Skip the Battery Tenders... The Harbor Freight ones are cheap, work great and you won't be pissed when you invariably run over/tear the cords out, etc...!!! Often on coupon sale for as little as $5. You can also get similar ones on eBay for cheap.
http://www.harborfreight.com/automat...ger-42292.html
And somebody bought a shipping container of Schumacher chargers of all types and is blowing them out in singles and lots on eBay for pennies or dimes on the dollar vs. retail if you are willing to bid/snipe.
http://stores.ebay.com/RugGalleryInc...4&_sop=2&_sc=1
None of these will fix a dead/dying battery, but will help keep them from getting that way.