Hi, I'm getting a message in the screen "low battery system will shut off in xyz seconds". There are two batteries in the trunk, a large main one, and a tiny auxiliary one (I assumed it was for start stop function but I'm being told it's too small to actually start the engine). The large battery was tested and it's in "great condition" according to the two mechanics I've showed it to, so I'll take their word and assume the big battery is fine (it was changed two years ago). The small battery was NOT tested in fear of causing electrical damage. The start stop function stopped working a few months ago 6 + I would say, but the low battery message started appearing recently. I should mention that my trunk latch is acting up and I have a separate post going into details (it doesnt open electronically and requires the key blade to open, once open it keeps popping back up after closing it softly using soft close function)

My question:

Is there some major electrical problem in the car? If the large battery is good, is it the small secondary battery that needs to be changed? When I was playing around with the trunk and had to lock and unlock the car about 8 times, the 8th locking beep was very weak as if the battery is about to die. I started the car immediately and it did so absent trouble - no lengthy engine cranking.

Is there a third battery I dont know about or is the small battery in the trunk (by the big one) responsible for all the electricals in the car.

The mechanic (private, not a dealership) hooked up his OBD scan tool, there were a total of 18 fault codes, one of note:
- Gear box fault
- trunk light fault
- trunk latch fault
and a few seconds later, the scan tool read " lost connection to module"

The mechanic advised that most of the unusual codes like gear box are possibly related to a bad battery or some major electrical issue (not an actual fault with the gearbox) he did not want to get into. He advised I take the car to the dealer.

Has anyone witnessed this battery related issue? Should I bite the bullet and take it to the dealer?

Any recommendations and advice is deeply appreciated.

Some extra info:

Single owner car, always babied and has been since day one. ZF transmission fluid was changed a year ago as a preventative maintenance. All parts used in the cars maintenance are OEM (usually purchased from the dealers parts department). The car is clocked at ~75,000 KM.

 

Small battery is for the stop start. My understanding is that it runs the electrics while the car is 'stopped' and the restart is powered off the big battery which can crank the starter. They do that so the electrical don't get spiked / stopped while car is cranking. More recently they just use a large capacitor instead of separate battery

So - if stop start isn't working it's the small battery. 5 years old - it has prob just aged out

The low battery warning is the big one. Don't trust the 'testing' - I've not found that to be reliable. The car is telling you the main battery is not well charged

Can you measure the voltage across the main battery when engine is off? Then start it up and measure again. Post back here

May be that it's not charging properly. May just be a bad battery. Sometimes there is parasitic drain on these cars. You could try a trickle charge overnight to fully charge it up. Also try fully disconnecting the main battery for 30s or so, reconnect and then fully charge it. Sometimes that stops the drain (resets a module that is draining it by not shutting down)

Also possible your latch issue is slowly draining the battery if it's constantly applying power to the latch

 

I have just had the stop start battery fail and it burnt out the alternator, my message was battery not charging. Some of the things I checked and learnt are:

1) I have the torque app on my phone connected to an odb2 reader and that always read 14.4v-14-6v when the car was running and when the warning message came up it read 11.8v (Get this if you have android as the app and module are about £10 combined and show stats like coolant temp, boost pressure etc)
2) When I checked the main battery with the car turned off it was 12.2 volts and when I started the car it was 11.8v
3) The start/stop battery was only 9v but jumped to 14v when I started the car.
4) There is a battery monitoring module on the earth side of the main battery (I don't know how to test this, but unplugging it showed an error in SDD)

First thing to check is have you put any battery draining devices on your car like dashcam etc.. if not check both your main and start stop battery voltages after a run and then leave the car overnight and check voltages on both batteries in the morning, if the start stop is much lower then it could be failing, if the main battery and stop start are much lower then if could be something remaining active while the car is off.

I would also check there is no water ingress to either the Rear Junction box in the boot (Above the main battery) and no water ingress to the Central Junction box behind the kick pad on the drivers side (right hand drive car) as that would cause all sorts of electrical issues.

 

Zereldo and sockpuppet, here's a little experiment you might care to try:
Completely and totally disable the stop/start system (and shut down the secondary battery) by disconnecting the earth/ground lead from the negative terminal of the secondary battery from where it bolts to the floor of the trunk/boot.
I suggest you will pleasantly surprised with the result!

 

I wish I had done that to begin with as I hate the stop/start system. Doesn't it throw an error code though?

 

It will raise a code but not an error message, you would need to plug in a code reader to see the code.
At most you might get a message on the screen between the dials saying something like "eco system inoperative" but it should go away after a few seconds.

 

Disconnected mine, all it does is show the start/stop indicator with a line through it on the dash,

I donl' miss it

 

I'll be taking the car for an oil change and annual check up. I'll get a third opinion on the battery problem and follow up with the rest accordingly. I dont want to open the trunk at this point because the damn thing wont close again. Could you do me a huge favour and check out these two clips I uploaded and share thoughts as to what I need to replace?



I changed the wire loom that is fed through the rubber housing b/w the trunk lid and the trunk itself since I was repeatedly getting camera unavailable followed by trunk not working (doesnt open through trunk release button in the cabin, key fob or switch under the chrome piece on the trunk lid)

 

If you are addressing your question to me then sorry I won't be much help as I haven't had an XF for coming up to four years now.
But there are plenty of bright sparks here with XFs who should be able to shed some light for you!

 

I was, and no worries! I didnt have luck getting any response though I've updated the dedicated post and hoping to hear from someone who can shed some light on the faulty latch.

Thanks for getting back to me.