USB pinouts

Would it be possible to buy a 4x AA battery relay from Radioshack and splice the output into a Micro USB cable to make a 6 volt emergency charger for a smartphone? Or do I need little electrical bits (resistors or somesuch) in between the output from the juice pack and the +5v pin on the USB?

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Miriax (author) 5 years ago
I was just wondering whether there is anything about USB that would keep the phone from accepting any juice from it at all. If it's as simple as applying voltage to the +5v pin, I should be golden. If it needs to be modulated somehow I'll need a more complicated plan of attack.

I know the phone can accept lower voltage, so I might go that route. I'll google the two circuits you mentioned. Thanks!
Try charging on 3 new AA batteries, as the actual voltage of the USB port on a PC tends to be a bit off 5V, so the charger should probably accept the lower voltage.

Also, some products that charge from USB will not accept a generic 'dumb' charger without the data lines. (I have heard that this is true for a number of apple products)
Miriax (author)  The Skinnerz5 years ago
Well I'd be charging a Droid X. Do you have any information on what makes a charger a "smart" charger? That's the heart of what I'm asking about.
If it is meant to be charged with a mains charger, then there shouldn't be any problems, and if there are, you can just charge it through the socket for the mains charger.

If I remember correctly, the two centre pins are the two data lines, but it is probably best to avoid having to try and send signals unless no other charging method is available.

verence5 years ago
As always: it depends.

It depends on how sensible your smartphone is about the supplied voltage. Some may work with a higher voltage, some might turn themselves into scrap immediately.

It depends also on the kind of batteries (zinc-carbon, alkali, lithium to mention only some primary cell types) and their state (new, old, already used). The 1.5V of a alkali battery is only a nominal value. Fresh from the factory it will provide a (slightly) higher voltage, during use, the voltage will drop.

And to make it more complicated, the voltage of any battery depends on the amount of current used due to the inner resistance of the battery.

To be safe, either use more batteries (a higher input voltage) and regulate it down (with a 7805 or similar) or go with 4 batteries and use a buck-boost regulator.

Or, if you are adventurous and can afford to buy a new phone (should it go awry), go and try it. Just don't blame me. :-)