1995 Renault Laguna battery not holding charge in cold weather.

Had my N reg laguna (56,000 total miles)for 5 years. In the summer battery OK holding engine running charge. During winter (cold weather) battery does not hold engine running charge. Changed battery twice since buying (first time thought battery was defunct), always the same. RAC and garages say enough charge showing and going into battery. If I do not start engine every day or take it for a run during cold weather have to jump-lead start. It seems to me (and it has been so for 5 years whether new battery or not)overnight cold weather drains battery. I am perplexed!!!!. We do exactly the same number of miles in the summer and winter, if not more in the winter. If it had happened with one battery or i had not had the car checked by the RAC and a garage I could understand it. Please help if you can.