I have a 2014 XV, regular model.  The original Panasonic battery lasted 6 yrs and 44k miles, just now replaced it.  Was reading 7.68V as I tried to start the car, and it just clicked a few times.  Replacing the battery is straightfwd with a small 10mm combo wrench.  Your electric rear hatch will not work once you disconnect your old battery, so don't have the new battery sitting back there.          Rear brake light bulb was also original and easy enough to replace yourself, but note a couple of things.  You want a 10mm socket to remove the two black screws that retain the entire tail light assembly onto the frame, beneath the rear hatch.  Once the two screws are removed, you probably want to be wearing grippy gloves as you carefully but firmly pull the entire tail light assembly STRAIGHT back from the care to remove it, NOT out to the side.  There are plastic retainer clips inside that you can't see or get at from the outside, and you will crack them if you try to pry the tail light off the car sideways.  Capiche?

Curious if the hatch release by the driver's feet would still work with the battery disconnected.