The RedArc DCDC is awesome and simply does its job. You can use it to charge an AGM, Lithium, and Lead Crystal using the AGM profile. IMO, there are still tradeoffs with any dual battery setup in the 200. Let's just look at a subset of options and the key tradeoffs.
Option 1: Dual-Matched FLA Batteries (a suggested in the first post)
Tradeoff: You're limited to dual Group 35 batteries which isn't a ton of usable power. It's a great, simple, solution if your power needs are low.
Option 2: AGM Deep Cycle Main Battery + AGM Aux Battery + DCDC Charger (my current setup)
Tradeoff: The good news here is that your aux battery will be happy but your AGM main battery will never be fully charged without intervention for all the reasons
@TeCKis300 pointed out. You'll want to hit your main battery with an external charger as often as you find it convenient. Why put up with the problem of having to charge the main? I do it because I've had FLAs die after extended periods of travel on washboard roads in the past (not in the 200, though). I've had good luck offroad with FLA Deka batteries in other vehicles and will likely try their Group 31 FLA tractor battery at some point when I'm tired of manually charging the deep cycle.
Option 3: FLA Main Battery + Deep Cycle (AGM, Lithium, whatever) Aux Battery + DCDC Charger
Tradeoff: This would be a bulletproof setup. Both batteries receive current how and when they prefer it. The only potential downside here is destroying the FLA battery in harsh conditions. While this has happened to me before, it's not catastrophic if you have a second battery and is still unlikely in general as long as the battery is secure.
Option 4: Any scenario using a diode to trick the voltage sensor (the ALT-S replacement fuse)
Tradeoff: This doesn't seem to work at all on MY16 and later but even if it did you're probably cooking the battery and its life will be reduced. On the flip side, it's likely that your AGM will probably receive more charge... though the battery may not be in the mood to receive the current based on the profile.
Option 5: Any scenario using a solenoid or the Blue Sea ACR (I used to use the Blue Sea ACR)
Tradeoff: The main battery (if FLA) is likely getting enough charge but your second battery will probably never be fully charged. This is very likely with an AGM second battery and still possible if the second battery is of the FLA varietal.
We carry a jump pack in each of our vehicles but I wouldn't put 100% faith in them. I have a recurring calendar appointment to make sure they're charged frequently enough and they're also on my "charge" list for a trip. I've never had one outright fail but I have needed one when it just didn't have enough juice.
