Havent been out in the car for a few days due to cold weather. Did start it yesterday and run Rear screen to thaw out and then switched that off and run demist control on full for a while. All the time the engine was running and no probs. This morning after another horrendous frost drove a couple of miles to shops and noticed my rear screen heater switched of after about 5 mins, I re set it and it did the same again. Could this be a sign of low battery? No warning lights on at all.