SBI12 or BCDC ??

Thanks for your inquiry. Firstly, all the products you've mentioned are suitable for charging auxiliary batteries. The SBI12 is a great way to allow alternator charge to your auxiliary batteries, and also isolate your start battery from any loads on the auxiliary system. With the SBI12 you also have the option to manually connect both batteries together, should you need to jump start or winch from both batteries.

The BCDC is designed to give the maximum charge possible to any type of standard automotive battery. In this setup both batteries will always be isolated from each other, so there is never a direct connection between two batteries of dissimilar chemistries. The BCDC also charges with a 3 Stage charging algorithm, specific to the battery type under charge, giving the best charge possible to your auxiliary battery. If you do not require the manual connection feature this is the unit i would recommend. Though you can still setup a manual connection feature in a BCDC setup with only a couple of extra components. Click Here

The BMS1215 is a complete battery management system and will charge your auxiliary battery from either 240VAC, DC from your vehicle or Solar. If you plan to have charge available from all these sources, this unit is the one for you. It will automatically select which input source to use, and do all the switching for you. It also comes with a Remote Monitor which gives you info on the system in an easy to understand way.

I am going to assume that you plan to run a fridge from the auxiliaries and probably not much more. In this case i would recommend the BCDC1220, as it will keep your batteries at or near 100% and will also prolong the life of your Gel batteries.