When the vehicle is in Ready, it is always working on keeping the 12v system fully charged, so I don't see any benefit in getting any kind of battery tender unless the vehicle is stored for a long time. If you are going to use a battery tender be sure to attach it to the battery properly to avoid doing any damage to it. Do not trickel charge the 12v battery, unatended at least, that may do more damage to the battery than letting it go unused for 3 weeks.