Volkswagen of America has recently released a Technical Bulletin (TB) to provide Phaeton technicians with more guidance - primarily a better overview - of the electrical load management function on the Phaeton.
As most of you probably know from having read the post Rear Window Defroster, our Phaetons have a very sophisticated system for shedding electrical loads to prevent battery discharge. It is a clever and well designed system, not at all unlike what is found on the most modern Boeing and Airbus aircraft. However - the energy storage capacity of a Phaeton battery is finite, and if the ignition is left turned on - without the engine running - for any period of time beyond about five minutes, the car will start to run low on stored electrical power. That is why it is so important to always have a battery maintainer hooked up to the correct location whenever work is being done on the car (see this post: NAPA Battery Maintainer for a Phaeton).
The attached technical bulletin tries to explain that if the Phaeton has gone into load shedding mode ('Intervention Load Management', to use the VW term) at any time in the past, a fault code will be stored in controller number 09 - the Central Electrical controller - indicating that load shedding has taken place. This is not a 'fault' per se - it is just a report that the Phaeton has, at one time or another, done what it was designed to do - shut down non-critical power consumers in order to prevent the battery from becoming discharged.
For us as owners, the trick is to try and avoid putting the car into a situation where it needs to start shedding loads. Load shedding itself is not a bad thing, and it is mostly transparent to the user. As you can see from the load shedding diagram, the least important functions get turned off first, and it is highly unlikely that we as owners would ever even notice that this is taking place. The big problem that we need to avoid is discharging the battery to the point that the car reaches the first critical low voltage threshold. We can recognize that this first threshold has been reached if we ever see a message on the Y24 display (between the speedometer and tachometer) that says PLEASE START ENGINE.
How do we, as owners, avoid reaching this level of battery discharge? It's easy, if you know how the system works.
1) Don't leave the ignition turned on with the engine off for longer than 5 minutes. If you need to leave the ignition on for longer than 5 minutes, either start the engine, or, hook up a battery maintainer.
2) If you or your Phaeton technician do hook up a battery maintainer, be sure to hook it up directly to the terminals of the vehicle power supply battery (the left side battery). Hooking a battery maintainer up to the terminals under the hood is (to borrow a simile from LBJ) like pissing down your pantleg - it might give you or the tech a nice warm feeling, but it doesn't accomplish anything other than that.
I've attached a copy of the TB for everyone's review, below. It does a good job of explaining how the load shedding system works, but (in my humble opinion) it doesn't make it clear to everyone how to avoid unnecessarily activating the load shedding system in the first place.
Example of a Diagnostic Report indicating that Load Shedding was active at some time in the past
Address 09: Cent. Elect.
Controller: 3D0 937 049 G
Component: STG.Bordnetz 5001
Shop #: WSC 01065
1 Faults Found:
00907 - Intervention load Management
000 - -
Truth is, this isn't really a 'fault', it's just a report that the load shedding system was active at some time in the past.
Phaeton Load Shedding Sequence
As you can see, the least important stuff gets dropped first. The car was designed so we as owners would not even notice when this is happening.
VW of America Technical Bulletin
Click on the link at the bottom of this post to download the PDF.