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Joined: Tue Apr 18, 2017 6:06 am

Information on the 12v charging system?

Mon Aug 06, 2018 1:45 pm

After about 5 months our second i3, will be on a third 12v battery....
First time it was swapped during the CPO process, then swapped out again after 1 week of ownership (battery warnings)...now the i3 is back in the shop again for a bunch of faults (in addition to saying battery has insufficient charge, brought up a bunch of other warnings like steering and braking...the center console flickered out a few times while on the SOS call) that seem to be related to a 12v battery failing....won't know for sure until the diagnostics are complete...but if I had to guess, who can be this unlucky with 12v batteries...perhaps the 12v charging/dc-dc system is killing them prematurely?

Does anyone have resources they can point me toward the go over the dc-dc/12v charging system?
Last edited by EVBob on Tue Aug 07, 2018 8:12 am, edited 2 times in total.

Posts: 4344
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Information on the 12v charging system?

Mon Aug 06, 2018 4:12 pm

The dealership should run a check to verify everything is actually turning off. Most of the things in the car are controlled via the computer in a distributed system. When you shut the car 'off', the main one sends a message out to all modules that tells them to go into low-power/shutdown mode. IF one or more of those modules doesn't get the message (loose connection, pinched wire, defective), it can stay on, causing the 12vdc battery to discharge faster than it should. The test would show how much of a current draw there is when the car is in the 'off' position and would be a good place to start verses throwing a new battery in there right away.

The DC-DC converter that charges the battery can be checked as well. I'd also want to verify that the voltage sensor attached is working properly, again, a loose connection, bad module, etc. could be telling the main computer that the battery doesn't need charging.

Have you added anything electrical to the car such as a camera, stereo, etc.? Maybe it is not fully shutting down when the car is off. ANything plugged into one of the USB ports or a 12vdc socket?

I'm assuming that your vehicle is driven regularly, but if not, there is a constant, but normally small, load, and the battery will slowly discharge even when everything is working as designed.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

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