Where did you find these batteries? Just curious since I’ve only been able to find the H4 from Rock Auto or BMW.
You have 2015 listed as your model year…so you are within the window of battery replacement…and if you’re having a lot of electrical gremlins that come and go…plus an actual
”vehicle under voltage” warning…I’d say that you are more than likely due for new batteries.
I’d also call my local BMW dealership’s PARTS DEPT to get a price quote. They may be comparable and if local taxes are less than paying shipping & handling..they may be a win.
I’d also double check before buying to see which main battery installed. If you have the H8 and replace it with an H9…then you’d need to CODE that change before registering the new battery. If you replace with like kind for each battery, then all you need to do is register the new one(s).
I’m still unsure if the aux battery in the X5 requires registration since it doesn’t have an IBS and is under the control of the PCU (power control unit). And none of the software I have lists anything for aux battery registration. But some threads do have owners having software/hardware that does register the aux battery.