Trickle Chargers...
 

I use a single battery tender at home and bought a 30' extender for the charging end, each battery I charge has the quick connect on it so it is easy to rotate. I charge 4 batteries at home this way, takes each battery a day or so to come up to charge, do it once a month.

I helped set up a new sporting clays course at the local Boy Scout here camp 2 years ago - we had to figure out how to keep 30 automotive batteries charged for the throwers. We bought 3 of these 10 bank charging stations and they work great.

https://www.amazon.com/Battery-Tende.../dp/B000NCOKVU

I use the Black and Decker ones, they are around $20. Home Depot I found them.
I’m not convinced using them prolongs the life of the battery. Since 1979 I’ve stored a car(s). I got 11 years from a battery that went dead a few times, yet on a trickle, one lasted 3 years recently.

Ive used just about all of them. Batteries got fried by a battery tender and a noco. Now I only use two. This ctek and this Optima. Both are excellent. I have em on 4 cars 24/7
https://smartercharger.com/products/ctek-us-0-8

https://www.optimabatteries.com/prod...rs/digital-400

My Delco battery finally went dead last year after 22 years. It got a 2 amp charge once every 2 months and that was it. Don't make em like they used to.

My electrical engineering friends swear by CTEK so that's what I've got and love it. Pricier but that means better-engineered charging algorithms to preserve the life of the battery, whether lead-acid or AGM (different charge modes).

Ctek

I saw several sizes of CNET chargers. What would work on tractors with 2 batteries?

The guy I work for had about 38-40 cars in his collection, 28 where I live and 10 or so in AZ. We use Ctek chargers , he probably had about 30-40 of them, only had trouble with 1 once and it was the first generation of Ctek.

The cheap chargers can some times not cycle and overcharge the battery’s, IMO.
Personally I have Cteks on my own stuff & the battery in my 2001 Gran Prix GTP lasted 15 yrs. That being said I replaced the battery with a AC Delco replacement battery and it only lasted 4 yrs.

I’m a fan of unplugging the Cteks and letting the battery sit for a few weeks to cycle them, it seems to help, plus I don’t like leaving stuff plugged in when I’m not at home. I think the battery quality has gone downhill lately, they just don’t last as long as they used too.

I have a 2 post Battery Tender I've used for several years. I NEVER leave it on all winter. I have 7 batteries I rotate every 2 months. Once the green light is solid, I leave the charger for another 2 days and then move to another battery. After they have all been charged, they sit for 2 months. These batteries range in age 6-9 years old now.