I think you'd have to try very hard to intentionally leave the headlights on because the car automatically shuts them off. I think if you turn the lights on manually after shutting down, they might stay on and shut off later, but I don't know how long that would be.

For your specific question, no harm leaving the lights in AUTO because they will definitely be off. If this is incorrect it's certainly news to me.

I seem to recall an electrical upgrade your dealer can do that affects charging and possible parasitic drain, so that is worth checking on. Hopefully someone more knowledgeable will jump in with more info on that. Other than that, the most common cause of battery failure is leaving the tailgate or interior lights on.

 

Having the headlights switch to auto will not kill the battery as long as you exit the vehicle normally, and you don't adjust the headlight switch after you've turned the car off.

When I say exit the vehicle normally I mean you don't do something where you Open the door with the car off but the ignition on, etc... Moving the key in and out of the vehicle and ignition on and off sometimes put it in weird states.

But normal parking and then walking away from the vehicle with the headlights in the on position is how the majority of Outback owners do it without issue.

 

On my 2015, with the stalk switch set to AUTO and ignition in ACC (or ON), the headlights will illuminate if it's dark enough. So, there's some risk there if you're unaware of this fact and, for example, you often listen to the radio with the engine off - depending on the ambient lighting, it's not always obvious that the headlights are on (but the headlight symbol in the cluster will light up to indicate this fact). I leave my headlights set to auto almost 100% of the time and it does not cause battery drain.

 

I leave my headlights set to auto almost 100% of the time and it does not cause battery drain.

Same here, on both my 2015 Legacy and 2016 Outback. Measured "dark current" on both cars is ~30 mA, even with the headlight switch left in AUTO.

 

Good morning! As soon as I bought the car, I turned lights to Auto and never thought about it again. No problem with battery, been 3 years, still original battery.

 

We have a 2019 Outback that is just recently acting up just like you describe, battery draining from some phantom load...we have no switches have been left on except lights on AUTO. I too found that language in the OM and think begs clarification when it says "...the battery MAY be discharged." What are those possible "MAY" conditions.

 

may conditions for headlights in auto to drain the battery:

1. leave the ignition on with car not running.
2. have it dark enough the headlights come on automatically as intended.

Thats it.

 

Over the last 6 months, my battery has discharged twice to a very low voltage (less than 8 volts). In each case, the car may have set idle for about a week, and I didn't find any switch left in the wrong position. My car is a 2018 Outback, and after the first occurrence I installed a new battery. I've used a clamp meter to check excessive parasitic drain, but can't really pinpoint a problem. I don't know if I did something weird and just didn't find it (like the powerlift gate not being closed properly; etc).

In any case, when looking at the OM again I came across this statement about NOT leaving the light control switch on auto with the car shutdown. You wouldn't think that it would cause any difference .... however that statement being in the manual bothers me. I wonder if anyone else has evidence one way or another.

Light control switch S03AH CAUTION . Use of any lights for a long period of time while the engine is not running can cause the battery to discharge. . Before leaving the vehicle, make sure that the light control switch is turned to the off position. If the vehicle is left unattended for a long time with the light control switch set to a position other than the off position, the battery may be discharged

I changed your bolding to the part that is actually important.
Its the lights themselves being on that kill the battery from the light switch. Not the fact they're in auto.

That said, your 2018 has no less than 5 different major reasons that can discharge the battery in a week.

They all range from both drains (hatch and other modules not going to sleep, since they always have power-on) to charging issues caused by "smart" charging stratagy. (ie: battery is low but smart charging system decides to not fully charge the battery as quickly as possible, leading to short driving hops discharging it lower over time)

 

One Guy's Anecdotal Evidence: I left Wanderlust's headlamps in Auto for four years. They never ran down the battery.

 

We have you beat, headlight's have been left in auto since we picked up the 18 3.6R Oct 2017.

Only time the battery was dead, was June of 2022, after she left a map light on overnight. I just put the bigger Group 34 battery in at that time.