House Battery for a 2026 Subaru Forester

 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.

Setting Changes

I switched the DC/DC absorption voltage up from 14.2V to 14.4V to match the Renogy manual and enabled adaptive absorption time. I raised the input cutoff/restart voltages from 12.5/12.8 to 12.7/13.0V to shut off when the voltage drops below high charge alternator voltage.

I noticed a car battery voltage as low as 12.3 measured on the DC/DC with the engine off, but it rose to 12.5/12.6V without an intervening engine start. There's reports on the net of high parasitic currents (~150-200mA) from the Data Communications Module (DCM) in Subarus leading to battery discharge when the car is idle, so this bears watching. Probably best to check when approaching the car after it has been sitting to avoid transients from around shutdown.

Turn on the Lights to Charge!

On the 2026 Subaru you can turn on a maintenance display screen by pressing and holding the both driver's side increase/decrease temperature buttons while double clicking the volume knob. Among other things it shows the battery voltage so you can track charging behaviour.

After starting, the alternator voltage slowly rises to 14.4V as you would expect, however it soon drops to as low as 12.0V, providing no new charge to the battery and no basis for the DC/DC to decide it can continue charging, so it switches off. If you turn off both the low voltage shutoff and the engine shutdown detection, the DC/DC will continue charging at the low voltage, boosting it up to meet the current setting. In this mode most of the current is probably coming from the alternator, as that big a draw would pull the battery well below 12V. Turning off the "smart" elements of the DC/DC runs the risk of charging when you don't want and draining the engine battery, especially if you forget to disconnect when done.

You can fool the "smart alternator" by turning on the headlights when you want to charge. It will maintain 14.4V on the alternator and thus continuous charging to the house battery by the DC/DC under its own algorithm. There will still be some charging under the smart alternator mode with the lights off, but not nearly as much!

Formula 1 Parallels

The smart alternator will jump the voltage up to 14.8 when you "lift and coast", harvesting some free power from dynamic braking, but otherwise hovers in the mid 12V range, probably taking a little operating current from the battery and relying on returning it during lift and coast phases or in the high voltage phase immediately after a restart from engine off at a traffic light. The result under normal daylight operation is probably a constant charge/discharge cycle near but not at a full state of charge and returning to the garage with a reasonable state of charge on the battery. The 2026 F1 cars have much more aggressive battery management and need special techniques particularly on formation laps and during qualifying to make sure they start with the ideal state of charge, essentially tricking the engine management software for these special situations.

The Subaru engineering team are probably young and healthy, counting on driving regularly with some night driving (lights on) to make sure the battery is occasionally fully charged. Anybody who leaves the car parked for extended periods or avoids night driving may not meet those expectations and less than optimal charging may lead to low states of charge and premature battery failure. We may also want to trick the engine management software for our own situations:

• Drive with the lights on when you want to have high alternator voltage for a DC/DC charger
• Drive with the lights on for a significant trip once a week to make sure the battery gets to a full state of charge occasionally, even if you don't drive at night
• Drive with the lights on for a while just before you park the car if you know you will not be driving it for a while and want to start that time fully charged

The engineering team may have some more sophisticated monitoring going on to make sure the battery gets a full charge now and then, but I can't find any documentation of those details, so I will run with the lights on to force the issue.