I've been using the HF Viking 4amp Charger/Maintainer through the trailer plug on mine about a year after getting new batteries in 2019. I've had good service from the unit. It'll work with all battery types (no lithium) and 6/12 volt. Cranking time hasn't changed but I do use the block heater when below 36 degrees on WIFI plug in case I need to rescue the DW. Have it come on 30 minutes before she leaves and off 60 minutes after she's left (would be at work by then). That way, I'm not hooking trailer and going up the road on cold engine. I know that it'll start with no issues and won't hurt, but why not use it. The HF Viking units come in a 1.5 amp, 4 amp, and a 2/8/15 amp unit. They are all the new smart charger types. Something to consider at the price point that they are at compared to the others on the market. The WIFI plugs really help! If I can get more than 5 years from this set of batteries, well worth the effort. Need to put meter on see what cranking voltage drop is. Haven't checked in a while. Have a digital volt gauge that carries 1 decimal point. Might wire to battery with a switch so not on all the time. Dropping $400 on batteries isn't what I want to do very often!