Sears has the Battery charger / tender I use is on sale at for $25
It actually comes with a 2 wire plug in that I've hardwired one side to my battery. I plug in Higgins any time it's down for 2 days or more.

 

Even at their regular $29.99 it's a good deal for a charger/maintainer. Less than a third of the cost of my CTEK.

Graham

 

Yep, had mine about 8 years with no issues.

 

FYI I have the nicer Platinum version and it sucks: Sears.com

When you first plug it in it will charge until the battery is full and then go into maintaining mode. The maintain mode doesn't supply enough current to keep the battery topped off and it doesn't sense the battery voltage dropping so it never flips back to charging. The only way around this is to unplug and plug the unit back in every few weeks. It will check the battery on startup and then charge it again. My car is locked and everything is off so I'm not sure what the deal is. Maybe I have a bad charger...

 

My XFR is never down more than 3 days, unless I'm doing something really major (Plugs and SC pulley) then I'll charge it with my 80 amp DieHard charger.

 

I've owned several classic cars and use battery maintainers religiously. I had a Sears unit that I bought decades ago that looked like that and still works. However, in 2008, I brought home a pair of vintage Corvettes and bought two more of those Sears units, but tho they looked the same, they were very different. I don't claim to know what's going on inside them, but I suspect they went to a much cheaper technology. The old one weighed as much as a brick but the new ones were light as a feather. Anyway, a couple days later I went out to the building to check something, it was very quiet and I could hear the faint sound of a battery boiling. One of the units wouldn't shut itself off. Luckily I caught it before any damage was done. I took it back to Sears and exchanged it. The next day, I went back out to the building to check on it and found the other unit humming quite loudly. That was it for me. I unplugged both of them and took them back to Sears for a refund.

I replaced them with Battery Tender brand. I've had one of those go bad too, tho it just went dead. The last one I bought, I got a CTEK after reading many recommendations and hoping it will be more reliable. But when you've got a car worth tens of thousands of dollars plugged into a device, and knowing as much damage as a battery can do when it boils over, I figure I don't want to take more chances than necessary. Maybe I wouldn't worry so much if I were out there every day or for several hours a day, but I typically don't enter the building except on weekends. Paying more doesn't always mean you're getting a better product, but this is one device where I wanted some peace of mind.