A trickle charger is an ideal solution for a vehicle that is left for long periods. We would recommend the CTEK MXS charger which we reviewed here: kit.honestjohn.co.uk/reviews/review-ctek-battery-c.../

The CTEK is a trickle charger we would recommend because it has a lot of safety features, comes with a five year warranty and also offers graduated charging, so it reduces the amount of charge fed to the battery depending on its current state. This means it is ideal for long-term storage and there's no limit to how long it can be connected.