My customer wants a warning
when battery voltage goes below 12.5V irrespective of whether the engine is
running or not. [...]

Personally I'd put the threshold to 12.5V nominal and have a large
time constant filter on it to catch glitches and momentary dips - this would
trigger at a minimum of 12.25V, max 12.75V.

Ask the customer how long a dip they would tolerate without a warning,
and why. An intellegent answer may require some dialog. And wouldn't a
large time constant filter =miss= momentary dips? That's what it's for
- to ignore the quick stuff and only warn on the more permanent stuff.

Jose
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