Why are your parents against a battery tendor? I think they are confusing it with a battery charger.
Battery tenders are small, completely safe, and are what you need to keep the battery at full charge while it sits for long periods of time.
Google "battery tendors", read up, then show your dad the links so he can read for himself. There is no danger with leaving a battery tendor connected to your battery. They have been used for autos, boats, lawn equipment, etc for years. They are safe!
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.