Whether you need a trickle charger or not depends on how infrequent your journeys are - most vehicles will be fine it left for a week without use, but if you need to leave it longer than this or are only making short journeys when using the car, a trickle charger may be a sensible precaution. We would suggest looking at chargers from PV Logic or Topsolar, or you could try an accessory store such as Halfords which has several different offerings. Using a trickle charger will not damage the battery or invalidate the warranty as long as the charger you use is from a reputable manufacturer and there are no specific exclusions from using a trickle charger in your warranty terms and conditions.