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Our i3 BEV is now three years old. Does anyone know how long we can expect the 12V battery to last? Since the car turns into a brick when this battery fails, is there a warning from the car when it is degraded? Should I start carrying around a 12V battery I can connect through the 12V outlet (see RoadPro Power Port) as a backup?
Since the car turns into a brick when this battery fails, is there a warning from the car when it is degraded?
Like in most hybrids and all EV's, there's no slow-cranking starter motor to indicate a failing 12 V battery. When the 12 V battery voltage is too low, an i3 can display a rather confusing warning about the battery charge being too low suggesting that the driver start the engine. This must be a warning carried over from BMW's ICE vehicles because there's no indication which battery has a low charge level, and there's no engine that would charge the 12 V battery when started. But this warning isn't always displayed before the 12 V battery's voltage drops too low to boot the i3's controllers which would prevent an i3 from being driven.
Should I start carrying around a 12V battery I can connect through the 12V outlet (see RoadPro Power Port) as a backup?
Doing so might risk damaging the high-voltage electronics. When I connected an external 12 V battery charger to our i3, I followed BMW instructions which require that the high-voltage system be disconnected from the 12 V system prior to charging. After 3 days of charging, our 12 V battery's voltage never reached that of a normal full-charged 12 V absorbent glass mat (AGM) battery which suggests to me that our 12 V battery isn't healthy.
I have installed a 12 V voltmeter in the center accessory power port below the climate control system controls in hopes of detecting a weak 12 battery before it is unable to boot the controllers necessary to drive our i3. I have discovered that the resting voltage of the 12 V system is always considerably less than that of a fully-charged 12 V AGM battery, and that the i3 boots 12 V controllers when almost any outside event occurs such as unlocking the doors, opening the frunk, charging the LI-ion battery pack, etc. When this happens, the voltage of the 12 V system in our i3 plummets to as low as 11.8 V before the DC-DC converter turns on to start charging the 12 V battery at which point the 12 V system voltage increases to ~14.3 V, the normal 12 V charging voltage. An i3 turns on the DC-DC converter very aggressively which essentially increases the capacity of the 12 V battery to that of the Li-ion battery pack and also masks a failing 12 V battery.
I have never seen the 12 V system voltage drop so much in a car without a starter motor. Our BMW service department states that the booting of several controllers requires a lot of current which is surprising to me since they are solid-state devices, and that this explains the significant, "normal" 12 V system voltage drop.
Fortunately, if our i3 fails to start one day due to insufficient 12 V voltage, our i3 will likely be parked in our parking space next to our EVSE. Plugging in an EVSE connects the DC-DC converter which will start charging the 12 V battery. This should charge it enough to allow the controllers to boot which would allow the car to be driven. However, should this happen when parked elsewhere, towing would likely be required although I now know how to charge the 12 V battery by connecting to a good 12 V battery or external battery charger. Doing so might be sufficient to allow the car to be driven assuming that the bad 12 V battery would accept a charge.
An undetectable failing 12 V battery is a weakness of EV's and many gasoline-electric hybrids. I have lived with this problem for 15 years with our Honda Insight. I was always able to replace its weak 12 V battery before it failed to boot its controllers, but I might not be so lucky with our i3 because of the way the i3 manages the charging of its 12 V battery.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE
Post by jpklmartin on Dec 14, 2017 16:45:30 GMT -5
Thank you very much for your thorough and thoughtful comments. Our i3 has had a pampered life spending most of its time in our garage next to our EVSE and the weather in SC is mostly reasonable so I'm hoping our 12V will last four years and will have it replaced just before the warranty runs out. I expect BMW to say there's nothing wrong with the battery (which is what they said last week) so I'll say the magic words - "Do it anyway, I'll pay for it". At the eight year mark we'll evaluate replacing all of the batteries. We kept our last car (1999 Lexus RX300) for 16 years.