Not one person mentioned a maintainer by Harbor Freight. Sure, right now you're saying oh those things don't work. I'd beg to differ with you. I have been using them on batteries ( I currently have 13 of them plugged in right now on various vehicles) and have had several of them for as long as 12 years. I just keep adding to the numbers of them. So far I haven't had to replace any of them and some of them are plugged in year around. I mounted one permanently on my 850 XP Touring. I forgot to plug it in one time this past year and after about three weeks, my battery was so dead it wouldn't turn the engine over. I thought, well that battery's shot but I plugged it in and it has been fine every since. And they don't cost near what the Tender's do. I've paid as little as $3.99 for them. But just like everything else, over time, they have gone up in price. You can still get them for $5.99 though. I wish I would have thought ahead back when they were $3.99 as I would have bought 20 of them. If I ever have one of them fail, then I would have replacements but so far as I've mentioned none of them have. I could name all the vehicles and other things I have them on right at this moment but this would be a pretty long post. So just suffice it to say, they work and work very well, just like they're supposed to. They keep the battery charged but not over charged. I've never ruined a battery yet. I bought an "03" Dodge pickup with a diesel new in "03". It has 2 batteries. I don't use the pickup much unless I need it to pull our 34' fifth wheel or haul the wheeler around. I don't use it as a daily commuter so it sits a lot. I only have 33,000 miles on it. I keep it in my shop and it still smells like new on the inside and looks like it's new on both the inside and outside. I keep the Harbor Freight chargers on it when it's in the shop. It's still got the 2 original batteries. I don't know how much longer they're going to last, but that's a long time for 84 month batteries. I'll put my $5.99 maintainers up against your $40 tenders any day.