My wife had a flat battery after 3 weeks. Called out the rescue service (in effect the AA). The AA man said that whilst there are some minor drains on the battery a decently charged battery should be fine for at least 3 or 4 months. However, what he said can discharge the battery in a shorter period is if you leave the lights and/or wipers on auto. So, if you normally have them on auto suggest you turn them to off. May or may not assist, but hopefully it will?