On Sat, 14 Oct 2017 20:18:36 -0400, rickman wrote:

> I was looking for a Li-ion power source adapter and found this in the
> process.
>
> http://www.ebay.com/itm/112419803835
>
> Looks like it would serve as a UPS for a rPi in many situations.  The
> listing isn't big on specs, but they seem to show it will drive well
> over an amp in a pass through mode.  They talk about a couple of amps on
> each of two ports in power bank mode.
>
> I know this has been kicked around a bit here, but I  don't know if
> anyone found an economical solution.  This unit is around $10 and a
> Lithium battery will cost around another $4 or $5.  You can either use
> the standard 18650 type or one of the Lithium polymer flat cells.  Both
> seem to be available cheaply with about 3000 mAHr capacity running a
> typical pi for around 2 hours.

Looks ok to me, if all you want is a power bank controller that can
charge and discharge at the same time.  You might find you can buy a
complete power bank (including the Li cells) for about the same price if
you shop around.  My local computer store was selling Romoss Sailing
models on sale for ridiculously low prices recently.

To be a useful, general purpose UPS though, it needs to be able to output
a signal to tell the load (Pi) that it's about to run out of juice so
that the Pi can shut down cleanly and go into a low(er) power state, all
without corrupting the SD card.

Assuming you can figure out how to derive such a signal (e.g. by
monitoring cell voltage, or by starting a timer when the charging input
fails), I guess you could try to couple it to something like this:
https://shop.pimoroni.com/collections/raspberry-pi/products/onoff-shim

"It watches the state of BCM pin 17 and, when pulled low (pressed), it
initiates a clean shutdown. Last thing, just before your Pi shuts down,
BCM pin 4 is pulled low to completely cut power to your Pi."

Allan

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