You might also consider a battery heater (or "blanket").
At -20° C., your battery has only 60 percent of the capacity it has at room temperature - at the same time that your engine starter is demanding several times as much current from it as it would at room temperature.

At -20° C., your battery has only 60 percent of the capacity it has at room temperature - at the same time that your engine starter is demanding several times as much current from it as it would at room temperature.