Everything I have read says DC to DC is the way to go these days. They are “Smart” chargers usually, which means they automatically detect and adjust the charge according to what is going to top up your secondary battery the most effectively; though I will say that with a caveat, I’m just learning about these systems and I may have oversimplified it a bit here. I plan on using a CTEK device in my rig as they are a fair bit cheaper than the Redarc systems, and although they provide fewer features, they have a lot of Overland miles on them with “good enough” results. However, my style is also not managing my electrical needs down to the nearest 0.5 amp hour, as some do (which is great as they have taught me a lot). My auxiliary power needs are pretty simple — a bit of lighting, charging batteries on cameras and radios, and the fridge. We bought way more battery than we need for this to allow us to sit for several days without turning the car on, but the worst thing that happens to us is the fridge beeps a warning and shuts down, and then we know to charge it up. And even if we don’t and our fridge goes warm, we might lose ~$30 worth of groceries as a worst case scenario, and we always have dried foods to get by with. In other words, our worst-case scenario isn’t bad enough to justify the energy, effort, and resources into a fancier system.
We also plan to wire in a battery maintainer that can be plugged into shore power for when we are at home; it gets very cold in the winter here which can dramatically impact battery performance so we are going to try to plug in the maintainer next to the block heater and see how that works for a winter season.
If you are depending on your dual battery for a CPAP machine, a fridge for Insulin, etc. Then our approach may not be acceptable and you might want to spend the extra money on more robust monitoring and control like the REDARC systems.
However, the overall takeaway is that these DC to DC charging systems are the simplest to install and use, and at only about $250, won’t break the bank. Even a “cheap” solenoid system will be 1/2 that cost before you include your time to get it working right, so I think the DC to DC is the way to go.
