I noticed the 12V socket near the rear gate is always on. So I brought a 5W solar car charger and plugged into that. The 5W solar panel can be slide on the rear side glass above the cargo area. I hope this will not only keep battery fully charged, also reduce the frequency using EV batter charge the 12V battery.
I have a van parked outside not driving most the year, a solar charger can keep its battery always full without any damage to the battery.