Battery dead. Again.

Good morning!

2016 Outback Premium - purchased used from dealer in 2022

Our 2016 Outback is having battery issues. The battery has been replaced 3 times in the past 1.5 years. The first was with a Walmart battery (since I needed a battery ASAP for short travel). The next two batteries were from Costco.

Battery 1 lasted about 9 months Battery 2, 7 months Battery 3, 2 weeks

2.5 weeks ago Battery 2 died. We made an appointment with the dealer. Husband purchased Battery 3, and installed it before we saw the dealer. Dealer checked out our car. They ran diagnostics and said no parasitic drain. They also disconnected the DCM. (Then they did an oil change. Replaced a belt, and a couple small light bulbs.)

This morning Battery 3 is dead. Car was good yesterday. No lights left on. No doors were accidentally left open.

I checked in with the service department this morning, to get more info on what they did, and how the car tested during the appointment. They said there’s nothing wrong with our car. Fantastic! But also, WTH??

We will be purchasing Battery 4. Our dealer is pushing Subaru batteries. Are they better? At this point I’m not feeling great about Costco batteries, but maybe Battery 3 was defective?

Do y’all have any ideas? Thank you for your time.