2014 i3 12V Battery charge

[Mat Kramer]

Hello. I have a 2014 i3 with no range extender. Recently I went to start it after it had been sitting idle for a few weeks and I started getting a slew of errors starting with "drivetrain failure". A little bit scary but after some googling I determined it was a depleted 12V battery. I tested the battery and it was low. I put it on a trickle charger overnight and in the morning it seemed back to normal. While I was charging I ordered an OBD reader and BimmerLink. I saw a bunch of errors, mostly due to undercurrent but all auto-cleared. I have been driving it normally since then.

So my question is, should I get a new 12V battery? I assume it's not normal for the battery to deplete after a few weeks, but I'm not sure if it was slowly degrading or not. I can keep an eye on the voltage now. I'm not sure how old the battery is. I bought the car used almost 2 years ago. I'm thinking this event indicates a weak battery and with winter coming (Minnesota) I should probably get a new one.

Also, it's not clear to me how the 12V battery gets charged. Does it only charge while driving and is it only from the regenerative braking or is there an alternator continually charging? Does it charge at all from the L1 charger connection?

Thanks!

 

[EvanstonI3]

Get a new battery. A healthy battery would have been fine sitting for 2 weeks.
The 12v battery is charged when:
1: The car is “ON” Driving or not does not matter.
2: When the car is being charged.
3: When the battery voltage discharges to 12.0 volts, the car will come on by itself to boost the 12v for exactly One Hour ( there is a possibility that this does not happen on some early models though)

There is no “alternator” in this car.
Instead there is a DC-DC converter that takes the High Voltage from the main battery and converts it to 14v to charge the 12v battery.

 

[mojo]

Does it charge at all from the L1 charger connection?

And yes, all the time that the car is on charge, the 12V battery should be charged too [edit: just realised that EvanstoneI3 has already said this]. Much of the car "stays awake" during charging so, for example, the internal 12V outlets inside the car stay powered-on all the time that charging is under way.

 

[alohart]

3: When the battery voltage discharges to 12.0 volts, the car will come on by itself to boost the 12v for exactly One Hour ( there is a possibility that this does not happen on some early models though)

I'm not aware of any documentation for this feature which my 12V system logger has recorded on several occasions. However, it has happened as high as 12.5V. I wish I understood the conditions required for automatic 12V battery charging to occur.

 

[Dutchi3]

3: When the battery voltage discharges to 12.0 volts, the car will come on by itself to boost the 12v for exactly One Hour ( there is a possibility that this does not happen on some early models though)

I can confirm that early models do this as well. My 12v battery lasted for 10 years (well, less but I did not notice) and never stranded because of no 12v power -but in the end I think the car kept topping up the worn-out battery, until I replaced it. It just kept some charge that way, enough to not noticing the worn 12v battery in daily usage. The constant topping up did start to drain the main battery though, so I got the "excessive drain" message that alerted me at last. Just in time, lesson learned.