Russ,
Every couple of weeks I put the Tender on them but as soon as the green light goes steady, I take it off. Usually only have it on a battery no more than 20 minutes before showing a full charge.
I know other people have had good luck with batteries from WalMart...wished I had also.
u need a charger that is designed for an AGM type battery. An AGM battery also needs to be intially charged differently also. The schmacher's walmart sells are excellent. the one for smaller batteries is basically the same as sold by yuasa which received a top rating from motorcycle consumer news. If it is a newer battery tender unit, u should be ok.