We just bought a new 2026 Forester and want to camp with it and our Dometic CFX35 fridge without running down the primary battery. Experience with the fridge suggest it might use 20-30 Ah of 12 volt power per day.
For a battery, I picked a Renogy Mini Size 12V 100Ah LiFePO4, on price, size and the expectation it would deliver at least 3 days of power for the fridge. It also has protection against low temperature charging, so it won't take a charge until the cabin and battery warms up, which shouldn't be a problem for mostly summer use.
To charge, I picked a Victron Orion Tr Smart DC-DC Charger non-Isolated, 12/12-30, on past experience with the brand, bluetooth, and the sense that 30A would be fast enough, but would not overload the alternator, while 50A might be pushing it. If I was doing it again, I might pick the new Orion XS. It's more efficient so it won't get as hot, and is software controllable for output current, but more expensive.
Although I thought about tapping power from the driving compartment fuse box to avoid breaching the firewall, I couldn't get the fuse tap plugs to fit well, and wound up bringing the feed directly from the battery terminal as recommended in the Victron instructions. The only way I could find through the fire wall was the main wiring harness entry and I needed to psych myself up to alter parts of my brand new car.
I nipped a small hole in the rubber cover above and to the left of the main entry, then pushed through a 12AWG wire (green in photos) with a Brian Toss Splicing Wand to have a blunt end (taping the wire to a large knitting needle might also work). I then used the 12AWG to pull through a 6AWG red cable, taped smooth to the 12AWG. The fit felt tight, maybe like I was pushing through some flexible sealant. I attached an in-line 60A fuse, tied off the cables to run around the outside of the engine compartment, as far from heat and rotating parts as possible, then sealed the penetration with white 3M 4200.
Inside the car, the wand had poked a hole through the rubber boot where it turned up into the dashboard, allowing the wires to be neatly tucked under the console moulding by hand. So far they have done well staying put without additional fastening and come out in the back seat.
In the back seat I added a high current Anderson PowerPole style disconnect and attached the 6AWG ground to the driver's side rear centre seat mount bolt. Note that the lug needs to be straight to avoid interference with the seat mechanism. All connections were made with copper lugs and a heavy duty crimper. The cable is solid copper, as copper clad aluminum didn't give me confidence despite the lower price point.
From there the cable goes to the DC/DC and battery in accord with the Victron instructions, including another 60A fuse between the converter and the battery, then a 10A inline fuse in the feed to the refrigerator. The DC/DC gets hot and I will be watching it, but it has internal current throttling to keep it from overheating.
For the present the system exists only to supply the fridge, but some additional outlets could come in the future. I set the fridge to the highest battery protection setting to avoid excess drain on the battery. I'll add any updates needed once we have used the setup for a while, but it's less than a month since I drove the car off the lot.
