12 volt battery in later i3s--watch out

[alohart]

I can relate to this. My 2018 i3 is now on its 4th 12v battery. I only have about 20,000 kms on it and the dealer says that is the reason the batteries are not lasting.

I drove our 2014 i3 only 13k miles (21k km) over more than 7 years. Our 12 V battery lasted for more than 7 years. This is probably because we stored our i3 for a total of ~3 years during that 7 years with the negative cable of the 12 V battery disconnected so that its only discharge was self-discharge which didn't reduce its charge level too much. If my driving habits were like yours, I might install a disconnect on the negative cable so that I could easily disconnect the 12 V battery while it was parked for longer periods. That would prevent the battery from discharging so much that it's life is shortened.

 

[Gonville]

When you open the doors, the DC-DC converter comes on and charges the 12v battery. It would normally time-out at around 15 minutes but if you closed a door and opened it again, the cycle repeats. With the car OFF and not in Drive mode, I can see how doing this over 2hrs could cause the car to trigger that warning, because the HV battery is "charging" the 12v while you are doing this, AND the car is "stopped".

I've noted further up this thread what my 12v battery looked when it was failing. It was self-discharging to 12.0 volts and the DC-DC converter would come on for exactly 1 hour to "boost" the 12v battery. I never got the "Battery Discharging While Stopped" message while this was going on though.

Pretty sure it isn’t activating the DC-DC charger unless the car is ‘on’. Easy to leave it in that state but opening the door makes it go to off - of course it stays on if you clip the seatbelt in (behind you) as it still thinks you are safely in the car. So with it ‘off’ but the rear open side doors not open it 100% discharges.

 

[Gonville]

I drove our 2014 i3 only 13k miles (21k km) over more than 7 years. Our 12 V battery lasted for more than 7 years. This is probably because we stored our i3 for a total of ~3 years during that 7 years with the negative cable of the 12 V battery disconnected so that its only discharge was self-discharge which didn't reduce its charge level too much. If my driving habits were like yours, I might install a disconnect on the negative cable so that I could easily disconnect the 12 V battery while it was parked for longer periods. That would prevent the battery from discharging so much that its life is shortened.

Is this also because you have very moderate temperatures? We have below zero centigrade frequently in winter and folks routinely get a failure if they have left the car for a week at Christmas/end of year holidays. If the 12v is already weak I guess that can kill it off more frequently.

I like your graphs and have borrowed them. Hope that is OK.

 

[EvanstonI3]

Pretty sure it isn’t activating the DC-DC charger unless the car is ‘on’. Easy to leave it in that state but opening the door makes it go to off - of course it stays on if you clip the seatbelt in (behind you) as it still thinks you are safely in the car. So with it ‘off’ but the rear open side doors not open it 100% discharges.

I have a power-port voltage reader plugged in and I can clearly see when it jumps up from 12.4 to 14.8 volts. The car does not have to be "on".

 

[mojo]

Same here - mine will definitely power-on the DC-DC converter just with the door being opened, though the rules seem to be quite complex (e.g. I *think* if the battery voltage is above a certain voltage threshold it won't turn on).

It's another aspect of the car - like the detailed charging behaviour - which is complicated and not documented for the user by BMW.