DSH1 wrote:I note that the i3 manual states "In order to operate the high voltage battery optimally, charge the vehicle regularly and completely in a compatible charger." However, a friend with an EV (not i3) says that his owners group claim that one should not regularly charge the lithium battery above 80% or damage may result.
It is undeniably true from an electrochemical perspective that any battery cell, including Li-ion of any chemical composition, is more prone to irreversible electrochemical side-reactions when it is at a high charge level which reduce the cell's capacity over time. However, the i3's battery management system (BMS), similar to the BMS's of most (all?) EV's other than Tesla's which allows the owner to charge to a very high charge level as an option, limits the high charge level to the low 90% range, thus minimizing, but not eliminating high charge level capacity degradation. So I don't typically charge to the highest level allowed unless I might need full range within a few hours.
However, reversible battery pack capacity degradation can occur if the charge levels of each cell begin to diverge over time, a natural occurrence as battery cells age and are charged/discharged. The BMS won't allow the cells with the lowest charge levels to be discharged below a safe charge level and those with the highest charge levels to be charged above a safe charge level, so the operating range of the entire battery pack would be reduced by cell charge level imbalance. The fix is to balance the charge levels of all cells. Electric Vehicles and the i3 (60Ah and 94Ah)
states that cell balancing is done only when the battery pack has been charged to its maximum allowed charge level, so if one never charges fully, cell balancing would not occur which would result in a gradual reversible loss of battery pack capacity.
So I will be charging fully more often than I have done previously because cell balancing is crucial for good battery pack performance. Unfortunately, it's not obvious when cell imbalance is significant or how long a battery pack must remain at full charge for cell balancing to complete. Possibly to avoid concerning a typical BMW owner with such details, BMW recommends to always charge to full apparently confident that such advice is unlikely to cause battery pack capacity degradation more than the 30% allowed by the battery pack warranty. However, those who plan to keep their i3's past the expiration of the battery pack warranty might want to minimize capacity degradation by being as wise as possible with charging. It would be nice if BMW would assist with this goal, but I don't expect this to happen.