Battery flat, boot locked
 

Having read about it happen to others it finally happened to me!
I experienced the amazing feature of Audi design, where if the car was locked with the key fob and the power supply is cut off, you cant unlock the boot physically with the key.

It was my fault really, I kept using the Webasto heater for 25 minutes before each day's driving, and I guess with the cold weather and everything the little lithium battery just was not getting charged enough after all that heater use.
So I finally pushed it over the edge and caused the low voltage cut off to kick in. Came back to the car and no power.
The battery low voltage cut off is an ingenious system with one flaw (due to being in the boot). The battery circuit switches itself off to keep enough capacity to start the car.
All you have to do is press the ‘on’ button :D
I could unlock the driver’s door manually and all the other doors. Luckily I have a ski-hatch with all the stuff removed. Using a wooden stick I managed to pry open the battery compartment door.
Still couldn’t see the button. So I taped a small mirror to the stick and moved it around until I could see the battery ‘on’ button. But how do I press the button? I decided I need a L-shaped stick long enough to reach the button. Luckily I had a cheapo metal broom stick with a plastic end and a hole to hang it up. Fishing around for something to jam into it to make a L-shaped stick I found an old plastic compass with a plastic screw-on cap to hold the pencil. So with a bit of fiddling this formed a ‘finger’ at the end of the broomstick to press the button.
After several tries (you have to hold down the button for a second or two to turn the battery back on) I managed to do it, pressed the key fob unlock and the boot opened.

Phewwwwww!

Now I know what a nightmare it must be to have the battery die and unable to open the boot. What a stupid design feature!

This cutoff and "on" button is presumably a feature of your specific lithium battery?

You deserve the Inspector Gadget or McIvor (was that his name?) prize for that Goran! +++

The lock will be seized, should be able to free it off with some WD40 (and some force, or strip and clean depending on your approach) and then use it periodically and problem solved +++

Indeed, there is a direct mechanical link from the lock barrel and, AFAIK no "deadlock" mechanism.
Sounds like you have some sort of mechanical problem that needs investigating, probably in the lock barrel or linkage.

Almost certainly the barrel - my previous fix on a number of Audi's involved a good dose of WD40 and some brute force on the key (mole grips) - goes with a crack and then it works fine afterwards. Risk of that approach is snapping a key - less risky approach would be to strip and clean the barrel +++

That's odd, because when I locked the car manually after removing the battery for a bit of home charging, I could lock and unlock the boot easily with the key.
I hope I wasn't just being thick, but the key definitely would not turn in either direction just after the battery went flat. When I have some time I'll try to re-produce the problem I am now curious what the boot actually does in a power loss situation.

The low voltage cut off is a feature of this particular battery, but I vaguely remember seeing some such add-on circuits for Lead acid starter batteries too.

I'll take that McGyver award thanks :D +++

Was it cold and just frozen?
You need to turn anticlockwise then a bit further and then push in to open.
Alarm will probably go off too, though with the battery "hibernating" who knows...

It was not frozen. The key would literally not budge in either direction.
Then with battery removed, locking manually and unlocking manually smooth as anything.

Odd. It sort of suggests you had either a different boot lock fitted to your car at some point, or you were trying to open with the valet (non flip) key, both of which would behave how you describe....

Anything is possible, I got 3 flip keys with the car and the valet key which does sound like a lot of keys? I definitely used the flip key.

I am still confused though, if there are definitely no deadlocks on the boot, and its purely mechanical, why would a flip key or any other mechanical key work after starter battery was removed, but not work after battery failure? Surely the conditions are identical?

Maybe I just didn't try hard enough to turn it, Lol that would be hilarious. I could swear I tried two or three times.

Lol. That would be amusing +++

That's a lot of keys... Are they all identical? Or is one different, and specific to the boot for example?

The lock is mechanical, and should turn both ways, anti clock to lock and disable the central locking iirc, middle for normal operation, and turn clockwise and push in to open....

Isn't it the opposite way to that Ian? Not that it matters much as it's easy to verify...

Definitely anticlockwise and push to open mine. Just tried it. ;)
Maybe one of them has a "valet" type of blade; IIRC that will not go into the boot or glovebox, only the door and ignition.

I have to look at these keys more closely, they all look the same at a casual look, but I never examined them closely.

I want to re-create the same conditions but with the battery outside the car with some long leads. Hopefully it was just me not trying hard enough :ROFL:

wouldn't the drill hole behind rear number plate unlock boot with the fine screwdriver be easier ?

Try all 3 keys in the boot first would be easier still. ;)

When leaving my car for long periods I always disconnect the battery and lock/unlock the boot manually using the key. My key also unlocks the drivers door manually when battery is disconnected.
The only time I found your problem was when I used the wrong key :Confused:

Yup definitely have to try this.
Confusing thing is, the same key did not work on the boot when it was electronically locked, and the very same key worked on the boot after I took the starter battery out.
If it was the 'wrong' key it would not have worked on the boot at all.

So either something fishy is going on or I just did not try to turn it hard enough anti clockwise after the battery failed. As I mentioned earlier the same key turned the lock as smooth and easy as anything with no battery in the car.

Unless there's something about the power boot release that "jams" with low voltage, but if you look how the mechanism works that's not really plausible - unless something is broken......

I'm thinking of doing this as our plans this year mean the S8 could spend several weeks at a time with no running.
Based on previous experience of dead/swapped battery the only thing you need to do is enter the RNS code again.

However I have two questions:
1. Are there any particular precautions to take re locking/unlocking the boot or anything else?
2. Has anyone ever used one of the battery isolator gadgets that typically seem to have a screw clamp to open and close the earth-side circuit?