Based on experience using the FRC batteries in a
previous robot we built, as long as you keep the batteries somewhere warm until a few minutes before they're needed, you'll be fine in 50F weather. The large quantity of lead in FRC batteries will maintain a sufficient core temperature until you start using them (after that point, heat from the chemical reaction usually keeps them warm enough to keep going).
If the temperature is closer to (or below) freezing, wrap the batteries in hand warmers and insulation.