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I found some technical information about the Battery Tender Junior. I imagine that other so-called maintainer chargers work about the same way. For the 12 volt "Junior", the battery is charged to 14.4 volts and then the charger switches automatically to 13.2 float volts. If the voltage drops to 12.6 volts, the charger resumes full charge rate until 14.4 volts when the cycle repeats itself.
The 14.4 volts seems undesirably high to me. I think that I will get one of these, but try to turn it off manually when the voltage gets to 12.5 or so. If I leave it unintentionally longer, the automatic float feature will offer protection.