Hi

I'm looking to design an Arduino controlled UPS for my RPI based NAS server and media center. It currently uses three RPI 3+ (one NAS, one for Kodi and one music player) and one HDD. The total peak power draw in a shutdown situation would probably be around 5Amps @ 5 V. So I'm looking to find three of-the-shelf circuits:
1: One which can provide this amount of power @5 volts from a number (say 3) of li-Ion batteries in parallel, for a short period, while the RPI's shut down
2: One charger circuit, which can maintain the battery charge, when power is present
3: Battery protection circuits. I understand that these are necessary, although I don't understand why.

Does anybody have any good advice on where to get by this (in terms of name of manufacturer and hardware kit. A supplier name - preferably in Europe - would also be deeply appreciated)

Thanks in advance, and apologies if this question has already been asked. I wasn't able to find any threads that matched

 

How long is a "Short Period".

Only 2-Cells are required if they have adequate Current Ratings,
but most of the generic Li-Ion Cells are maxed-out at around ~1-Amp of Output.

If you use a Li-Po Battery, 2-Cells will handle your 5-Amp-Load easily,
but the Battery-Capacity vs Time factor will have to be calculated first.
I am referring to the type of Batteries normally sold for Radio-Controlled-Model-Power,
which are specifically designed to be able to withstand heavy abuse.

Drawing excessive Current from a Li-Ion, or a Li-Po,
over their rated maximum discharge capacity,
will shorten their life expectancy.

Special considerations will have to be made when using either type of Battery
for continuous "Stand-By" service.
Roughly 3.8-Volts should be the maximum Charging-Voltage, per Cell,
this will result in roughly a ~25 to 30% reduction in the total rated Amp-Hour-Capacity.

Lithium based Batteries can not be maintained at ~4.2-Volts continuously
without sacrificing some of the Life-Expectancy of the Battery.

If You expect to need UPS-Power for more than ~10 to ~15-minutes
a ~6-Volt-Sealed-Lead-Acid (SLA) Battery would probably be a better choice.
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I am a bit confused about this statement: " The total peak power draw in a shutdown situation would probably be around 5Amps @ 5 V. " It might mean the normal operating current at the time the mains fail, OR it may be describing the power draw while shut down, which is not reasonable.
To do an urgent shutdown when power fails a system must first be able to save and close all of the open files, which may take quite a few seconds. So the first step is to know how long the system needs to be supported for a shutdown. Probably the music part can just close the files, or even just release them and park the disk heads, presuming disk memory for the music.

 

What crawls into my mind is to do the uninterrupting upstream of a 5V buck converter. Use a 15V power brick as the main source, with suitable charging and protection boards to keep a 3S lithium pack charged. Use a pair of 10 amp rectifiers to isolate the battery output from the 15V DC. Under normal conditions, the buck converter transforms 15V down to 5V, then when power fails, the battery's output of about 12V will take over. I'd expect the converter would handle that drop in input voltage without a glitch, but if in doubt, add a big capacitor ahead of the buck converter. A relay or optocoupler connected to the 15V output could provide a "power fail" signal to the RPi; the buck converter may have an input to make it shut off.