Aside from the tailgate issue, I think the battery is a bad design, or frequently defective. We have a 2019 Ascent Touring, purchased in March of 2019. Our first incident of dead battery had nothing to do with tailgate being left open. My wife came out of a meeting at church and there was barely enough juice to get the door open. AAA rep reported dead cell in battery. Jumped it, drove five miles to home, drove to dealer next day - 8 miles, maybe 15 minutes. They said battery checked out ok but since I had printout from AAA saying dead cell, dealer replaced battery -- with OEM, saying they could not replace with anything else. Called Subaru of America and registered a problem report.
Forward now to incident 2 - wife came out to car after getting hair done. Door opened, but when she tried to start car - all dash lights flashed randomly and car wouldn't start. Called the Subaru road assistance - and got notified they were sending someone from a company with the word Upholstery in it. Sounds like a shop that does towing as a sideline. I got there before the 'assistance' and got the car jumped with portable battery jumpstart kit (after reading the forums, bought a kit for each car). Cancelled the assistance request.
Talked to dealership - to their credit, they ended up giving us a loaner and the service manager drove our car for four days back and forth to work. He didn't have any issues and everything checked out.
Now - incident 3. Packed car for trip, and during the packing had doors and tailgate open frequently. Got the house locked up, and the next step was to start car and begin the trip. Only problem was that the car wouldn't start. Flashing lights all over the dash. Subaru 'Assistance' response was 'well we can have it towed into the dealership.' Sure, packed for trip and they want to tow it into the dealership. Fortunately the service department was still open at my dealership. Won't go into my conversations with them, but within the hour the service manager and a tech were at my house checking things out. I give them credit and thanks - they've been very responsive. It was good to have purchased locally. I had made one attempt to start car with my jumper kit and it didn't work. Didn't want to cause any other issues, but it may have worked if I tried again - was down to 65% and I did have another kit in my other car. Anyhow, they got it started with their own jumper kit and checked alternator, etc. Problem was attributed to simple acts of having opening and closing doors/tailgate too much while packing. All looked ok and decision was made to go on trip and if I had any issues on trip, get another battery and they would reimburse. They said they would replace with a non-oem battery on our return.
No issues on trip. Car even sat unused in parking garage for a week while we were on a cruise. I did leave the car running if I ever was opening the tailgate for more than a simple open/close. When we returned, the dealership installed an Optima Red Top Battery. It's been a little over a month now with no other issues. Question may be asked - do we get better mileage? Can't say. At they same time they replaced the battery, they did two recalls that dealt with ECU. All seems good and we love the car. I have purchased, but not yet installed, Nick Blackman's switch for the fuse that controls the tailgate, just in case I do want to leave the tailgate open for an extended period.
I have not been able to find a reference to show the specs of the OEM battery that came with car, but whatever they are - it's not enough to handle all the electronics, or they are faulty to begin with.
I also want to add that my wife is handicapped and I complained to Subaru of America about not being able to open tailgate if battery is dead. Her electric wheelchair is in the way of the access cover inside, and besides she wouldn't be able to crawl back there to open it. She can walk in/out of a store, home, church, but going to mall, COSTCO, etc, requires the chair. And they want to tow the car to dealership - with the wheelchair inside? That is a non-starter. However, the same issue is in other cars. When we were car-shopping, I asked the same question at the Toyota dealership when looking at a Highlander - "How to get tailgate open if battery dead?". They couldn't tell me. I think the manufacturers should be required to have a manual key access as a backup to open tailgate.
OK - I'll get off my soapbox. Thanks.