Yup, idling will actually drain these batteries. The bike needs to be moving (engine at 4k+ rpm) for there to be any extra juice to be able to charge the battery on top of dealing with the immediate electrical needs of the bike.
I'm a little surprised that 35 miles of riding wasn't enough to get the battery back in shape enough to start the bike at the end of the day, but I guess if it's completely dead, that's just not enough. You're absolutely right, the tender is the way to go.
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