When replacing the battery, I noticed a wire with a connector that is hanging loose from the harness. It looks like it was from the factory. It is small, white w/ blue, and has a female-spade crimp terminal. It is near the positive side of the battery.

Anyone know?

 

Battery transport isolator/protection circuit.

It is used when the car is on the boat and disconnected and part of it is discarded when it arrives here.
Vector

 

I have seen this question and answer a million times by now, and guess it will pop up an other couple of times .
However, I still wonder what it actually did, do you happen to know?

 

Keeps the car asleep and the battery safely isolated.

 

So, I presume they power the cable while on board, then an insulator in the car becomes active?
Just out of interest, and as I can't seem to find it in the electric diagrams, is that how it works?

 

I have one of those relay/isolator
Here is how it work:
this "relay"got on top of the + side of the battery
The + battery cable connect then to the relay
The small blue/white wire connect to the negative side of the relay coil
The + side of the relay coil is connected to the + side of the battery.
The small wire is supplied ground from the BCM to energize the coil and close the circuit between the battery + and the positive battery cable
When the car go to "sleep", the ground feed to the small wire is interrupted, and the coil open the circuit

Now, how do they re-power the coil to start the car beside removing the relay?
There is another connector on the relay that supply 12v all the time but not sure what it is used for

 

Here you go.

 

Nori:
This PDF is not for an X308 but rather for an X350

The relay look exactly the same but not the wiring.
The X308 only has the auxiliary connector "D" (the little blue/white wire) and not the by-pass feed connector"B"
As I mentioned in my post, the relay on the X308 has the connector for "B" but no car harness/wiring going to it

 

Ooops! That's the earliest diagram I can find.

 

No problem, This, and Luc's post give a perfect explanation of how it works.

Always wondered, each and every time the question on the mysterious trunk wire came up.
Now I wonder if refitting such isolator is a useful addition for those of us who use the car only every now and then ... seems a perfect automated device for those of us who are not hooking things up to a tender.

 

A ctek-style device is better as it will keep the battery better than just letting it stand (though it costs more than the relay thing).

If you keep disconnecting power you'll lose learned things (radio, windows, fuelling, ...).

 

I am aware of that, and had all my motorbikes on C tek's.
However, I have no electricity in our underground parking garage, and now disconnect the battery for every longer term parking.
Very annoying, so that's why I was thinking out loud.

 

These items have positive reviews from the UK classic car world. Maybe something like that would suit you Eric?

Products Archive - Battery Brain

 

I can't see one of those helping & they're expensive.

A (hefty) disconnect switch might be useful as there's no power for a ctek.

 

Well I've seen these and similar systems work well for a number of years now.

Personally I'd just buy a high quality battery and leave it to it. If you can't leave your car a month without it going flat, you've got either a crap battery or another issue. I use Varta batteries, which Jaguar supply OEM, and never had an issue with leaving standing. My Triumph hadn't been started since November but the other day she fired first time, as did the Audi Coupe which had also sat over winter. Both on Varta.

 

A month should (note the word) be OK but if you leave it a long time you're gradually flattening the battery and at some point will reduce the battery life. Either way, when starting make sure to drive long enough to full recharge it or else you're again killing the battery.