I bought a couple of cheap $20 “battery maintainers” at Walmart and they didn’t work. I left one on over night and the next morning there was an acid smell in the garage. I suspect it didn’t turn off when the battery was fully charged and boiled the battery fluid.
I prefer the Detran Battery Tender (
www.batterytender.com). I have a 6 volt model That the previous owner gave me when I bought my Roadster. I recently bought a dual 6V/12V Battery Tender at Costco for $40 so I can use it on my modern cars as well as my Model A. Both models work very well. I plan to buy the model that that maintains two cars at one time. We have four cars and due to the Pandemic they tend to sit for long periods of time.
David Serrano