Yeah it’s good for for less than $5. Wait for the sales. They’ve made one for at least eight years. The early ones really sucked. There used to be a lot of guys posting the circuits up over the years. Some of the early ones were really poor designs.
I haven’t cracked mine open but I did measure the output. When I connected it to my car you see a negligible change in voltage on a 4 digit DMM. I was too lazy to haul over my 6 digit DMM so I just measured the current. It sources 500mA at around 30% duty cycle. That will never boil your battery off. Old school Schumacher or Sears chargers would have 2A and 6A settings.
It is a little more than my 500mA at 1 hr a day lamp timer trick. 30% duty cycle is like turning it on 8 hours a day.
It’s a pretty dumb float charger but it works. Good for lawn tractors and motorcycles. It is worth about $5. Any more $ than that you’re better off with the DSL router wall adapter and ikea lamp timer trick.