Reviving this thread to add a few details.
The thread didn't really talk about how many amps posters are using to charge or maintain their batteries, so I did a little research.
I know my old battery charger has 2a normal and 10a start settings, and the 10a is supposed to only be used for 20 seconds or so when cranking.
Since all I'm looking to do is maintain my batteries, no charging at this point, and I want the maintainer small enough to fit into my battery compartment so I can simply plug it in when I park.
I don't live in a really cold climate, and I'm not planning on using my bus in a really cold climate. The coldest it gets in the Seattle area is high 20s. Yet, the colder it is, the less efficient a battery is at taking a charge.
Here's a pretty informative article on batteries, charging and cold climates. https://www.interstatebatteries.com/...th-jeff-barron
I was looking at a .75a Battery Tender maintainer, which would maybe work. Yet, since I won't be starting and driving the bus much during winter to really keep it fully charged, I need something that has some extra umph when it's around freezing. So, I'm going with the 4a model.
If it doesn't need all 4a, cool, but if I'm doing any kind of KOEO testing, at least it will help keep it charged during that too.
Hope this helps someone.
Best of luck.