DIY external battery USB charger


Old 07-27-12, 03:53 AM
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DIY external battery USB charger

Hi, I'm trying to do a small diy project but I have very minimal to no knowledge in electric circuitry.

I'm basically trying to make an external battery pack for my phone. My phone's battery is rated at 3.7v and I plan to use the same type of battery for my external pack. The basic idea is that my external battery would be rechargeable via USB from one end of the pack. The other end would be the cable that connects to my phone. The pack would be able to charge the phone when the external battery has a charge and is not connected to a power source as well as being able to charge the phone even with a depleted battery when the pack is connected to a USB power source.

If any of you are familiar with electronic cigarette passthroughs, it's basically the same idea. Or basically a USB charging cable with a rechargeable battery stuck in the middle. It would function like a normal USB charging cable when attached to a power source and an external battery pack when not.

Is it as simple as connecting the USB wires to some sort of current regulator which converts the 5V current to the needed 3.7V, have that regulator connected to the battery and connect the phone's wires to the battery as well. Logically, it seems like everything should be connected in this sort of fashion. Current from a USB source would travel into the regulator, then travel into the battery then travel back out through the other cable to charge the phone. When not connected to a power source, current from the battery would travel to the phone. But since I have no knowledge in this field, I'm certain I'm missing something.

My ideal goal is to get whatever circuitry that is required to be as thin and small as possible. I would ideally want everything to be printed on a flexible circuit board as to try and get the board to as close as 1mm in thickness. Would something like this even be possible? I don't intend to make the circuit board myself, but I would like to know if it's even possible.

Any input and advice as to how to go about building this would be greatly appreciated.
Thank You.
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Old 07-27-12, 05:15 AM
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No reason to reinvent the wheel. There are many emergency phone charger already out there. Example: Duracell Instant USB Charger with Lithium Ion Battery/Includes Universal Cable with USB and mini USB: Health & Personal Care
Old 07-27-12, 05:52 AM
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I understand that such devices already exist, but I'm actually more curious as to whether or the all the circuits required for such a charger can be printed onto a flexible circuit board, effectively making the the entire circuit board close to paper thin.
To be more exact, I'm sure that the circuit board for such a device requires resistors, capacitors and even current regulators. Are there capacitors and resistors that are less than 1mm thick and capable of handling these types of currents?
Old 07-28-12, 04:17 AM
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There are surface-mount devices (SMDs) that are soldered to the board instead of passing leads through the board, but they are extremely small and difficult to do by hand.

In any case, you find the strangest things in the strangest places. The circuit in the link below is on an e-cigarette site. (Scroll down.) It's consistent with others I found, but most of them don't include the PC board layout.

USB 3.7V Battery Charger Circuit t5698
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