I'm driving my 2008 XF Jaguar a lot less now that I've retired and I've been having battery problems. I'm hoping the brains trust can help me figure out what is the trouble. Bear with me whilst I fully describe the situation.

Back in Feb we spent a couple of weeks away, when we came home the battery in the Jag was stone cold dead. I charged the battery, all good, then the car sat for about 6 days and the battery was again flat. The battery was purchased in April 2024 so I took it back to the seller, he charged it and then tested it with his load tester, said the battery was good. I figured I must have a parasitic drain in the Jag, so I tested this- a bit of a task as the car has to be locked and then enough time allowed for all the systems to shut down, so with the battery being in the boot it's a bit of a circus. The residual load after everything shuts down was 28 milliamps, which is exactly what the Jaguar workshop manual specifies as the maximum allowable.

2 weeks ago I went away (not in the Jag) for 3 days. The day after I came home, I went to go out in the Jag and the battery was dead flat. It appeared that I had forgotten to lock the car after retrieving my sunglasses from it at around 3am before heading off. These cars are known to have heavy residual current if not locked, as some systems stay active. I cursed myself gently and put the battery on the charger overnight- it was so flat I had to boost it briefly before the charger would recognise it. All seemed good after charging the battery.

Last week I again went camping for a couple of days and again found the battery in the Jag to be dead flat when I got home. This time the car had been locked so back to the base load of 28 milliamps. I attempted to charge the battery, this time my charger recognised it was attached but kept indicating a 'fault'.

I took the battery back to the retailer on Saturday morning, young bloke checked the voltage and it was about 9.5v, said he would put it on charge. I went back this morning (Monday) and an older bloke found the battery, I told him I suspected it was down in capacity. He attached it to his load tester which indicated the CCA was pretty much bang-on spec (780). I had a bit of a discussion about how it was draining in such a short time with a 28ma residual current; he basically said that if his machine says it's good, it's good, not his problem.

Is it possible that the battery will show OK with a load test, but have reduced amp-hour capacity? 28ma for 72 hours equals about 2 amp-hours, shouldn't be anything like enough to drain a 160a/h battery.

I have set the battery up on the bench with a 480ma load attached (a strip of LEDs), my maths suggests it should be able to run this lamp for nearly a fortnight (160a/h / .48a =333.33 hours) so I'm interested to see what it does over 24 and 48 hours.

 

I think your bench with load is a good idea.

I suspect it'll go flat (*) but please post back what happens.

(*) and if so is toast...

 

POD, I think my background is important in this discussion as I have dealt with batteries for a long time and batteries on a very large scale (like capable of powering a small town, or a nuclear submarine, whichever you prefer). Knowing how it works was vital for me to ensure the nuclear reactor remained safe. So, I am going to keep things simple, yet, get the point across. I think you are getting a few numbers crossed and I am going to help you straighten them out. First off, CCA or Cold Cranking Amps is simply a measure of how much current the battery can flow at a given temperature (normally around freezing) before the battery falls to a given voltage (current for 30 seconds before the voltage falls to 7.2 volts). The benefit of a higher CCA is that in theory, you can maintain a higher voltage on the battery, making it easier for the starter to develop the current needed (ie, torque) to roll the motor. I normally tell people to pay attention to how the car starts. Most modern day cars you only need to roll the key to the start position for a second (1 or 2 revolutions of the motor) before the engine catches and fires up. When a battery starts to have issues, you will find that it will take 2-3 seconds for the motor to catch. Not a big deal as the car is still starting, but this is an indication that your battery is starting to go or you have other issues.

Now for your 160 number. I think you misread the specs as this is reserve time. This is a measurement similar to amp-hours, but is a bit different too. reserve time is the amount of time (normally in minutes) that a new battery at full charge can last before hitting 10.5 VDC (ie, the minimum voltage deemed possible for starting the car) when loaded down with a 25 amp draw. So, in theory, you have a battery with a 160 minute reserve, the battery can be run for 160 minutes with a 25 amp load and then remove the load and the car should roll over slowly, but start. Where amp-hours is calculated by taking the reserve time (minutes) and multiplying it by 25 (amps) and dividing by 60 (minutes/hour). This will give you amp-hours. I am going to divert for a minute to get a little into battery theory to help explain the following. An ideal battery (they do not exist) in theory outputs a constant voltage regardless of the current you pull from it. In the case of car batteries, you will find that under atleast lower currents (say under 20 amps), the general rule is a 0.1 VDC drop in voltage for every 10 amps pulled (this varies based on the chemical reaction in the battery and is well beyond what is needed here). So, another way to look at it is to have the ideal battery in line with a 0.01 ohm resistor and all of this is between the 2 battery posts. The one variable in this is going to be that the 0.01 ohm is based on a new battery at full charge. As the battery gets older and/or the battery discharges, this resistance goes up. In most cases, we are looking at going from 0.01 to 0.03 ohms. But, this plays into things. So, in other words, for the same current being applied, the battery voltage is going to be lower on an older/discharged battery.

So now, lets get back to amp-hours. Amp-hours for a battery is relative. What I mean by this is if you have a higher current draw, the battery is not going to last as long, ie, drop to 10.5 VDC. So, with a 25 amp draw, you get 92 amp hours, but you put a 100 amp draw on the battery, you would want to say that you put 4 times the current on the battery, therefore the time would drop by a factor of 4 (or a 100 amp load would only last for 40 minutes - 160 minutes divided by 4). When have you ever been able to crank you car for 40 minutes straight. In reality, your amp-hour rating drops. So, at 100 amps, you may only get say 40 amp hours, or 24 minutes (even this is being generous). But, on the other side, you lower the current down to say 1 amp, you can get say 120 amp-hours out of the battery. So, you can power a 1 amp load for 120 hours (almost a week). There is a lot of characteristics of a battery that feed into how this works and if you really want to know this, there are plenty of articles out there.

Now, to your problem. Keep this "ideal battery with the resistor" in mind. You have 6 cells in a battery, each has 2 plates (a positive and a negative). If you take a standard starting battery (what should be in your car), every time you deeply discharge the battery, you damage these plates. So, when you recharge, these plates to do not back to 100%. This causes that resistor to start climbing in value. In some cases, it can be goofey and be one resistance at a low current, yet, another value at higher currents. So, you can have a battery that tests fine under low load conditions, yet, fail miserably at higher (starting) type loads. This is where I think you are.

What I would say is to go to a new store and ask them to do a load capacity check of your battery. Jags are very picky and they tend to only like batteries with a capacity of 75%. Less than that, they start acting goofey. I think you are going to find your battery is below this capacity. Therefore time to get a new battery.

So, to summarize what all was said above, your battery is good at full charge, but you have 1 cell that is damaged and you discharge it some, it causes the battery to fail miserably.

 

hi Thermo, thanks for taking the time to post that detailed information. I have to confess to still being confused about the situation with my battery. After several days with just the 24 milliamp base load of the vehicle, it's not in a state where it won't sufficiently crank the engine- it is stone, motherless flat- won't even unlock a door, won't light up the start button, 9.5 volts flat. If I understand you correctly, that is possibly due to one failing cell, but that is not showing up on the seller's equipment. I'm getting credible, qualified advice on another corner of the internet that this strongly suggests that the battery is badly sulphated from my habitual short-trip use of the car. The retailer charges the battery, does the brief discharge test and the battery reads ok on his gadget. How is the load capacity test that you suggest, performed? This battery is now 13 months into a 24 month warranty. I'm unsure if it is a warranty issue or a user issue?

 

POD, sulphating of a battery is where the plates develop little "fingers". With no discharging (more than a full charge state starting of the engine, more on this in a moment), the fingers grow until a finger touches both a positive and a negative plate. At this point, a cell undergoes what most would call cell reversal (ie, instead of it producing 2.1 VDC, it sucks up 2.1 VDC and then promptly starts discharging the battery until it is completely dead. Where this does not necessarily fit your problem is that once a cell reverses, it is permanent. It doesn't flip back and forth.

As for the capacity test, you should start with a battery at full charge and then you put a heavy load on the battery and you are essentially figuring out the capacity (amp-hours) of the battery. You then compare the calculated to the rated.

The test that I would tell you to do is to completely disconnect the battery (ROTATE THE TRUNK LATCH SO IT IS MADE UP AND KEEP IT THAT WAY UNTIL DONE!!!!!!!). Now, using a battery charger, charge the battery up to 100%. DO NOT RECONNECT THE CABLES!!!!!!!! Immediately after removing the charger, if you measure the battery, you are going to find it up around 13.4 VDC or so. This is normal. After about 10 minutes, it should be at 12.6 VDC, not 12.5 or lower. This would be an automatic fail. Now, let the battery sit for 12-24 hours with nothing connected to it. When you measure it again, it should still be at 12.6 VDC. If it is down at 12.5 VDC or less, then you have an internal problem and you need a new battery.

If you find that the battery is at 12.6 VDC, then what I want you to do is find a 1 ohm/1 watt resistor (you will also need some other hardware to attach the lead to the battery post and the battery terminal, alligator clips should work for this, but may need a large pair for the battery post). What you are going to do now is connect up the negative post like normal. Now, you are going to connect one lead of the resistor to the battery post and the other lead to the battery terminal (make sure the battery terminal does not come in contact with the body of the car). Now, at this point, you should be able to read the voltage across the resistor and because it is 1 ohm, 1 millivolt is equal to 1 milliamp. What is the voltage across the resistor? If you were correct earlier, it should be 28 millivolts (0.028 volts). I think you are going to find that the draw is going to be much more than the 28 millivolts that you are anticipating, indicating that you have something in the car not turning. Now, this assumes the car is completely shutdown and all doors are locked (having the trunk open is not a problem because the car is going to see the latch made up and think the trunk is closed).

When you are done, you simply hit the open button on the outside of the car and the latch will roll to allow you to close the trunk once you have returned everything to normal. The big thing is when you have the resistor in series with the battery, do not turn on anything big. That resistor will get pretty warm and it will make the car do funny things as it is going to see a low voltage condition.

As for preventing a sulphating (aka, mossing), this can be cured by simply running the battery once a year through a 10 amp load for an hour without the car charging the battery. On other cars, I would tell people to simply turn the radio on and wax their car for an hour. But, our XJ's are too smart to allow that. So, you will have to get a little creative. Then with the battery slightly discharged, start the car and let it give the battery a hard charge for 15 minutes or so to get it back to 100%. This heavy charge will burn the "fingers" or "mossing" off and ensure even plate build up. The easiest way I can think of is to get like 2 H1 halogen bulbs, wire them up in parallel to each other with leads that would go to the battery. That would meet what you are trying to do.

 

Charge the battery fully (24 to 48 hours)
then test for:

• State of Health: Assess the overall condition and lifespan of the battery.
• State of Charge: Determine the current charge level.
• Cold-Cranking Amps: Measure the battery's ability to start the vehicle in cold conditions.

Pretty much everything you need to know and it's not rocket science it's just a small testing unit.
You can also us a de-sulphating unit to attempt at least partially recovering the battery.
No math, no test bench, no power supplies, no additional wiring. These are simple and cheap device devices.

 

After a long working life in the physics of electronics, I like to keep it as simple as possible . . . so, I endorse the brief but very accurate summary that John offers in post #2 . . . either your car has developed electrical gluttony (and <30mA while correctly asleep disproves that) . . . . or your relatively new battery is toast!

Failure of an otherwise new battery is NOT unknown. Rare? Yes, but not unknown.

Despite what your older salesman contends is not HIS problem, Australian Consumer Law has quite a surprise awaiting him . . . IT IS THE SELLER'S PROBLEM!

Bottom line? Your tests are well founded and unambiguous. Like Monty Python"s dead parrot, your battery is not just resting . . . it is a DEAD battery! Please do yourself (and all of us in Oz) a favour by insisting on either a replacement or full refund (your choice; not theirs). If the latter, I suggest you take your refund and your business elsewhere. If they offer less than a helpful solution, do let us all know who and where, by posting in the VIC regional forum as well.

Best wishes, mate.

 

Thanks for all the replies so far. At the moment I'm testing the capacity of the battery with a 480mv load attached and periodically checking the voltage to see how long it takes to discharge to 12.0v. Plenty of info now with other tests to run when this result is known.
The test described above with a 1ohm resistor appears to be testing for the same thing I tested for when this problem first occurred, which I did by locking the boot (trunk) open and allowing the car to go to 'sleep' with an ammeter in series between the battery and neg cable, as described in post 11 of this thread https://www.jaguarforums.com/forum/x...-drain-283683/ and I see my recollection of the residual current was incorrect, it was 24ma not 28ma. I will check it again to ensure something has not gone awry since then.

 

Here is what Jaguar has on current drains or Quiescent currents.
Your right in the range for normal operation as you state? Your seeing 24-28 ma.
The chart says XF's should be less than 24 ma.
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24-28mA would take an age to flatten a properly working battery.

Either it goes up to a much bigger current or the battery is failing.

 

POD, I mention the resistor check as I have found that a lot of multimeters are not very accurate when measuring milliamps. By converting the current to a voltage, you can get a much more accurate reading. Hence why I said to add in the resistor. Yes, it is the same sort of check, but a different way about getting the information.

 

I've had a 480mA load on the battery and it is now down to 12.009V as of 9am today, 69 hours (4,140 minutes) in. My calculations, extrapolating from the 50% discharge and dividing the minutes by 52 (480mA is 1/52 of 25amps) suggest that this equates to a Reserve Capacity (RC) of 159.23 (159.23 minutes to full discharge at 25 amps). The label on the battery gives RC as 160.

This suggests that the battery capacity is pretty much what it should be, which suggests that my problem lies elsewhere in the vehicle and that I need to repeat my test for parasitic drain.


Please chime in if I've got this wrong.

I'll do this test next, now that my discharge test is complete. Off to hook up the charger.

 

The way you did the capacity test completely negates the value you received. Remember, the test is from fully charged to 10.5 VDC, you only went to 12.0 VDC. So, your capacity figure would still be on the test device. Also, the heavier discharge rate would help to pull the battery value lower, shortening the time.

 

I'm getting sightly conflicting advice from a couple of different sources (who know a LOT more than I do about this!) and trying to apply it with the available equipment....the other chap has advised that discharging the battery to 12v is 50% discharged and that the 50% can be extrapolated, but that my small discharge current cannot be extrapolated due to paekert's law or something...whilst I had the battery still out of the vehicle, I've done a second discharge test with a 8amp load (apparently this being 1/20 of the stated capacity is a standard test) and the battery discharged to 50% in 3 1/2 hours, indicating drastically reduced capacity...I've had the battery on charge for 24hrs so I'm about to re-install it and test again for parasitic drain, assuming that has not changed and is still within spec (yes a big assumption, hence my intent to re-test), I'll take steps to de-sulfate the battery and try to regain the lost capacity. Also going to set up a battery maintenance charger via a solar panel to try and avoid this problem in the future.

 

I would trust the 8 amp load test you did more than the 480 mA test. YOu need something with a bit of a current draw to it to really test the battery I will liken it to having a marathon running seeing how fast he can run a marathon by how fast he can wiggle his toes. There is probably some relation, but not a real good measure. Now, you make them walk as fast as they can, probably a bit more relatable.

 

With the problems these cars have with batteries I would keep the car on a plug in charger. Just to eliminate another variable.
The most recommended one is CTEK which is the brand Jaguar uses in their show rooms. In fact you can buy a very expensive CTEK that is branded Jaguar too.

Here is a thread and we have many if you search. Has a good picture of the Jaguar branded version.
CTEK Battery Maintainer
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I've had a recommendation to use a solar panel as a battery maintenance charger; apparently the inherent hours on / off (i.e. day / night) works to recover a sulfated battery and prevent the process. One of the jobs I did yesterday whilst the battery was charging was to install a fused connector to the battery compartment to connect a maintenance charger,
Earlier this year we left my wife's car in airport parking whilst we went interstate for a couple of weeks. No way would i be game to do this with the Jag at present. Being stranded in the airport carpark in the middle of the night after a long flight does not appeal at all.

 

Up to you but with all the experience on this forum I would just get the CTEK and be done with it. If the battery is sulfated just change it. Not worth messing with a marginal battery.

I have left my 2014 XJR at the airport for 6 weeks because of a family medical issue and it started right up with no problems. My factory battery lasted over 10 years and I only changed it because well it was 10 years old and I had never seen a battery that old that was any good.

If you use off airport parking which I completely recommend they all have run into this many times and they will have a jump set.
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well that took 5min and only 20 bucks for the tool to use over and over in the future.
I just hate to waste so much time trying to out think a simple problem. I like to keep it simple.
This didn't even require a full page ad in the post for beard stroking. Mine is bigger anyway

well that's a bad battery

ok this one is still good just 10% low charge

 

I've seen very different state of charge for AGM than that - are you sure it's correct?