Dead Battery

[garfield81]

So I try to start my X and all I see it a airbag sign lit on the dash and nothing else. No crank no light no nothing. Checked the voltage on the battery it was somewhere below 5V. Then I jump start it with my other car and take the X for a ride. Everything seems to working fine. Come back turn the X off and check the battery voltage it was above 12V. So looks like my alternator is doing it's job.

The question is I have CPO remaining and also have maintenance plan from BMW so does BMW cover the battery or should I go to Costco and pick up a new battery?

Or do I take the cheap route and fill the old battery with mineral water (Really don't know how mineral water helps in batteries?)