I'm still on my OEM battery after almost five years, but only because I use a maintainer almost every time I charge. Are you leaving your Leaf plugged in for hours before and/or after charging? This is the easiest way to kill the accessory battery in a 2013 Leaf. The "good" news is that if you use a maintainer, you can leave the car plugged in as long as you like.
Scarlet Ember 2018 Leaf SL W/ Pro Pilot
2009 Vectrix VX-1 W/18 Leaf modules, & 3 EZIP E-bicycles.
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.