I have been overly plagued with this leaf battery problem to no end on my 2015 SL. My 2014 SV never had a problem. It started doing this just after the 3G telematics upgrade, the battery would run down to 3v, I would charge it, then it would happen again. I got a USB voltage logger "Lascar Electronics EL-USB-3 USB Voltage Data Logger" which was great, I could read the data from the logger and print the graphs and show the dealer what was happening. You can set the log time as short as 20 seconds to pick up spikes. Their opinion was to put in a new battery for $250, so I did. The logger showed it happened again after that, so I found a software update for the 3G unit that was supposed to prevent the battery drain and had the dealer install that. It is described here: https://mynissanleaf.com/viewtopic.php?p=535176#p535176
They charged me $180 to do that. But It happened again after that, drained to 3v. Then the electronic ABS Brake booster unit failed, a loud clunk noise when you press on the brake pedal. They wanted $3500 to fix that. My 2014 was wrecked when I was hit by a drunk driver, I still have it, so I used the ABS unit from my 2014 and put it in myself, had to get a consult-III tool to calibrate the brake pedal position. Now the new battery was bad again from deep cycling. My suspicion is that the low voltage caused the ABS unit to go wacky and burned itself out. I put in the battery from my 2014 and it was holding above 12.5v most of the time, verified by the logger, and things seemed ok for about a year. Then the telematics quit working. After six months my wife wanted it fixed, she likes to turn on the ac before she goes out to the car from work. She won't drive a smoker anymore. Once you taste electric you're addicted. So I took it to the dealer and they said the telematics unit had failed, wanted $500 for a new one and programming. I took the car home and searched the blogs, also put the logger in again, I was thinking of stealing the telematics unit from my 2014 instead of buying another one. But then I found the battery had dropped to 12.18v. In the meantime I had been working my own DIY lithium battery solution, and had purchased a battery from Energized Outdoors for $145 "12V 20Ah Lithium Iron Phosphate Battery". I think their name has now changed to Amped Outdoors. So it seemed like the time to finish my project, I hooked up some SAE terminal posts from auto-zone because I didn't want to modify the battery cables on the car, drilled some 3/8 holes in a small metal plate from home depot to mount the SAE battery posts to the battery, as the lithium battery has small 5M screw posts, it is not a car CCA rated battery, it is a reserve capacity battery. I put a piece of wood under the battery mounting bracket to match the height, and got it installed in the car. The battery now stays above 13v, charges at 14.5v, everything seems fine now. When the battery gets under 12.5v all kinds of wacky things start to happen on that Leaf. I'm surprised the dealer didn't check the battery when they were diagnosing the telematics again. Also that telematics unit that the dealer said was bad is working fine now. If my DIY project fails I'll report it, but for now, it seems like a cheap solution. Amazon also has a 30ah version for $150, I probably would have got that one but it wasn't there when I got mine. Thanks to all you bloggers for sharing, maybe this helps someone as well.