> PIRs, but can also be smoke detectors and all sort

PIRs will work at 5V or more (lens selection is just as important
as operating voltage by the way), and most, if not all, domestic
Americium smoke alarms run on a 9V battery, so there's no
problem there. I'm sure you'd find a sensor for any application
that will run at 5V, or certainly under 12V

IMHO I would aim for all and any sensors working properly at or
below the battery's terminal voltage, whatever you determine that
to be. With a light load this could be around 11V. But you need to
have 12V/3A (and also 5V/3A if you intend to use just one battery)
available, presumably for alarm devices. Trying to draw this much
current from a nearly-flattened battery is going to cause a serious
dip in voltage. It is not good for lead batteries to go below 10.5 for
extended periods. You'll need to do some testing to find out how
low battery V can go before the required amperage is no longer there.
And then as best as you can, stay away from that limit

Operating temperature must be considered too. A cold battery has
a higher internal resistance (= lower Ah capacity) than a warm one.
At 0degC there'll be maybe 65-70% of the nominal 25degC Ah capacity,
and if you expect the unit to be outside in winter, you'll have to allow
for this with either more efficient high-load devices or with a bigger
battery

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