tman66 wrote:Is there any particular reason why the battery should be at ~50% and not let’s say 100%?
A Li-ion battery cell degrades more rapidly as its charge level varies from ~50% (i.e., more rapidly at very low or very high charge levels compared with mid-range charge levels). The i3's battery management system doesn't allow its cells to reach 0% or 100% charge levels to protect them from rapid degradation. According to the excellent book Electric Vehicles and the BMW i3 (60 Ah and 94 Ah) by David J. Bricknell, a 60 Ah battery pack can't be discharged lower than ~10% (a displayed charge level of 0%) or charged higher than ~95% (a displayed charge level of 100%). So if a battery pack is allowed to remain at a displayed charge level of 100%, the actual 95% charge level is still quite high and results in cells degrading more rapidly than at lower charge levels.
Many of us who plan to keep our i3's past the battery pack warranty period do what we can to minimize the inevitable battery cell degradation rate. Minimizing the time a battery pack is allowed to remain at a high charge level is one of these strategies.
For those leasing an i3 or who don't plan to keep their i3's for many years might feel that manually managing the battery pack's charge level is more trouble than they want to endure, so they just leave their EVSE's plugged in to their i3's whenever they're parked at home.