Dead battery. Replaced with new. Now won't start

[andrewwynn]

He just doesn't understand and with how much details are thrown back and forth it's easy to miss. How it "soft failed" (where it worked some times) is very strange.

My test for BST is: use a jumper cable from the car battery to the B+ jump point under the hood. If the car starts it's almost certain the BST has opened.

There is an automatic disconnect that opens (think fuse) in the main power cable at the battery + terminal.

BMW makes a repair kit search for that but it's pricy.

The cheaper solution is to get one from a wreck (but confirm with Ω reading it didn't blow during the wreck).

You would have to cut out the section of bat+ cable from the battery to under the back seat. Find the directions online for repair BST. It shows exactly where to cut the cable.

With a heavy duty butt splice and a working section of cable it's not a difficult repair.

You cannot drive the car once you start with a jump because it also disconnects the alternator no bueno. (if I was stranded somewhere I would just leave the jumper cable from the trunk to the hood to make the car work until I got somewhere safe).

I would be very interested in seeing that old BST to do a proper post mortem. Was the car ever in an accident that you know of? If the BST opens on purpose there is a set of barbs designed to keep it from closing it's a one shot device.

I want to figure out the exact mechanics of how the thing works and if it somehow opened without the explosive charge it's very likely I could restore proper function and could in-effect make a BST repair kit somebody could use

[jagarundi]

Originally Posted by bhennrich

Andrew already explained this to you! Why ask for help and disregard the help already given?

DUDE!! I'm NOT A CAR GUY so I don't understand what the below MEANS!!


jumper from battery pos to B+ under hood.


If you're not going to help WHY THE HELL RESPOND???

[jagarundi]

Originally Posted by andrewwynn

He just doesn't understand and with how much details are thrown back and forth it's easy to miss. How it "soft failed" (where it worked some times) is very strange.

My test for BST is: use a jumper cable from the car battery to the B+ jump point under the hood. If the car starts it's almost certain the BST has opened.

There is an automatic disconnect that opens (think fuse) in the main power cable at the battery + terminal.

BMW makes a repair kit search for that but it's pricy.

The cheaper solution is to get one from a wreck (but confirm with Ω reading it didn't blow during the wreck).

You would have to cut out the section of bat+ cable from the battery to under the back seat. Find the directions online for repair BST. It shows exactly where to cut the cable.

With a heavy duty butt splice and a working section of cable it's not a difficult repair.

You cannot drive the car once you start with a jump because it also disconnects the alternator no bueno. (if I was stranded somewhere I would just leave the jumper cable from the trunk to the hood to make the car work until I got somewhere safe).

I would be very interested in seeing that old BST to do a proper post mortem. Was the car ever in an accident that you know of? If the BST opens on purpose there is a set of barbs designed to keep it from closing it's a one shot device.

I want to figure out the exact mechanics of how the thing works and if it somehow opened without the explosive charge it's very likely I could restore proper function and could in-effect make a BST repair kit somebody could use

Ok, thank you. So to be clear, I take a jumper cable and connect the positive on the battery in back to the positive nut in the front, leaving the negative cables unattached?

[bhennrich]

Originally Posted by jagarundi

DUDE!! I'm NOT A CAR GUY so I don't understand what the below MEANS!!


jumper from battery pos to B+ under hood.


If you're not going to help WHY THE HELL RESPOND???

Do you know what a "hood" is? Do you know where the battery is located? It has already been pointed out but under the hood on the firewall (that is the metal that separates you in the car from touching the motor) there is a metal post that has a red + on it, that is the B+ Under hood. You run a jumper from the battery positive terminal in the rear of the car to the B+ under the hood. If that lets everything start as normal you BST is bad. The BST is there to help reduce the risk of the battery grounding out causing a fire in the event there is a crash.

[wpoll]

Originally Posted by jagarundi

Ok, thank you. So to be clear, I take a jumper cable and connect the positive on the battery in back to the positive nut in the front, leaving the negative cables unattached?

This image may help - it's not 100% correct but that doesn't matter right now.



See the little orange BST thing near the battery - this is the supected problem.

Take a jumper lead and connect it from the battery (+) terminal and attach the other end to the (+) jumper terminal on the bulkhead under the hood/bonnet. BE VERY CAREFULL TO NOT TOUCH ANY OTHER PART OF THE CAR WHEN DOING THIS!!! Best idea is to attach the lead to the front B+ teminal first - less chance of doing any accidental welding...

Leave all other wires as they. This jump cable simply bypasses the BST. If it starts - you know the BST is bad.

[andrewwynn]

lots of jumper cables will not be long enough to reach especially going around the car I would pull down the backseat and make a fairly direct path even go out the drivers window but yes you can hook up a jumper cable as wpoll described.

BMW reefers to the red/pos jump point under the hood as the B+.

Look at the diagram that was posted above learn how the X5 gets power. There are two primary positive powers one that goes on the right side of the vehicle and runs everything in one that goes up the left side of the echo and only attaches to the starter and the alternator.

since the starter cable is unfused and capable of supplying over 1000A, BMW came up with this battery safety terminal to disconnect that dangerous supply of power in the case of an accident.


Sent from my iPhone using Tapatalk Pro

[jagarundi]

Okay, I disconnected the battery cables from the battery and attached the jumper cables directly onto the terminals. I then attached the jumper cables to the B+ and ground up front. Tried to start the car and same thing. No crank, no sound.

[wpoll]

Leave all the battery cables attached to the battery when you add the jumper cable....

[andrewwynn]

Don't add additional steps. No mention of disconnecting things. The car gets power to run everything from the battery terminal you just disconnected.

At the battery the power is split into two cables. One that feeds everything except starter/alternator and the other that feeds just those (but through the BST).

[oldskewel]

Sorry to hear of the frustration there jagarundi. Hope this can help.

"B+" in almost any circuit, whether on a car or not, refers to the positive (+) connection of the battery (B). So your starter, battery, alternator, etc. all have B+ terminals. The alternator's B+ connection is an M8 stud, with a thick cable attached to it that runs back to the starter, and then another cable runs from the starter to the jump port in the engine bay, then another thick cable runs from the back side of that jump port all the way back to the battery. So if we're talking about the alternator and say "B+", that means that M8 stud. Saying "B+" without further clarification to someone who does not know what you're talking about is ambiguous.

"Jumper" in electrical circuits often refers to something like a wire that connects two points with a conductor to make their voltages essentially equal. A paperclip in some applications can be a jumper. Jumper cables are commonly used to connect the batteries between two cars when one battery is low. So in this case, since we're talking about high current transfer in a car, jumper means a jumper cable. But not both cables (red and black), just use one of them (either red or black, does not matter), leaving the other one unconnected to anything.

These X5s have the BST (battery safety terminal) which is a pyrotechnic safety device that cuts the electrical connection from the battery to the starter and alternator in the event of a crash, for safety reasons. It is the major thick cable that can carry a lot of current. The theory is that if the cable that runs from the battery in the rear to somewhere in the front is puctured and grounded at any point, the battery will be shorted to ground at that point, allowing the entire battery to discharge, creating heat and a further fire hazard at that point.
https://bimmerscan.com/bmw-battery-safety-terminal/

The rest of the car still takes power from the battery at the battery's B+ terminal, but not through the BST. This way, even in the event of a crash that blows the BST, you'll be able to open doors, windows, etc.

So the theory behind doing this test is that if your BST has blown (due to a crash being detected or some malfunction), the rest of the car may appear good, but with a blown BST you'll be getting nothing to the starter or alternator. If the BST is not blown completely, but has malfuntioned somehow, so at low current, you can still measure 12+V at the B+ jump port forward of the driver on the firewall (that separates the engine bay from the passenger compartment) in the engine bay. But at high current, as called for by the starter, the BST may not allow enough current through, and the starter may appear completely dead.

====================
OK, so that's the background, here's the test that is suggested, and you'll see is very easy to do, once you know what is being explained:

Put everything back together as it was before trying to do this test. Battery connected, etc. Leave the hood up. Leave the battery exposed (but with the terminals on the battery posts as normal).

Try to start the car. This is what has failed before. Confirm it still fails. It should be just as it has been. Try to pay attention to any noises, etc. as it fails to start.

Connect one end of a battery jumper cable (say the red cable) to the Battery positive post / terminal. Leave the black cable at that end not connected to anything.

Connect the other end of that battery jumper cable to the B+ jump port in the engine bay. When you make that connection, nothing should happen, because those two things should be connected anyway. There should be no spark or sound or anything else - pay attention for those things.

Now try to start the car and see if anything is different vs. without that jumper cable in place.

If the car starts, it would point to a problem with the BST. In fact, if anything at all is different, it suggests an issue somewhere on the path between the BST and the B+ engine bay jump port.

If you find the results are the same with the jumper cable in place vs. not, that would suggest the problem is elsewhere.

This should have been a 2 minute test to do, but I completely understand how it may have been confusing from your perspective.