Anyone recently replaced their 12v battery. My '15 P85D (47k miles) just displayed 12v battery needs replacement warning. Got a mobile appointment next week for replacement but no word yet on it's cost. Looks simple enough to replace if I can find a replacement battery, saw one last year on ebay but looks like they don't sell it anymore and saw a post somewhere that the software needed upgrading by Tesla if you replace the 12v battery. Any thoughts before I bend over and pay Tesla to replace.