2018 X5 40e - help with faults/battery

[NormalDude]

So my 2018 X5 40e is having issues. 78.5K miles.

Despite being at or close to full charge (eg 80%+) it kicks out of full EV mode when going up a hill of any decent size or accelerating at highway speeds, etc. It also simply just doesn't retain as much EV charge, I can feel that in my daily commute, despite it being 5 years old and cold weather. It never used to be like this, not prior winters or etc.

I took it to the stealership for diagnostic. Thinking maybe I could get it covered under warranty. I pulled some codes from OBD via bimmerlink before I took it in. I looked-up those codes but couldn't' fully tell what they meant. Definitely seemed a problem with batteries and charging system. I will post those codes.

Anyway, stealership said they need to replaced my normal 12v batteries first and recode the car, also update SME control unit. They want about $2,400 for that. Seems they are just doing what an amateur like me would do: replace the lowest hanging fruit and see what changes? I will post their estimate here as well.

So I am thinking I will replace the 12v battery myself from either a BMW dealership parts counter (about $500) or more likely from Autozone, NAPA, etc. (at about $300).

First, one battery correct? the dealer said plural, but I believe one 12v battery. Next, all the BMW parts diagrams online show the 12v was a 105mAH battery but discontinued and replaced with a 50mAH battery? if anyone knows how or why I'm curious.

Next, I can recode the battery with BimmerCode (since different mAH) and register with Bimmerlink, correct?

Then, is there anything I can do about updating the SME control unit? Must that be done in a BMW service Center? And if needs updating, is it normal to charge the customer for this?

Lastly, any advice or hints or help on what to do and how to do it try to get the EV batteries replaced by BMW before the 8/80K warranty expires would be greatly appreciated.

Thanks.

[stocktree]

Yeah I'd start with the 12v battery. If it's original, it's definitely due.
Lift the floor cover in your trunk, and remove the plastic panel covering the batteries. You'll see if you have one or two batteries. And if they are original, the manufacture date will be stamped on top of the battery post.