GerryAZ said:
A few comments regarding several posts:
1. Plugging the car in will not charge the 12V battery and the traction battery if the 12V battery is so low that it does not activate the main DC power contactor.
2. 12V batteries can fail without warning. I once started my Jeep at the airport after it had been parked for several days (everything seemed normal). I stopped at an ATM on the way home after driving at least 15 miles on the freeway and turned it off. The battery was completely dead when I tried to start it after withdrawing cash. Fortunately, I had jumper cables and someone else using the ATM was nice enough to help me. I drove home without stopping again and started carrying booster batteries in each vehicle after that episode.
3. You can jump start the LEAF from another 12V battery to get the main DC contactor to close so the car goes into READY mode which allows driving and also charges the 12V battery from the traction battery. You can also connect a booster battery to provide 12V power and then plug the car in to charge the traction battery. The 12V battery will then be charged while the traction battery is charging.
4. I have had good service from Optima yellow top (deep cycle AGM) batteries in my two previous LEAFs and will install one in my 2019 as soon as there is any indication of weakness with the OEM battery. The 12V OEM batteries in my 2011 and 2015 both failed after about 2-1/2 years use.
5. As someone else noted, driving more miles per day does result in more charging of the 12V battery because it is charged while driving and while the traction battery is charging. Higher mileage use results in more driving time per day and longer or more frequent traction battery charging so the 12V battery gets significantly more charging time. This might be why I have never needed to connect an external 12V charger to any of my LEAFs.
6. I believe you can access the mechanical hood release mechanism from underneath the front of the car by removing the plastic splash shield. You will need jack stands, ramps, or a hoist to raise the car enough to get underneath it.
Thanks for the nice rundown and consolidating most of what has been said in this 3-page thread into one post, I'd like to add a few comments to a couple of your points:
Point 1-good to know, so one really needs a 12v external battery charger, the built-in charger and EVSE is not capable of charging a Leaf with a very weak 12v battery, in this case the 12v battery needs to be charged with your external 12v battery charger at least until the point where the car will accept power from your J1772 cable.
Point 2-This is why I purchased a while back and always carry in my Leaf a smaller portable 12v Lithium battery jump starter, they also come in AGM style but as lithium batteries hold their charge much longer than AGM which seems to go flat after not too long, I didn't want to risk it being dead if I really needed it, so I went with lithium.
Point 3-And I realize this may only affect a very small percentage of the people with a dead/weak battery but if the 12v battery is so low the car won't power up, and you discover you cannot open your hood, I think you could really be screwed. Luckily in my brother's case, the battery had enough charge to turn the car ON, just not enough pull in the HV contactors to start the HV to 12v charger. Because it had enough power to partially turn the car on, it also turned on the cigarette lighter socket which allowed me to hook up a cigarette lighter to cigarette lighter jump starter cable I had, which after a few minutes gave the Leaf 12v battery just enough power to pull in the contactor and start the HV to 12v charger and I could unplug my jump starter cable. I was happy to be able to use my little jump-starting cable, purchased it many years ago and in this case anyway, it probably paid for itself.
Point 4- And while a 12v lithium battery is probably the best in moderate climates it does have issues in areas like mine where it can get to -20F. Lithiums just aren't meant for the extreme cold without an external heater like our Leafs have on their traction battery and a heater just isn't practical for the 12v battery. No for people in areas that get very cold(sub-zero) I still believe Pb batteries are the best with the AGM type being best for our Leafs.
Point 6-I didn't try accessing the hood release from under the car, I might have to try that on my brother's Leaf, I just know I saw NO way of accessing the release from the top, including through the charging port area. I think first I'll try pulling harder on the cable with a needle nose pliers and then try going through underneath the car.
Going off Leftie's point in his previous post. So if I understand you correctly, turning on the WW motor only for just one cycle seems enough to initiate the higher 14v charging for a 1 minute or so period after, so in my brother's case where he wanted to charge the weak 12v battery as fast as possible, it sounds like one way to do that without taking extra power to run the WW motor constantly it to put it on a long intermittent setting. This would keep the 14v charging active and yet not waste 12v power running the WW motor constantly, does that sound correct?