This morning, when I ignited my X350 Portfolio 2006, the engine started without difficulty (although the battery seemed a bit weak, I don't run the car that often and last time was 10 days ago).
However, within a couple of minutes, the dashboard illuminated with all possible warning lights, and "engine anomaly" as well as "handbrake issue" warnings appeared.
I stopped the engine and measured the voltage at the battery: 12.30 V, which is on the low range.
Igniting the engine was possible again, and voltage measurement at the battery immediately showed 14.50 V, meaning the alternator is properly working and would charge the battery.
When stopping however, voltage dropped down to 13.0 V and then started to slowly decrease, 12.99, 12.98, 12,97 etc.
And the orange engine light remains.

I decided to put the battery unde charge with a Ctek MXS 5 "test and Charge" for a full cycle. When plugged in, no red light anomly appears on the Ctek (meaning the battery is still alive and should be rechargeable) and 2 then 3 diodes appeared, meaning the charging process started correctly.
I will have a look tomorrow, hoping that all this was only due to a weak battery, the X350s' electronics are very sensitive to low voltages.

I am nevertheless a bit intrigued by the fact that the Christmas tree did not appear instantly and remain after ignition, but only after a couple of minutes, when the alternator was already charging the battery under 14.50 V.
Is that usual, kind of lag for the electronic devices to send default codes?

I also would like to know how to eventually delete the orange warning light and erase the default codes.
Does it happen naturally after driving for some time or should I erase the default codes using a SDD-IDS /Mongoose device or alike?
Also, would a disconnection of the battery and contact of the + and - leads to discharge the capacitors be a good idea?

Thanks for your advices

As you state, a weakening battery can cause all kinds of mischief, so I would start by replacing the battery. IIRC, there have been numerous instances reported here where a battery tests 'OK', but a new battery was really needed.

Our 06 SV8 did this when my wife was driving at night; she said she hit a bump just before it happened but not a big bump. She stopped at a gas station and called me and I went down and started it and recorded and then erased the codes with an ultragage I have on it semi-permanently and there were no more problems on the 6 mile drive home. I think the battery checked at 12.5 volts the next day.

I later did the "disconnect the battery cables and touch them together" thing because of some window one-touch issues that I thought it might fix, and I checked the grounding studs on the battery negative cable; the end one was barely finger tight (so I snugged it up well) and the center one was still good and tight. When I started the engine after the cable touch It again had all the dash warning lights on and every warning message scrolling in the message center and it surged at idle and even died once or twice. It took several re-starts and short drives and erasing of the engine codes to get it back to normal which is an "air suspension fault" message about half the time but apparently normal suspension heights and no other fault messages or codes. The "christmas tree" effect has not recurred since.

Don't be surprised if you have another christmas tree after the cable touch, but if my experience is common it will not be permanent. Look up ultragage on the internet - a not too expensive way to check and delete engine codes plus you can set up seven different pages of gages to monitor your car's engine and performance via the OBDII interface. Now if it could just read chassis codes.....

BTW, Serge, I also have a 1986 Daimler Double Six that was originally sold in Belgium and imported into the US in 1991 by the original owner. A Jaguar dealer there was helpful in tracking down its history.

Battery might recover. Don't be shocked if it doesn't.

The car relies on the battery not the alternator more than people tend to guess so all you're seeing is normal for a low battery.

Lead acid batteries like to be kept full not partially discharged - which is what happens when a car is not used much and isn't kept on the likes of a ctek.

If it's the MIL (as it probably is) you have code(s) stored - fairly meaningless ones probably. It'll go out after a few warm ups (engine cold to hot) unless the faults keep occurring.

Thanks to everybody, very helpful answers.
I will look at my car this morning and will report.

Phil, thank you especially for your info concerning the Jag dealer.
I am in contact with a very kind guy from Jaguar Drogenbos who helps me a lot "against Jaguar Belgium".
Th process is still ongoing, I received successively three draft certificates, all including gross mistakes such as wrong length, diesel engine instead of V8 SC, wrong colour, Dutch language instead of French requested, and many empty boxes!
I am still firm in pursuing my request.

So, in a nutshell, for the time being the car is OK.

Now in detail:

After a full intelligent charge cycle, the voltage before ignition was 13.30 Volts, as expected for a fully loaded and sound battery.

Before igniting the engine, I carried out an inspection using my IDS-SDD v130 and clone Mongoose. I must confess that I am hesitant to run that animal because it is not very user-friendly.
After putting the key on ignition, the orange engine malfunction light appeared, as yesterday. The VIN was recognised.

Inadvertedly I started the engine, which went nice.
I then decided to go on using sSDD while the engine was running.
After a few searches, a code was detected: P0087-00, fuel rail/system pressure - too low, and the recommended action was to test the fuel rail pressure sensor (looking in particular for shorts to grounds), however warning on technician responsibility when conducting the diagnostics!

So at that point I decided not to go further, took out the Mongoose and went for a short drive.
In driving, everything was OK, engine smooth, responding well on acceleration up to 4000 rpm with apparently no fuel feed issues. Only issue was that remaining orange light.

Back to the garage, stopped the engine and ignited it again. And then... no more orange light! Voltage at the battery was 12.90 Volts and seemingly stable, so corresponding basically to full load.

I will look at it again to night, to check if the code is still there and to eventually erase it if I find the suitable way to do it. Either I have a real fuel pressure issue or that code was possibly triggered by the battery anomaly.

I will report.

Glad to hear it has worked itself out (so far at least). Our "christmas tree" has not returned either.

Thanks.
No time yet to do another SDD check, but still OK so far.
Definitely X350 doesn't like low voltages!

With certain codes, if the condition that threw the code disappears, the fault light is displayed for a few more engine cycles (start to stop), , but then the fault light no longer comes on, unless the fault condition recurs. The code still stays in memory, however.

Yes, kept just in case - but deleted after 40 (or so) warm ups.