My guess is they wanted a fully charged, functional ECO battery to test the PSDB, as that is where the LIN lies. The canbus picks up at the module gateway. Regardless, you are right. I completely removed my ECO battery with no ill effects other than the ECO light. It was only when I removed the whole PSDB that the heated windshield, steering wheel, and (maybe) seats stopped working.

Even though, I am easily persuaded and now want to test my alternator as well.

 

Just another update, more daily driving and hundreds of miles later and not a single CAN bus error, battery holding proper charge and alternator voltage remaining steady. At this point I feel confident this particular issue has been resolved. Stopped by the main dealer today to pick up a part and the service advisor asked me if I took the truck to another dealer. I said no need, I fixed it. How? she asked. I installed a new alternator, I replied. She seemed stunned.

On to the others now...

 

Further two months and no issues, at least not until today. I had stopped monitoring battery and alternator voltage as both had remained consistent and where they needed to be. But today I had over 30 CANbus DTCs and had to keep stopping the truck to reset it, though without success. After limping home, I checked battery voltage and it was 12.2 volts! Alternator stayed consistently at 14.3 volts. Placed battery on charge with the CTEK for several hours until all lights were illuminated indicating it was complete. Measured voltage again and this time it measured 12.6 volts. Normally the battery would be in the mid 13s. It's Interstate's best AGM battery and only about 8 months old.

Thoughts?

EDIT: After CTEK said battery was fully charged at 12.6 volts, sitting locked 6 hours later the battery measured 12.3 volts

 

How long after shutting the car off did you wait before measuring the battery voltage? If I go into the car to pop the hood, some things come “alive” in the car, and the battery voltage at that point might measure around 12.2 or so. But if I wait a few minutes (like maybe 10 minutes or even longer) and keep the key away from the car, the voltage will come back up to around 12.6.

If you’re really losing juice while its sitting, that suggests parasitic drain or bad battery. But parasitic drain would not explain the problems you had when you were driving. Could be a bad alternator diode, but not likely since you just changed it.

Sorry - no other bright ideas other than the obvious - bad battery, bad ground someplace. Several people have had problems with water getting into the passenger side floor - either behind the glove box or under the seat - creating issues with those circuits. I’ve also heard of a ground wire in the passenger fender well that can raise havoc - haven’t set eyes on it personally, just saw it in a video someplace (can’t recall where).

 

At 12.6 or less, it's too low which's likely why the truck was having a complete CANbus meltdown the entire day. After replacing the alternator, the battery was staying at the 13.5 volt range which is where it's supposed to be.

Something I haven't mentioned in any of the threads is the truck has been babied, never left central Florida, has absolutely no rust anywhere on it and has never been driven in water. Watching service videos for these trucks typically shows massive amounts of rust and corrosion which this one has none of.

Voltage has been checked after it has been sitting for several hours or overnight.

I have not observed any parasitic drains and there are no water leaks anywhere that I can tell. It's puzzling the CTEK said the battery was fully charged at 12.6 volts when previously it was in the mid 13s and the now 3 year old AGM in the Jag holds steady in the mid 13s as well.

Could be the battery but this would be going on battery number 5 now in 5 years. Could be the new alternator but I would be really surprised. Could be this all circles back to a bigger electrical issue involving the erratic idle problem I have had for over 2 years. Could be the numerous main and ECO battery issues of the last 5 years have been an indication of a related problem. If this is the case, it is beyond my abilities as well as the abilities of 3 main JLR dealers as well.....

Anyhow I am now keeping it on the CTEK when not being driven and so far for the last 1.5 days the problem has not returned though the battery is still sitting in the low to mid 12s.

 

Everything I’ve seen says 12.6v at rest is considered fully charged in most batteries. My battery is a T8 Interstate Megatron AGM. Maybe some are made to have a higher resting voltage. But mine rests at 12.6 and I’ve had no issues. When running, charging system runs around 14.2-14.5. I think if you’re battery is sitting at 12.6, and alternator about 14, your problem is probably someplace else.

 

Unless I've misunderstood your reply, I politely disagree.

When I replaced the alternator in January, I monitored battery voltage for a couple of weeks just to be certain the alternator replacement had solved the problem. During that time, battery voltage stayed at 13+ volts.

A couple of days ago, when the vehicle failed for the ENTIRE day, the battery was at 12.2 volts which is WAY TOO low, JLR vehicles and other modern voltage intensive vehicles are known to have a myriad of issues related to low battery voltage.

While I am puzzled as to why the CTEK will no longer charge this battery past 12.6, since I've charged it, there have been no further issues related to module failure.

And yes I can confirm the battery in the Jag is at 13+ volts and the battery in the Rover used to be and is supposed to be at 13+ volts.

 

You can search the info and pull up many, many references that say around 12.6-12.8 is normal fully charged voltage with the engine off, and that you should see above 13.5v with the engine running. I have no explanation as to why your batteries were holding at 13.5 - but I guess there’s nothing necessarily wrong with it. Still, 12.6v seems to be sufficient. My battery is at 12.6v with the engine off, 14.5v with the engine running, and I have no problems.

 

My point was at this time, I'm seeing less than 12.6 volts and 12.2 volts is TOO low and likely why it wasn't driveable that day. There's no doubt based on my diagnosis prior to replacement and after replacement the original alternator was bad. What I need to determine now is why less than 6 month old battery is reading too low, why is it staying in the low 12s, even after a charge. Sure I could likely have them replace it, but that would be main battery number 5 in 5 years.

EDIT: Tried the crank test this morning and battery dropped to 10.1 volts but after cranking it bounced back to 12.4 volts.

 

After sitting for several hours, battery still reading voltages as low as 12.1 volts to as high as 12.8 volts. After charging with the CTEK, voltage reads anywhere from 12.5-12.7 volts. Have not observed any further CANbus related MILs.

I will again repeat that in January, the CTEK was able to charge the battery to 13.5 volts prior to the alternator being replaced, now several months later, the CTEK can't seem to get the battery higher than 12.7 volts and sometimes no higher than 12.5 volts. No idea if this means anything.

As I've mention previously I've been a JLR owner for 20 years and a very active participant in the Jaguar forums for over 10 years and there you will find numerous threads related to battery voltage. It appears fairly consistent that newer, good batteries sit at above 13 volts and that batteries below 12.6 volts trigger all sorts of electrical anomalies and MILs, and the closer they get to 12 volts, the worse it gets.

I'm no expert but surely experience counts for something.