I have a 12v automotive battery I need to maintain against the currently unused car’s vampire draw. I’ve bought a number of the stupid harbor freight ones over the years, and I’d like to get something that is “good” even if it costs a bit more.
Or should I just get the $20 from Walmart?
Do those not work? There’s not really much difference between them, far as I know - as long as they’re a three stage one that drops to float once it’s charged, it’s about the same as any other from my point of view. I’ve used a range of them, the only time I’ve had problems are if I try to use too small of one for the battery/loads. Get a 1.5 or 2A one for a car and you’ll be perfectly fine.
… half of mine support 6V batteries too. 
Awesome! I keep forgetting how advanced microcontrollers have become and how even the yumcha stuff is better than top tier 20-30 years ago.
I like the NOCO brand battery maintainers. They do the fancy pulsing to prevent sulfating, etc. not just charge/float. I’ve bought 4 so far; one for myself, the rest as gifts for people with boats, cars, etc, and have had no bad feedback about them.