Draining Di2 Battery

Hey everyone, reaching out because I’m losing my mind a bit with my 105 Di2 (R7150) setup.

Last year I had a major issue with parasitic battery drain. I replaced the internal BT-DN300 battery in June 2025, and that fix everything for the rest of the season. Fast forward through winter: the bike sat on my trainer (Zwift Cog) and didn't get shifted once, also i didt ride much on the trainer. The battery eventually went flat while sitting there.

Now that I’m back on the road, the drain is back. It’s not as aggressive as before, but it’s definitely losing charge much faster than it should when parked.

To make things weirder, my right shifter is throwing a "Critical Battery" error on my head unit. I’ve already swapped in a brand-new CR1632 battery, but the warning just won't go away. The weirdest part is that the shifting works perfectly fine.

Everything is updated to the latest firmware and the bike hasn't been crashed or altered. Anyone have experienced something similar?

Would love some advice before I start tearing cables out!