) into two segments of 0.3V. So the bottom op-amp detects a voltage of 3.6V, the central op-amp a voltage of 3.9V (3.6V + 0.3V) and the top op-amp a voltage of 4.2V (3.6V + 0.3V + 0.3V).

Each op-amp drives a LED via a series resistor. The LEDs could be any model, ideally the bottom one will be red, the central one yellow and the top one green.

While Vout (voltage at the 5V regulator's output) is below the threshold, the op-amp's output is at -Vbat, so the LEDs are off. When Vout is equal to or greater than the corresponding threshold, the op-amp's output jumps to +Vbat, so the LED turns on.

So this indicator will light the red LED if voltage is at or below 3.6V. If voltage is between 3.6V and 3.9V both the red and the yellow LED will light. If voltage is above 3.9V, all three LEDs will ight.

If needed, I can add more stages to the circuit. U1 is a quad op-amp, meaning one op-amp is not used in this design. I could add a fourth stage, changing the divider network accordingly. Good thresholds would be 3.6V, 3.8V, 4.0V, and 4.2V. But I think with thre LEDs (three stages) it is good enough.

The circuit as it is will probably fit inside my current case, but I'll have to rearrange some elements. I'll change the switch I'm using with another, smaller one and of a different design, because the one I'm using has been switching on and off by itself whenever it bumps against othe objects or against my backpack's walls.

I'm also thinking of a way to increase the heat sink capacity. I think I'll cut part of the case's bottom wall and replace it will a large flat heatsink I have, this will provide a larger area and will save some inner space.

Konrad, I really appreciate your suggestions, in fact you've given me a lot of things to think about in all the threads we've "hijacked" . The 723, for example, is a VR I often overlook because I'm used at assembling quick & dirty supplies based on the 78xx or LM317. But this project clearly could benefit from a 723... or a switched supply. <drooling>

Ok, I'll go shopping tomorrow afternoon, then I think the boss will send me back to the mountain and will I will probably be offline the rest of the week. I'll try to finish these mods during that time. On second thought, I think I'll arrange for a computer or two to go bad at the office, that way I'll be able to stay in the city (and online) for a couple more days

[raspabalsa]

Hi all! I've been away for ages, it seems! It's good to be back. It's been a month of all work and no play, which we all know makes raspy a dull boy , so here's the (almost) culmination of a fun project:

I finally found the components for the battery's voltage gauge. I made several changes to the schematic I attached in my previous posts (mainly corrected mistakes, I mean). Check the first and second pics attached below for the final version of the charger and SOC. I decided not to change the 7805 for a LM-317, mainly out of laziness, but also because the advantages in terms of extra power were not as good as I first estimated. The SOC is very similar to the previous one I posted, but I changed the point to be measured. The SOC still is powered by the full battery voltage (Vbat), but the measurement is taken in the middle of the battery pack (Va) , so the SOC takes half the battery pack's voltage and compares it against the threshold points set by the voltage divider network. The rest of the circuit is the same: the zener's reference voltage (5V) is divided by the network into the thresholds: 3.6V, 3.8V, and 4.0V. Remember my pack is made up of six Li-Ion cells, connected in parallel by threes, then both sets of three cells connected in series. So the corresponding full voltages would be 7.2V,7.6V, and 8.0V. I'm just halving the voltage to be measured by the SOC. Each of the threshold voltages is fed to an op-amp, and compared against Va. When the battery's voltage exceeds the threshold, the corresponding LED turns on. So when battery is fully charged all three LEDs are on, when at 2/3 charge both yellow and red are on, when at 1/3 charge only the red is on, and when empty all are off.

Third picture is the prototype assembled in a breadboard. As you can see the circuit is very simple, just 1 IC, two pots, six resistors, one zener and three leds. The circuit on the lower right of the breadboard is an adjustable regulator built around a LM317. I used this to calibrate the SOC, but this does not go in the final circuit. In the picture, the voltage being measured is just a tad under 3.6V, so the red LED is still on, but dropping the voltage a bit turns it off.

Fourth picture is the SOC assembled in its board. Again, there's the adjustable regulator for a final calibration. Here, voltage is just above second threshold (3.8V), so both red and yellow LEDs are on.

Admittedly, the board is big. I could have made it smaller, but not by much, mainly because it's not a printed board. Instead, connections are made with jumper wires on the back. It's kind of hard to make a decent PCB when in the middle of nowhere, with no-man's land to the left and a big "there be monsters here" sign to the right . Anyways, the components are also quite big, I couldn't find smaller pots in the right range. The length and width of the board were no problem, it fit well in the device's case. The main problem was the height of the board when components were assembled.

Picture five is the initial attempt to install the board inside the case. As you can see, length and width are no problem here. The problem was the height of the board. I needed to increase the height of the case by a little over 1 cm (about 3/8"), and this would leave a huge empty space in the front part of the case, between the batteries in the bottom and the case in the top. Also, I would have needed to reinforce the whole structure of the case and this would mean much more weight. So I decided to do things a little different. BTW, this pic shows what a typical raspabalsa's mod looks like: lots and lots of splice tape, epoxy putty and super bonder

In the end I came up with a nice solution, seen in pictures 6 and 7. I did raise the height of the case, but only in the area occupied by the SOC board. I glued panels made up of perforated circuit boards (the only suitable material I had at hand), filled in the holes, then used epoxy putty to level up all imperfections and sanded with 180 and 240 sandpaper for a nice, smooth finish. I haven't painted the device because I just arrived today in the city and haven't shopped for the adequate paint. The device still needs a final sanding with a finer grain sandpaper before painting, but what you see is the final implementation. I changed the original on/off switch because it protruded too much and the pack would turn on by itself when placed in my backpack. You can't appreciate in the pics (60% lousy photographer + 40% lousy cellphone camera = 100% lousy pics), but the new switch is inside a small hole in the case wall, so its top part is flush with the case and now it won't be turning on by itself. Pic 7 shows the LEDs working. You have to press a little button for the indicator to light up. The button is in a small hole so it won't be accidentally activated, and the hole is big enough for the tip of the stylus or any pen to fit in and reach the button.

All the above work means the device's weight has increased, from about 200 grams without the SOC, to about 350 grams with the SOC and all the body work. It's a considerable increase, 75% more weight, but the added functionality of the SOC more than compensates for it. It's still very light and doesn't add much to my standard field backpack.

That's about it. I don't plan to make any further mod to this project. Just some sanding and painting, but nothing more. I'll post a pic of the device if I can paint it before going on vacation.

All in all, I'm very, very satisfied with this little gadget. I think I spent almost two days pressing the SOC button just to see the LEDs turn on . Battery power is awesome to say the least. I'm sure with a good paintjob it will look great

[Dick Tracy]

Sweet! Please don't charge the monsters on the right.

[Konrad Pierce]

Clapping, bowing, whatever it is we're supposed to do now that we're not repping ... all Hail Raspy

You've probably done a better job than I would have - my part count (and heat losses) would've been twice as high

[RickAgresta]

Sweet! Please don't charge the monsters on the right.

<RA wonders if the sign looks like this....>

[expressviewer]

Hi Rasp,
Great work and looks very interseting to try out. may be a dumb question but do you have the pin connections you used for the USB connectors?
thanks

[Konrad Pierce]

USB pinouts are easy to find on google. Here or here, among other places.

For this application you're not interested in a "real" USB device that properly enumerates itself and such, just something that plugs into a USB connector to draw power from the USB host (a so-called USB decoration, strictly speaking). You need only worry about Pin 1 (VCC, +5V Red) and Pin 4 (GND, Black) ... for this application you would gain little advantage from adding great complexity.

[yorker]

This is sweet! Really nice job raspy!

[roony]

hi, awesome job, found your thread googling for my own external psu, got some embarrassing beginner questions, i've been trying to find the housing for the batteries, the case with the springs, whats it called or some links to pick some up.

Also will the battery case directly work with this http://linitx.com/viewproduct.php?prodid=10917

from the site : M2-ATX is an intelligent, high power, vehicle (car / boat / electric cart) 12V DC-DC ATX PC power supply designed for car pc and battery powered applications

or this practically the same : http://www.morex.com.tw/products/pro...l.php?fd_id=62

[raspabalsa]

Hi all! I forgot to check this thread last time I was online. Thanks all for comments and nice words.

Hello roony, welcome to Brighthand! I'm not sure what you're planning to build, but the M2-ATX and the PDB060R looki like interesting devices for car-based applications. I'm not sure how they relate to my application, which is charging a PDA or cellphone with a battery pack. Would you like to share your project idea?