On 19 Jun 2019 22:15:11 +0100 (BST)
Theo  wrote:

>Mike Scott  wrote:
>> On the down side, I've heard it said that some won't supply power while
>> they're charging themselves. I have no idea if that's true or not, but
>> it has rather put me off the idea :-{ Can anyone confirm/refute this?
>
>I bought one similar to this:
>https://www.ebay.com/itm/4-in-1-DC-5V-9V-12V-5W-UPS-Raspberry-pi-18650-battery
-Charging-Step-up-Board/323270800137
>I confess I haven't got around to testing it yet.
>
>There are some more options described here:
>https://github.com/rootzoll/raspiblitz/issues/263
>
>Theo

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

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.

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.

Under normal conditions the original supply is higher than the battery voltage
so the battery's output diode is backed off. If the mains fails, the voltage
sags to the battery voltage minus the diode drop, which will still be plenty.
At the same time the light bulb will go out, and the series diode prevents
power being push in reverse to the power supply.

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

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

--
W J G

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