kinda both, it allows long term parking when running something like a fridge but if you have a large capacity AGM battery you will also need to maintenance charge to prevent sulfation (or upgrade to a higher output, higher voltage alternator)
Its necessity and convenience.
Necessary because... AGM requires maintanance...and...I work shift work. For me that means a 30 minute drive to work, a 12 hour shift, and then 30 minutes home where it stays parked until i go back to work. With a fridge running 24-7 the voltage dips. Every other weekend im off 3 days. If I stay put all weekend, im sure the voltage will dip. I havent had the set up long enough to see how far. I have already ordered the voltage boost diode and am anxious to see how that helps...if at all. The on board charger would make it super easy for me to just connect an extention cord to the truck one day each week to keep it topped off.
Convenient because.... I live in a 1950s era cabin on a lake. I dont have a garage or even a carport to be able to set up a charger out of the weather. Having an on board unit would solve that. Also, I do a lot of travelling and camping in remote areas. I have only used one campsite for one night that had a 110 receptacle. It was nice to be able to recharge EVERYTHING! Im not opposed to pulling into a state park somewhere and reserving a powered campsite if im in the middle of a trip and have noticed low battery voltage, or need a shower, or laundry, etc. an onboard unit is always with you. Shame to run a battery low and have your charger 7 states away at home.
I do own a good charger that has an agm setting. I havent researched to see if its a good choice for the odyssey. I use it to maintain the wet cell on my party barge. Its an older unit made by black and decker.
Another option, of course, is to power off the fridge except for trips. Id be okay with that too, but honestly its been nice having it running to keep a stock of drinks in and to haul cold items from grocery trips. A week ago tomorrow I made the final connections to power up the fridge. So this is all new for me still. Im trying to be vigillent to determine what sort of attention the set up will require. I have a single 34r odessey, a dometic cfx50, and its a 2017 t4r. I considered a dual battery set up but didnt think I stay parked long enough on trips to justfy needing that much reserve. I also considered a group 31 single battery, but my research led me to discover that not only would the group 34 suffice for my needs, but that maintaining the group 31 might require even more attention. So this is my set up. Future option includes solar panel and charge controller.
