I just replaced mine today. It's a pretty simple job, remove the right front wheel, then remove the access panel that's secured with (I think) 8 bolts and 3 nuts. There are a total of 4 bolts at the top, 2 that are obvious and 2 more a little farther up. I found it to be a bit easier to remove the headlight access panel too (2 more bolts) because you need to slide the main access panel around a little to get it out and with the headlight panel removed you can see what you're doing a little better. I still had my original Chinese battery in there so it made it 3½ years. It's a standard size motorcycle battery, 12v 8AH. I picked up a US made replacement at Batteries Plus for about $80.
I had my annual service about 6 weeks ago, then about a week later started seeing the "12 volt battery needs service message". I'm not sure if they would have replaced it at no charge as part of the annual service since my car is out of warranty.
Anyway, it's an easy job, about an hour, not worth taking the car in or paying for a Ranger visit.