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)
hi,thank you.
antipico6 years ago
Hi! Great tutorial. I just got one of these working with 3.7V Li-Ion cells, but I have trouble charging my iPod. The charge icon turns on and looks to work fine for a minute until it stops charging. Like there weren't enough current. I've tried this with alkaline batteries also but still the same prob. How big currenct could be pulled out of these things?
RUBiksCUbe9 years ago
Just curious...how did you take those pictures? Both of your hands are clearly in the shot.