Teensy 3.0 battery powered + USB recharging? PCB schematic

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Hey guys, this is my first post on PJRC. Loving the Teensy board! So much so that I want to try creating a PCB with it.

I want to have Vin be powered by a battery connection - a 3.7V LiPo. Then I would like to be able to plug in the USB such that it charges the battery. Also I want to make sure everything is getting the right voltage/current/power.

I don't know very much about battery electronics. Here is what I am initially thinking:

- Use this charger as a starting point: https://www.sparkfun.com/products/10217
- For the purposes of my PCB schematic, replace the USB connector with a simple wire that connects to the output power, the 3.3V pin of the Teensy (can 3.3V even charge the battery like this?)
- This is super hard / frustrating to describe. Here's a schematic with annotations of what I'm thinking:


My other big concern: the Teensy 3.0 voltage input (which I take to mean Vin) can only work with 3.3V. If my battery is 3.7V LiPo, is there a simple step-down DC-DC converter I can drop into my schematic?

In my schematic I found an Eagle library from here:

I understand this is a rather involved question and I'm still learning circuits / PCB design, so big thanks in advance! Bonus points if anyone shares or can quickly whip up a schematic I can use to do this ;)
Adafruit li-po chargers

For an off-the shelf solution, Adafruit has 3 li-po chargers that you can plug in the battery, and then connect it to the board, and possibly charge the battery with 5v (or 6v solar or up to 12v):

Now, obviously if you want to learn how to do the design, etc. go for it. But if you just wanted something to work with a minimum of fuss, this might be the way to go.

I believe that if you hook up to the VIN pin, you can put in voltage from 3.7 to 5.5v, so you can use the li-po directly, and it uses the Teensy's voltage regulator. If you hook up directly to the 3.3v pin, you must do the regulation yourself. If you power the Teensy via the VIN pin, you must not also connect USB power at the same time to prevent, voltage feeding back to your computer's USB port.

You can cut the trace between the VIN and VUSB pins, and then the USB power will not power the Teensy, you will need to power via VIN. You could attach a wire from VIN to VUSB when you need to get power from the USB. Here is a post on how to do this: http://forum.pjrc.com/threads/19228...ng-VIN-from-VUSB-Teensy-3-0?p=44024#post44024

Somebody else mentioned this to me. I bought it, but I haven't wired it yet, but it would allow you to automatically switch power sources between 2 sources: http://www.pololu.com/product/2594.
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