Here's what I use:
Schumacher Battery Companion .
I bought mine 6 yrs. ago and it's worked great. I got mine at ACE Hdwr. for under $20.
I have three car batteries and two mc batteries on my bench through the winter every year and use only the one maintainer between them.
I switch it every week from one battery to the next - you don't need to keep it constantly on a single battery, and all 5 batteries have come through the winters in great fashion, each 6 yrs. old now and none have any issues at all.