If you wish to prolong the life of your 12V battery, here's a post I made quite a while ago on another forum :
It is important to make sure that the 12V (accessory) battery in your BEV is fully charged regularly so that it won't go bad. (Older Nissan Leafs were notorious for destroying 12V batteries in a short time, 6-18 months, due to their horrid 'keep battery charged' logic, which was crap.)
If one would like the 12V battery in an EV to last 8-12 years (instead of 1-4 years), I highly recommend a smart 'battery maintainer' to make sure that your battery is FULLY charged a couple of times a month. A 'fully charged' (saturated)12V lead-acid battery *should* read around 12.65-12.7V (with NO load). A lead-acid 12V battery that is 'low' (under 80%) and frequently under load (even a small load) will start to sulfate (crap sticks to the plates). One can prolong the life of the battery by doing a full charge every now and then, as a FULL (saturation) charge will get (almost) all of the crap that was stuck to the plates to dissolve back into the lead-acid solution in the cells.
Driving an EV only a 'short distance' daily may do the battery in after just a short time (a year or two) as there isn't enough time to fully recharge the 12V battery. Driving an EV enough to require 3+ hours of charging every other night might keep a 12V battery pretty close to full and thus in decent shape (since the 12V *should be* charged/topped-up while the main Li battery is charging). I measured the voltage across the battery terminals in my Spark EV at different usage scenarios (car on 'ready to drive', during L2 charge, during DCFC charge) but I seem to have lost the paper I scribbled the info on. I seem to recall that 13.6V was the voltage provided to the acc battery from the main drive (L-Ion) battery when the car is being driven, but I can't swear to it (13.6V is low, IMO, if the battery needs a real charge - it's fine as a 'float' if the battery is close to full).
I bought a C-TEK battery maintainer, which I use 2-4 times a month to make sure the 12V battery in my BEV is absolutely, fully charged. It is a super-slow, auto-sensing, trickle charger (0.8A) with all the built-in bells and whistles. I used a voltmeter to check on charging behavior, and it is safe for AGM (absorbed glass mat) as I never saw the voltage go over 14.4V in ANY of the 6 charge modes. (The Battery Tender brand unit I tried *did* go over 14.6V, which is fine for old-style flooded lead-acid batteries, but not advised for AGM.) I use a standard 15A "smart" charger if a battery is darn low and really needs a significant charge; the CTEK is simply used to gently and safely top-off (fully saturate charge) the battery. Since the C-TEK has a "maintain" mode, and is pretty much, but not quite completly, sealed (although not safe to dunk) and fully automatic, I can just plug it in and let it trickle charge (as needed) overnight and unplug it the next morning when it shows that is has fully charged the battery and has switched to 'float' (maintain) mode. There are several good ones on the market - I got the C-TEK because it is AGM-safe (and has not only "de-sulfation" "bulk", and "absorption" basic charge modes, but also "float", and "pulse/check" modes, so I could leave it plugged in for weeks).
