Hello Folderol,

> Anything based on lithium batteries is going to be complex. SLAs are far
> more easy going, especially when on constant trickle charge.

Yes.

> There is a relatively simple way this can be achieved, and get constant
> genuinely uninterrupted power to the Pi.

> You use a 12V SLA charged from about 15V via a series diode and small
> 12V light bulb. The light bulb not only gives you current control,
> it also gives you visible indication.

That light bulb is placed in parallel I asume?
What capacity in Watt's you need?

> Both the battery, and the original supply have further diodes, both
> cathodes going to a small DC-DC converter to give you the stable 5V out.

Make that 5.1 or 5.2 Vdc=, as 5.0 V is to low for the Pi to operate safely
in the long run.

> Under normal conditions the original supply is higher than the battery
> voltage so the battery's output diode is backed off.

Yes, the highest Voltage will feed the load.
Victron Energy in Almere.NL makes a battery combiner with two
80 A Shottkey diodes for 12 or 24 Vdc= usage ;-)
Used for dual power safety critical users, like VHF, AIS and ECDIS.
I build an ECDIS with a Pi and OpenCPN chart viewer software.

> If the mains fails, the voltage sags to the battery voltage minus
> the diode drop, which will still be plenty.

5.0 Vdc= minus 0.6 Vdc= diode Voltage drop is to low for the Pi.
You need 5.1 Vdc= with 3 Adc= at the Pi's power input.

> At the same time the light bulb will go out, and the series diode
> prevents power being push in reverse to the power supply.

Yes.

> I've used variations of this on many, many occasions going back into the
> 1970s and never had a problem.

Only when the power outage is during too long ;-(.
But then you can start a generator, or better use sunpanels and/or windmills.

> If you want to be really fussy, add a relay to the output that drops out
> if the battery is below a certain voltage.

A relay makes it more complex and less faultproof.
KISS = Keep It Stuped Simple ;-).

Henri.

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