No Power except through jump leads?????

[firemedic8047]

Ok, I searched and searched this and other forums and haven't found a solution yet. I have a 2008 535xi, 101k miles. Noticed a few weeks back that I had to keep resetting the clock when I got in and first cranked the vehicle. I then started getting an increased battery discharge warning message. Looked it up and a lot of people here said it was the IBS. Disconnected the IBS and all was well until last week it would not start. Ordered the new IBS, took out the battery, (which was drained), and placed it on a charger. New IBS installed, freshly charged battery (that was bought in Jan of 2018) was but pack in and now.... nothing. No dome lights, door locks, nothing. Hook up the jump leads under the hood and , viola! Dome lights come on, car goes through its normal pre ignition phase, but will not start. You can hear the starter engage once, but then nothing. Take the jump leads off and all power immediately dies. I have checked all fuses, made sure the new negative battery cable that came with the IBS was grounded, tested the battery with a meter, and now I'm at my whits end. Any ideas would be great. I really don;t want to tow this thing to a shop. Thanks in advance guys!

[02Pilot]

Check all the grounds - many electrical problems are caused or compounded by bad grounds.

[bmwdirtracer]

Check battery voltage across the two battery terminals. Then check voltage between the underhood jump point and a good ground underhood.

Make sure all wires at the battery are plugged in. Make sure battery cables are tight.

Is the positive jump point underhood tight? You should check the main hot feed from the battery to the junction box (fuse box)

[firemedic8047]

Check battery voltage across the two battery terminals. Then check voltage between the underhood jump point and a good ground underhood.

Make sure all wires at the battery are plugged in. Make sure battery cables are tight.

Is the positive jump point underhood tight? You should check the main hot feed from the battery to the junction box (fuse box)

Voltage across terminals are 12.6. The underhood terminals are getting power, as its the only way that I can get any power to the vehicle at all. Is there a power module between the battery and the fuse box I'm not aware of?

[MarcoZandrini]

There’s the battery safety terminal (BST) at the battery. This, when activated, kills electrical power to almost all systems. It’s triggered when the car is in an accident severe enough to set off the air bags.