si2t3m wrote:Regarding the dead 12V batt, I'm I over simplyfing it? I ordered and cigarette lighter plug with USB and voltage display. This should give me a clue?
I've been driving with a 12 V voltmeter in the auxiliary power port under the climate control system controls for over a year hoping that it will detect a failing 12 V battery. However, the i3's 12 V system behaves so differently from any I am familiar with that I'm not so confident that this voltmeter will detect weak 12 V battery.
Because of the low capacity of the 12 V battery, the power required to boot several controllers when the doors are unlocked or opened or the hatch or frunk opened pulls the 12 V system voltage down as low as 11.9 V before the DC-DC converter boots which then begins charging the 12 V battery raising the 12 V system voltage to ~14.3 V.
When I first installed the voltmeter, I was concerned that this behavior indicated a weak 12 V battery, so I asked my BMW dealer to check the 12 V battery. His shop foreman told me in November, 2017, that this is normal i3 behavior and that my 12 V battery tested good. In March, 2019, the same 12 V battery continues working as it always has.
The DC-DC converter is very aggressive keeping the 12 V battery charged, so it's difficult to measure the battery's resting voltage. I finally connected a voltmeter to the 12 V terminals beneath the cargo floor by running the leads up through the underside of the car. This allowed me to monitor the 12 V battery without opening the hatch, doors, or frunk which would turn on the DC-DC converter. The resting voltage seemed to be within the normal range. However, I did not leave these leads attached to the 12 V terminals, so I can't easily check the 12 V battery's resting voltage whenever I want.