DROK 5A DC-DC Converter recommended by Gdetrailer.
I use these as battery float and conditioning chargers for my group 27 & 29 batteries, using old 19.5VDC laptop chargers as the input. Easy to dial in the proper voltage and are very stable and reliable. No guessing at what voltage/current your battery is getting, 100% controllable. Not recommended for bulk charging because it is limited to 5A. Excellent battery maintainer, not charger.
Really handy to have. I have built a few for friends for their boat batteries or they have built their own after seeing how I use them.