You can always try a Schumacher charger, this one:
http://tinyurl.com/2f9dm8xI have a Battery Tender Jr, a regular Battery Tender, and this Schumacher charger, and I prefer the Schumacher.
It's a bit more intelligent than a Battery Tender, recognizing when it is hooked up wrong, recognizing if the battery is getting too hot, and it has additional lights to show what it's doing. It has the same connectors as a Battery Tender, so you can use it on bikes with the battery tender plugs attached.
They originally sold for $18.00. I got mine for $20.00, but the price has gone up again. It's still a good deal.