Over at VFR World, ran across a thread on 'Is it ever too cold to use a battery tender?' Two of the answers were
It is too cold when the battery has cracked in half and is in two pieces.
That means you must be in North Dakota.
I would say -273K (-459F). At that temp your battery and the wiring all go superconductive, so you'd be ok and not need a battery tender.
And I stopped reading at
Never used a battery tender but I know that at absolute zero resistance is futile.