Hi guys

I have a 2006 HSE (Jaguar 4.4 v8 engine). I occasionally get the odd HDC or DSC errors. Sometimes the car wont shift higher than gear 3. When these kinds of errors happen, I'll just pull over, turn it off, wait a few seconds then start it up again.

I've also noticed that sometimes the blower motor turns itself down (the speed), which I'm not worried/concerned about, just saying in case it rings a bell.

Whilst I was trying to park up today, the car shut itself off. I fired it up again and it shut itself off again. I turned off the AC blower and fired it up again, this time it was ok. Parked, did what I needed to do, came back to the car, and it started by fine. Tried not to run the AC blower on high after that as I didn't want to be stranded.

I took it to a garage who tested the battery and their tester said my battery only had 47 CCA and it needed to be replaced, although the tester settings only went up to 900 CCA, and mine is a 950 CCA AGM 105A Varta. They told me to go to their bigger branch so I went there and they tested it with a better tester (Midtronics MDX-651P AP) and it said the battery was at 12.1v with a measured 759 CCA. The machine says the battery is "GOOD, RECHARGE", with battery health showing around 30%. He measured the voltage with the car on and it was coming up as 13.2v.

I came home, a few hours later went out and popped the bonnet and without starting had around 12.15v across the terminals. A few seconds after starting I had around 12.89v across the terminals. I turned the blower fan all the way up and the headlights too. Got same, around 12.89v across the terminals. Then I turned on the high beam and fog lights as well. This time I got like 13.8v or so across the terminals. Measuring with a Draper digital multimeter with a good battery.

The battery currently on the car was fitted in May/June 2021. It was a brand new Varta 950 CCA 105AH AGM. I use the car around twice a week, usually for runs at least 15km (each direction).

I live in Qatar so car batteries usually last around 2 years here (as it's super hot here). Standard batteries usually last 2 years max. AGM ones usually last 2 years minimum, sometimes 3 years.

I'm thinking I might have an alternator problem. The shop I went to (reputable guys for batteries, tyres, etc, but not specialists) said that the voltage across terminals should be 14v minimum when the car is running. The Land Rover independent mechanic I spoke to here says that the voltage can be between 13.5v to 14v. Common knowledge says 14.4v is the target. There are plenty of happy people on these forums with 13.8v etc.

I'm not sure what the deal with my car is. Shortly after starting I got a reading of 12.8v. When I turned on everything I got 13.8v. The shop measured 13.2v. What's the deal here? Why so much fluctuation? If it was constantly 12.8 or 13.2 I'd say change the alternator, but the fact I saw it at 13.8 with everything turned on makes me hesitate. What are your thoughts?

The voltages look OK for a managed charge system (as on your vehicle), the voltage varies - not just for battery SoC but also for ambient temperature and engine load, you say you had a replacement battery fitted, is it the same type as the old one? the wrong chemistry can cause problems.
What is the typical journey length once the vehicle is started?, repeated short journeys (<20 mins) can deplete the starter battery to the point where an alternator cannot fully charge it and requires a bench charge, preferably on something that has a 'de-sulphation' function which will help recover the batteries capacity.

Managed charge system? I didn't know I had one of those. I had pulled a bunch of fuses for the entertainment system etc around 3 years ago as I was worried there was a parasitic drain on the car. Sometimes when I locked the car the entertainment screen would be on. Could the pulled fuses be affecting the managed charge system?

The original battery when I bought the car (bought used 3 years ago) was a crappy aftermarket one like 75AH or something, really small. I kept trying to charge it up and jump start the car until eventually realised the battery was the issue. I probably did a lot of damage to the alternator during that time, not sure. I eventually had a Varta 950 CCA 105AH AGM fitted and then everything worked fine. In May/June last year, I had the battery tested by a shop who advised to replace it, so I did. It was around 20 months old then.

So basically
when I got the car it had a useless crap battery
Since then I'm on my second Varta 950 CCA 105AH AGM battery

Typical journey length is around 20 minutes, around 15-20km. Most of that time is spent either cruising at 80-85km/h, in gear 6 at around 1600rpm or so (it's auto), or at traffic lights.

The charge system controlled by the Engine Control Module based upon various factors, your journey length as an average suggests there are a fair number of journeys shorter, I've been to that part of the world a few times and car journeys are high electrical load due to the weather (it's always odd being in mid-30's temperature with the wipers on due to the humidity!)

The Midtronics always seen to err on the 'good - recharge' side of things when the battery is on the top edge of being measured as faulty.

Despite the received knowledge about charge voltages, anything above 12.6V is maintaining the battery state of charge (note, maintaining, not improving), above 13.2V is improving. That you observed 12.9V on startup and then with an increased load (don't forget that putting back the charge removed from the battery by starting the engine is also a load) the voltage was maintained, suggests the alternator is OK, the gearchange issues and other aberrations suggest the underlying concern is low voltage on cranking - typically a sign of a low SoC or reduced capacity of the starter battery.
If you can, charge with a charger specifically rated for an AGM battery for at least 24 hours, check for hotspots or general heating of the battery after a couple of hours, and again after a few hours more, if the battery or parts of it are heating significantly, then stop charging it as it has an internal defect. If it charges OK and you get the same concerns within a couple of days, it's most likely a defective battery, if it takes a week or two, it may be that the usage cycle is a significant factor.

Thanks a lot for your detailed response, it was very helpful and made sense.

I will try getting an AGM charger and do what you said and post back. It might be in a few weeks though as it can sometimes be difficult to find stuff around here!

Thanks, I've uploaded the charge system diagnostic instructions for the LM which you may find useful.
https://www.fullfatrr.com/gallery/albums/u...ostics.pdf

Also from the 'Generator description & operation' page:-

"The 'smart' regulator controls the output voltage from the generator to protect the battery; at low temperatures battery charge acceptance is very poor so the voltage needs to be high to maximise any re-chargeability, but at high temperatures the charge voltage must be restricted to prevent excessive gassing with consequent water loss. The Engine Management System (EMS), which controls the regulator, will calculate the voltage set point required for the ensuing conditions. The 'traditional' regulator controls voltage against generator temperature, which means the battery temperature will lag a long way behind so there will be significant periods of operation when battery charging is compromised. With this system, the EMS can set the voltage by inferring the battery temperature from information received from it's various sensors, hence voltage will accurately follow the battery's needs."

Note that this vehicle was originally specified with a Lead-Calcium battery, EF & AGM batteries will tolerate the charge pattern for Pb/Ca batteries but it does seem to shorten their life.