I've flown at down to -35C. (Heck, it was -22C here just last night
As far as battery voltage, I use the same programed minimums that I use in the summer. The battery temperature however is important. You need to be over 18C, or you'll get warnings.
The battery is enclosed and it creates its own heat. I've found that after take-off the battery will always get warmer while flying. But, at about -25C, (and lower) you need to do some spirited flying. If you simply hover for any length of time the battery temperature will start to drop. Again, don't let it get under 18C.
I've found that the cold has little affect on flight times. Even at -30C, I get about the same number of minutes in the air as I do in the summer. The battery is quite happy at temperatures of 22C. Personally I think that's better for them than the 50C I often reach on hot summer days.
Just for interest, in sub zero weather, I fly from my garage. It has a big picture window that faces a large expanse of fields across the road from my property. I heat my garage, so I don't get my fingers and toes frost bitten