Battery OEM Hyundai or OEM Mercedes

the only time it failed me was when the battery died after 100k miles.

When you say, \u201ctwo new batteries\u201d are you speaking of the big ones? There is a small one that operate your convenience/comfort items. It looks like a small garage door battery.

Original battery in my GF’s Elantra finally died today after 10 years.

The batteries Mercedes uses seem to be really poor quality. I have a daily class and it seems they are changing the battery every 1.5 years so far on average…

I've dealt with this issue on both of my newer AMG vehicles. My older ones never had this problem. From my understanding, this is caused by the new mbux system being a bit of a power hog. I eventually had to invest in a trickle charger, and I have to charge both of my AMGs 3-4 times per year. It gets worse as your battery gets toward the end of its lifespan.