Michael: sounds very like my problem with my Wisper, and the warm room doesn't help either. Mine is kept in a centrally-heated house, but give it 3 minutes out in those bitter cold winds and it loses its 'oomph'. I'm sure David/Wisper will sort it out if it IS a duff battery, but in my case in certainly was the cold - now it's around 10°C in the south of England all is back to normal.
Allen.
Allen.