## Step 8: The Process: Prototype Testing

Now we test to see if it works! With the two batteries inside, I measure the voltage on the USB connector: about 5V, which is good. I send off this version to a friend with once of each kind of iPod, including the newest 4G video iPod, for real-world testing: Both to verify the iPod will charge and also how long it will run with the additional pack.

Its also time to verify the math for efficiency: how good is it, after all?

So, in theory, we should be able to calculate the efficiency of the boost converter from datasheet info. We're basically boosting 2.5-3VDC -> 5VDC at around 50mA-100mA. Looking at the MAX756 datasheet, note the efficiency graph.

So we should be getting around 85% efficiency, perhaps a little more. I think the only thing that can really change this number a bit is the inductor. (Below, I verify I'm getting 82% efficiency)

If we're getting 82% efficiency conversion from 2 x 3000mAh Duracells, that means we get (2 * 1.5V) * 3000mAh * .83 = 7.38 Watt hours. Compare that to a single 9V as we calculated before: (1 x 9V) * 500mAh * .65 = 2.93 Wh. So we're going to get about 2.5x more power out of these two AAs than a single 9V.
With rechargeable batteries, we get (2 * 1.25) * 2200mAh * 81% = 4.45 Wh (about 50% more than an alkaline 9V and 3x more than a rechargeable 9V)

Next, lets verify the efficiency using test equipment, and try out the different inductors to see if they make a difference. Instead of using batteries, I'll provide 3V from a bench supply that will also tell me how much current is being drawn. And instead of an iPod I'll fake the load with a resistor. Since the standard USB current draw is 100mA from 5V, that means I need a 5V/.1A = 50 ohm load. I can't just use a tiny resistor because 5V * .1A = 1/2W and most resistors are 1/4W. So instead I take two large 100ohm 'power' resistors, and twist them together. I also check the resistance to verify that together they are 50ohms. I also find a 20ohm power resistor. This will allow me to not only test a 100mA load but also a 250mA load.

I perform 4 tests with 2 inductors: 100mA load for both 2.5V in and 3V in (rechargeable and disposable batteries) and 250 load for both.

My results are summarized in a table attached as the second image

It looks like inductor #2 is little more efficient, probably due to the fact it has a lower DC resistance (30 milliohms instead of 70mohm of the other inductor). It's also a bit cheaper so I'll go with that inductor.

Regardless, it looks like the efficiency is around 82% which is about what I expected.

Another thing to note is that I don't put an on/off switch in like you'd need with a 9V+7805 regulator. That's because the quiescent current of the MAX756 is very low, on the order of 100uA (0.1mA). I measured this myself and got about 75uA.

That means that the self-discharge rate is ~2000mAh / 0.1mA = 20,000 hours, more than 2 years. Most batteries don't last that long! Therefore we don't need a switch, when nothing is plugged in, almost no power is being used.

(in the end, i found another radial inductor that was cheaper and as efficient, which is what I use in the kit)
<p>What is the holder you are using in the pics?</p>
<p>Would this project work better with a 9 Volt battery, instead of the 2 AA?</p>
<p>good idea thank you</p>
<p>does anyone know if you HAVE to solder the wires? And why mine burns up before I even plug in the USB??</p>
I made this from the kit but on my iPhone 5s it charges for about 20-30 mins then it says &quot;This accessory may not be supported&quot; and stops charging. Any advice?
<p>can i use a coiled inductor of 22 mH? Its hard to find the one mentioned in the project!</p><p>please reply soon</p>
It will probably be fine. I'm pretty sure the inductor is just an AC choke, but you'll probably experience some efficiency loss due to the higher DC resistance.
<p>how long does that battery normally last?</p>
<p>Could I stumbleupon http://www.umi-zero.com/</p>
<p>A nice effort and I salute your spirit for <br>developing such a nice product. Well, I even purchased one from Amazon <a href="http://amzn.to/1BrwTY9" rel="nofollow">http://amzn.to/1BrwTY9</a> <br>that can charge up to 6 devices including the latest iPhone 6/6 plus, Galaxy <br>Note 4, Nexus 6, iPad Air 2 simultaneously in just 2 hours with its unique auto <br>detection feature. Currently the charger is under 20% discount after applying <br>this coupon code: 20OFFANU</p>
Where are the parts? No idea which caps i should us if I do make one... It is genius. My 9V battery charger lasts for a few minutes.
<p>The parts are in the .sch</p>
<p>This is very well-done and sexy! I want to make something very similar, and I was thinking it would be cool to use rechargeable batteries AND a small solar panel. The thing could be left on a car dashboard (assuming the sun isn't too bad for the batteries) and it would slowly charge the rechargeable batteries. THEN you could take it along and use it as an emergency power-source for phones. No expensive battery replacement, and no worrying about whether the sun is powerful enough. <br><br>Also: I'm not the most perceptive person, so correct me if I'm wrong: but why on earth do they not sell things like this in stores? People whose phones are dying are a pretty damn big demographic (and a desperate one at that!) They should sell these things pre-loaded with batteries at airports, supermarket checkout lanes, etc. What's going on with that?</p>
<p>I found solar-battery chargers on Amazon... check out PowerAdd and Levin SolStar to name a couple. they're about \$20-\$30 and work fine for me (when I RTM). I don't remember seeing chargers that take AA batteries though. </p>
Mine heats up a lot got any suggestions?
&nbsp;do any of u know how to add a switch and led to this kit
<p>a switch you would just wire directly onto the red cable coming formt he battery, the LED you would want to place right behindm the switch, probaply you would need a resistor to make sure the LED doesn't get a too high voltage.</p>
<p>indian not max ic max ic equivalent ?</p>
<p>just wonderful..a detailed project...</p>
<p>Great innovation!! But I still prefers the usual <a href="http://www.sfcable.com/portable-power-bank.html" rel="nofollow">power banks</a> because of some reasons that is it is not gonna last for a long time, and these batteries can't be able to charge my i phone even to 80%...</p>
<p>u could also use 2 dc batteries and put them in a watch case or can but the same idea just alittel more work needed to make.</p>
<p>Could I use hot glue instead of foam tape? Also, would this work? </p><p><a href="http://www.amazon.com/DROK-Power-Converter-Module-Charge/dp/B00C93Z8JY" rel="nofollow">http://www.amazon.com/DROK-Power-Converter-Module-...</a></p><p>Thanks.</p>
<p>can i use 4x aa batteries?? thanks..</p>
Hi..I am a student pursuing my third year in engineering..i have taken minty booster as my project..i have successfully implemented this circuit on the breadboard and got the output..but in the PCB m not being able to get the output..it would b really helpful if i get the layout of PCB..thanking you!
Did the popcorn come with the stuff? if it did what site do you order from that's good customer service!
when will u make a solar powered one
Hey Can dis circuit be used for a 4-volt input battery(18650 kind li-ion cell,,2400mAh)???? <br>coz i got six of dem from a laptop battrey..... <br>It would be great if you would help.......thanks
i don't know for sure, but oh my god! Use proper english! its not that hard to type an extra 3 or 4 keys on your keyboard is it?
Only as hard as hitting the shift key to capitalize your sentences. Also, Yes, This can be used with a 18650 Laptop cell. Got a 3AH one hooked up to mine and used a cheap gutted 18650 charger and a 6V solar panel to charge mine. No issues for the past 7-8 months.
I was looking through the instructable and decided that I wanted to add in the low battery LED. I found the circuit diagram in the version 2 schematic showing how to wire it. <br> <br>My question is regarding R3, the value is not mentioned? Also why is it neccessary if your using two 1.2V cells in series since it gives you 2.4V then while the LED that I want to use has a max forward voltage of 2.4V. This falls within the rating? <br> <br>Or is the resistor used for when your using 1.5V batteries i.e. 3V? Anyways was just wondering if someone could help me out with the value? Thanks in advance. <br> <br>Really enjoyed this instructable, still busy with it, but thanks LadyYada for this instructable.
what is he using to hold the pcb? id love to know what type of vice that is
She* <br>Not certain on the exact name, but i have one very similar i bought at a electronics store called a PCB vise...
Hi, great project. Does anyone by any chance know the name of this components in Spanish?<br>Thanks a lot.
Products still tend to be sold under their english names... however the part numbers should at least be usable..
If I wanted 2 parallel banks of 2 AA's to increase charger time, would your circuit work still?
if you have 2 sets of 2 AA's in series, and these sets in paralell (i.e. so the voltage is still 3V ) then yes it will work, you can also use AA or C or D size batteries.. <br> <br>:)
This is one of the best idea I have never seen &quot;IF&quot; I saw this design 5 years ago. <br> <br>No disrespect at all, but, I had my first 2G iPhone back to 2007 and I already bought this product in the market 5 years ago for my iPhone. <br> <br>You didn't mention this is the original design, but if there is any product similar to yours. Why pay more for your product? All the big brand such as GP. Their retails price is even lower than yours. <br> <br>Good luck mate.
Hello, <br>If you hadn't noticed, this was published 30th may 2006 - over 6 years ago! <br> <br>Back when this was designed and available for sale, down here in Australia there wasnt a single commercial charger of this design available anywhere! <br> <br>I have about 4 of these now, and they still work perfectly (2 x V2 and 2x V3), this design then inspired diy users to modify and improve the design, the company's saw this and developed their own. <br> <br>Also, personally i would pay for something that i built, i know how it works, and i know wont fail anytime soon. This product is not cheap &quot;Made in China&quot; crap. <br> <br> <br>Please Check your dates before posting. <br>- Ralim <br>
Looking at this, the 9v versions are way cheaper to make yourself, and if you're using rechargable 9v's in it is it really that much worse? Especially if you use the high-power ones that you can get now... <br> <br>Nice project though, I like the presentation v. much!
How much would it cost me to get one of these pre-assembled? I would love to get my hands on one of these, but I solder like an epileptic squirrel at a rave party. I have looked around the internet for a usb charger to run my headphones with, but they never show me the actual power specs for the device output; I do not trust said products.
Mouser is 20\$ shipping .. too much!
This is probably one of the coolest things I have ever seen. I downloaded the layout files and I am looking at various PCB Manufacturers... <a href="http://www.customcircuitboards.com/" rel="nofollow">Custom Circuit Boards</a> looks pretty good for these type of prototypes. <br><br>Any other suggestions for prototype projects like these?
I am from the uk and i am looking to build this kit. i was just wondering whether this is the right boost controller: <br>http://search.digikey.com/uk/en/products/MAX756CPA%2B/MAX756CPA%2B-ND/1130177
Looks about right... I haven't actually looked at the parts list, but all the voltages and stuff on your link are correct
i see how this works it uses 2 1.5 volt batteries and then a voltage doubler finely a resistor to bring the voltage from 6.00 v. to 5.00 v. very simple.
what r the parts caled i would like to know riten now so i can go buy them befor radioshack closes<br>
Forget about that. I hadn't checked the user manuel on the original website
I would like use this to power an arduino with a 3.7V Li-Ion cell. Would I need to make any changes to the components.
You say a 9V battery has a high internal resistance and then quote ~2ohms. That is incredibly low resistance - it's almost a dead short. Did you mean it is more like ~2Mohms or something?
batteries are close to an ideal voltage source. An ideal voltage source has a resistance of 0 ohms. 2 ohms is actually pretty high for a battery. Most AA batteries are like 1/4 to 1/2 of an ohm. Shorting it would produce about 3.4 amps