If your i3 has spent its entire life in Phoenix, its battery pack could have been hot long enough to increase the battery cell degradation rate. Unlike a Tesla Model S and probably Model 3, the i3's battery pack cooling system does not automatically operate when an i3 is parked, so an i3 parked in the sun on hot pavement or in a hot garage won't automatically keeps its battery pack cool. If battery pack preconditioning is on, cooling would operate only long enough to cool the battery pack immediately before a set departure time which would not keep the battery pack cool at all times. This might require an EVSE to be plugged in as well, but those more familiar with battery pack preconditioning will correct what I've written if it is incorrect.
So it's possible that significant irreversible battery cell degradation has occurred, especially if the previous owner always charged the battery pack to an indicated 100% (an actual 90-93%) and allowed it to remain full for extended periods of time during hot conditions.
If you're lucky, the previous owner may have rarely charged to full such that the accelerated cell self-discharge rate at higher temperatures has resulted in the charge levels of the cells varying considerably (a.k.a., cell imbalance). Unfortunately, cell balancing occurs most rapidly when the battery pack is left at full charge for an extended period, the very conditions that result in an increased permanent cell degradation rate.
If our battery pack seemed to have a low capacity and I knew that it had not had much opportunity for cell balancing (i.e., it had not remained at full charge for an extended period of time), I would try charging it to full and leaving the EVSE plugged in to keep the charge level full as cell balancing decreases the pack's charge level, all under cool conditions, if possible.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE