Mine lasted 9½ years on the original factory fitted battery and was just replaced recently.
I’ve struck a couple recently (the X5 being one) that didn’t give any hint that they were going to die like I’ve seen in the past, I don’t know whether this is just unlucky or this is the way batteries are designed these days.
At 8 years old you probably won’t get much more out of it so I would look at replacing it as a preventative maintenance.
In hindsight I should have done the same to mine as Murphy’s law dictates that it will likely fail at the most inopportune place/time.