Hello Lew,

> How about one of those cellphone battery banks? They can simultaneously
> supply power while being charged from an AC converter, and will continue
> to supply power (for a reasonable time) after the AC has cut out.

> Outside of the smarts, and the DC-AC converter, they seem to be capable
> of acting as a UPS for a Raspberry PI. Just plug the Pi into the battery
> pack's DC output port, and the wall wart into the battery pack's charger
> input port.

The problem is that they give you only +5.0 Vdc=.
The Pi wants 5.1 Vdc= and at leasts 2.0, 2.5 or now even 3.0 Adc=
for the Pi 2B, 3B and now 3 Amps for the new Pi 4B.
These machines have no good capacitors to buffer a low Voltage drop.
That's the biggest problem, hence the wish for 5.1 or even 5.2 Vdc=
and at least 3 Amps continuesly.

But beware not to use a high Amperage power supply to any (old) Pi.
When you take to much power at i.e. the 2 or 4 USB2 ports,
then the Pi's power input port will damage (fuse blown),
even if the Amperage of the power supply is suitable to feed the USB demands.
The Pi 1B, may only be feeded with 5.1 Vdc= at 2.0 Adc= total
So you can only draw 1.0 Adc= from the two USB ports total.
Do not use the new 2.5 or 3 Amps power supply for that old machines.
The Pi 1B+, 2B, 3B and 3B+ may only be feeded with 5.1 Vdc= at 2.5 Adc=
The old 2.0 Adc= power supply is to light for a Pi 1B+, 2B or 3B
with all 4 USB2A ports filled, each drawing the max 0.5 Amps per port!
The Pi 4B wants 5.1 Vdc= at 3.0 Adc=.
So the best Idea is to mark those power supplies for each model Pi
to prevent using the wrong power supply at every Pi.

If in doubt, use a sufficiently separate powered USB2 hub,
i.e. with at least 2.0 Amps for a four port USB2 hub model,
and ad 0.5 Amps more for every port extra above 4.
I saw some people using a 7 ports USB hub with only a 2 Amp supply, arg ;-(.
Murphy will catch them the sooner or later.
Amperage calculation is not the strongest point of some users.

Henri.

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