Battery Tender brand. Be careful with some of the cheapies.
I have a track toy, a Mazda MX-5, that is track prepped with racing suspension and supercharger. Loads of fun. I installed a new battery at the end of track season and I got what I thought was a "smart" trickle charger from AutoZone. But it only charged to a full charge once then shutdown, never checked the voltage again and never provided any more power to the battery. Next spring I had a new, very dead battery.
After that I picked up a Battery Tender brand off Amazon and it's been great, it monitors the charge and trickle charges as needed. I've since bought a couple more for the tractor and any other incidental battery that may need a charge or vehicle that only gets minor use.
Kevin