[EricH]

I'd wait until power-up, just to be safe.

I've seen what cold can do to a NiCad (admittedly not a lead-acid) battery at "warmer" temperatures of around 20 F--went from about 3-4V or so down to 2V or thereabouts in a matter of minutes. Yes, it was being used, but that kind of drop is peculiar. To make matters more fun, it went back up after I went inside for a couple of minutes. The lead-acid(?) 12V batteries were somewhat discharged already, but refused to work right until put inside and charged, so we grabbed a car to supply the power needed.

At 50 F, you might get away with it, but I'd keep it in the vehicle until right before you need it.