tonyowen wrote:Page 203 of the 2017 manual is very clear about vehicle storage, which implies a "full" charge is not an issue, no?
My view is that BMW has a viewpoint that might not be in full alignment with that of owners. BMW's main goal is to avoid expensive battery pack replacements under warranty and the negative publicity which would occur. Thus, BMW has designed the battery management system to minimize the risk of a battery pack losing 30% of its capacity during the warranty period. BMW has calculated that its storage recommendations would not significantly increase its battery pack replacement numbers while being the most convenient for i3 drivers ("Just plug an i3 into an EVSE during storage and don't worry").
Many i3 owners would not be happy with up to 29% capacity degradation during the warranty period (i.e., degradation that would not qualify for a battery pack replacement under warranty). Many leasers don't care about battery pack care because they don't own the car and won't have it after 2 or 3 years.
So how would an owner minimize the capacity degradation? By following the principals of electrochemistry which state that a Li-ion battery cell at a very high or low charge level is more likely to experience irreversible side reactions that decrease the cell's capacity. Even though the i3's battery management system does not allow the charge level to reach the absolute maximum charge level or full discharge, the charge level at "full" is likely ~95% and at "empty" is likely ~10% at which irreversible side reactions are more probable than at moderate charge levels.
BMW's storage recommendations contain an incorrect assumption: the battery pack's charge level could decrease considerably when an i3 is in storage. The Li-ion battery pack is electrically isolated when an i3 is parked and off (i.e., there are no electrical loads that could discharge the battery pack). Li-ion battery cells have a very low self-discharge rate, so the battery pack does not discharge appreciably on its own while in storage. So if an i3 is stored at 50% charge level, it will be at ~50% charge level a year later (the self-discharge rate isn't 0, so a very small loss of charge over time would occur).
The problem with storage is the discharge of the 12 V battery because several 12 V loads are always on: burglar alarm system if the car is locked, keyless entry system, cellular phone transceiver, entertainment system. To avoid ruining the 12 V battery while an i3 is in storage, it must either be disconnected or charged. The 12 V battery is charged when the car is plugged into an EVSE, so that's the easiest way to prevent the 12 V battery from being completely discharged and ruined, but the side effect is that the Li-ion battery pack would be maintained at a full charge which is more detrimental than leaving it at 60 ± 20%. The i3's 12 V battery is an adsorbent glass mat (AGM) battery which means that it has a much lower self-discharge rate than a conventional flooded-cell battery. The i3's 12 V battery can be left disconnected for many months without losing significant charge, so disconnecting it is better for the Li-ion battery pack than plugging an EVSE into an i3 for months on end.
We own our i3 BEV. We have stored it at least 6 months every year starting in 2015. I want to minimize battery pack degradation, so I store it at ~50% charge level and disconnect the 12 V battery. When I take our i3 out of storage, the charge levels of the 12 V battery and the Li-ion battery pack are never significantly lower than they were when we put the car into storage.
So I don't believe that BMW's storage recommendations are the best for owners who want to minimize Li-ion battery pack capacity loss.