Srivenkat said:
1. If I understand correctly, with a REX, one could test SOC down to 6% without fear of becoming stranded. I am wondering how a BEV can be tested to these or lower levels.
I occasionally drive our BEV down to very low charge levels without any problem so far. One needs to be certain that an available EVSE or DC fast charger is within the remaining range to avoid becoming stranded.
Srivenkat said:
2. I have seen reports of wrong SOC being displayed by the i3. Has this been fixed?
BMW hasn't explained the problem AFAIK. It could be due to battery cell charge level imbalance such that one or more cells drops below the minimum charge level allowed by the battery management system (BMS) even though the charge level of the pack remains above the minimum charge level. To protect the low charge level cells, no further pack discharge is allowed which stops propulsive power (i.e., the car won't move under its own power). The fix that the dealer could implement is to rebalance the cells' charge levels.
The i3's BMS implements passive cell balancing that occurs at high charge levels. Those cells with the highest charge levels are slowly discharged until their charge levels are near those of other cells. However, if an owner never charges fully and allows her i3's battery pack to remain at a high charge level for an extended period, charge imbalance could occur over time.
BMW has not documented how its passive cell balancing is implemented. Must an i3's battery pack be at its highest allowed charge level for cell balancing to occur? I'm guessing that some cell balancing occurs at lower charge levels but at a slower rate. Must an EVSE be plugged into the charge port for cell balancing to occur? I'm guessing not.
Srivenkat said:
3. From a Li-Ion battery perspective, would frequently bringing SOC down to 6% or lower be good or nightly top-offs would be better?
I've driven our 2014 BEV just over 6k mile since November, 2014, so I don't drive much. Except when I need full range, I avoid very low or very high charge levels, neither of which is ideal for a battery cell's longevity. Even though the BMS does not allow an i3 battery pack's charge level to discharge below ~10% (an indicated "empty") or to be charged above ~95% of the maximum charge level (an indicated "full"), irreversible electrochemical side reactions occur more rapidly at higher charge levels, so these occur much more slowly at an 80% charge level than at 95%.
The exception to this rule is charging full and allowing our battery pack to remain full for an extended period of time occasionally to facilitate cell balancing. BMW suggests always keeping the charge level as high as possible by plugging into an EVSE whenever one is available. However, BMW's priority is different from a long-term owner's priority. BMW wants to minimize battery pack warranty repair costs, so it has designed the BMS to assure that the battery pack capacity remains above 70% during the battery pack warranty period. As a long-term owner, I want to minimize the loss of battery pack capacity and would be very disappointed if our battery pack capacity drops to 70%.
But as with all series-connected battery packs, it's probably good to occasionally discharge to a low charge level followed by a full charge which helps calibrate the charge level calculation and "exercises" the battery cells although Li-ion battery cells do not suffer from the memory effect that plagues NiCad battery cells.
Srivenkat said:
4. REX to BEV conversion: If I get a REX and the Engine becomes junk out-of-warranty, is there a simple way to force the i3 to not bother about the REX? Such as via coding?
A 2014 REx owner who uses his i3 as an Uber taxi suffered a serious, expensive REx failure at ~100k miles. He chose not to repair it an just continues driving on battery power alone although with an illuminated check engine light. I'm not aware of any coding parameter that would allow a REx to be converted to a BEV.
