I have had 3 LEAFs (2013,2018,2018). Each of them had 12V batteries that lasted around 5 years before either throwing error codes or going dead and requiring a jump. Should these batteries be expected to last longer?
It's hard to say if they should last longer than in an ICE car as we just don't have the decades of data across dozens of brands and thousands of models like we do for gas cars. The use of the 12V battery is significantly different than it is in a car with a starter load, but I can't tell you if that will shorten, lengthen, or have no change on battery life. Lead-acid battery design is based around their use in ICE engine bays, but I can't say the features that prolong life in those conditions negatively or positively influence it's life in an EV. Though generally, with the decreased peak temperature it experiences (no 900F exhaust system inches away in some cases) I would generally expect it to be similar or longer.
The lives of these batteries appear to be similar to my previous ICE vehicles. If the batteries in the examples above are only lasting 2 years then I suspect its more of a manufacturing defect in the 12V battery of the battery instead of charging logic OR some small load that is incorrectly kept on all the time.
After seeing the same behavior in 3 batteries on my car, I don't think the defect is in the battery itself. Charging logic is suspect, though I can't tell if it is the logic itself, or the data coming from the voltage and current sensors that are potentially faulty (sensors failing or being defective).
Example of a small load: one 2018 MY I still have...had its 12V replaced as I left a obd2 reader plugged in for over a week. From what I know about lead acid batteries even small charging cycles over a few years will eventually cause the battery to stop taking charge.
Considering that I drive and charge my car daily and still saw the decrease in charge of the battery over time, it's not the draw that's the issue, it's the fact that the car never tops off the battery, or even fully replenishes the charge to the state from the previous day (see the graphs in the first post).
I disconnected this plug at the beginning of February and have left it unplugged since that day, and I've had zero further issues with the battery charge/voltage being maintained where you'd expect it to be for a lead-acid battery.
Until Nissan decides to evaluate the ongoing failure of the 12V system in many of these cars, I'll keep mine unplugged. As they keep saying they're going to be discontinuing the model, I don't expect them to do anything. Considering we've seen this issue on the LEAF for many years now with just the excuse that the battery needs to be replaced (my first battery lasted less than 2 years, but since the car was over 36K miles, they refused to replace it under warranty), I don't expect Nissan to ever issue a TSB or recall to address whatever it is, whether that be the programming, sensors, or other components that are the root cause.