Every time we get in the car this message appears on the dash panel, indicating to me that the battery is nearly discharged. The car is driven at least 20 miles per day. Dealer says alternator is OK and battery is not defective. Anyone know anything about this? Has there been a software fix for it? Dealer is useless. Thanks..

 

What year is the car ? This issue was on early cars, and is, I believe fixed, but don't know how early cars get fixed. I would think it is a software problem.

Other thing to do is to always lock the car so all electrical systems are shut down, as just being left unlocked in your garage or on the driveway keeps a lot of modules active.

 

20 miles is not very far. If the battery is fairly old or if there's a drain then 20 miles won't be enough.

You can check for a drain and/or put it on a ctek (or similar) charger.

 

OP's Personal Profile shows his XJ is a 2015 3.0L SC. I'm pretty sure it has 2 batteries, a large primary and a small secondary for the Intelligent Stop-Start system. The Stop-Start battery doesn't last as long as the primary battery. Since both batteries are over 3 years old, I'd have them load tested at the local auto parts store. Most national chains (AutoZone, Advance Auto Parts, etc.) will do it for free while you wait.

 

I have the exact same issue with my 2014. I just live with it and when it gets annoying enough, I put it on the battery charger overnight and then it is good for another few weeks. I think the battery is actually low but not really low enough to be an issue. These cars have such low gearing that I think they alternator does not spin enough to keep the battery fully charged. I also drive around 20 miles a day.

 

The alt spins plenty fast.

Some people don't drive enough miles, others have a small current drain they leave unfixed.

 

It is not as far-fetched as you might think. Most alternators do not put out 14 volts or higher until an engine speed of 3000 RPMs. Under 14 volts output is not really enough to recover a depleted battery over a short drive. IOW, the alternator may not be supplying the battery with enough power to overcome the battery drain on short drives. I see this in my car in that I need to manually charge the battery about once a month when I start seeing the telltale "start car immediately" message. A day on my charger and I am good for another month or so. My charging system has been tested by Jaguar and is 100% within spec. The only conclusion is that the alternator is unable to provide enough power to keep the battery charged on my typical 1 hour/day driving.

BTW: I do not use "S" mode but suspect it would make up for some of the lost alternator power if one regularly used it since it significantly increases engine RPM over the entire power band.

 

Do you always lock your car, even when it's parked in your garage? If not, that contributes to battery drain since convenience mode keeps those computers up and running until they time out. Shut them off manually by locking your car and you'll minimize battery drain. And use a CTEK battery maintainer to keep your battery fully charged.

 

We're not talking about "some engines" but instead about the kind in your car.

Measure the voltage and see...

 

This message has been on my car's screen for a month. It has and still cranks great, so I am wondering if the message concerns the secondary battery. I have noticed that the stop/start feature hasn't been operational as of late, a blessing.

John

 

I will check this with my multimeter and see what it says but I think I would need to test while the engine is under load and I am not sure how to do that. I do have one of those power outlet voltage adapters (shows a continuous readout) and wonder if that would be accurate to what the battery is receiving.

If not the alternator, and the charging system is working correctly, and externally charging the battery resolves the issue, to what would you attribute the battery charging apparently not being enough to satisfy the car's diagnostics?

 

As I understand from my tech at Jaguar the batteries on the 2013 to 2015 model years the smaller battery is there to run systems while the car is at a stop and uses the main battery to run the starter when the brake is released. If the system determines that the small battery is not strong enough to provide the systems then there can be low battery displayed and/or the stop/start feature is disabled, but the main battery can still be in good condition.

 

I have never charged the small battery but when I charge the large one, the low battery messages go away.

 

JagGate I still think your small battery is slowly dying and usually does not last as long as the large battery. The warning may be due to total available battery power. Have you checked the small battery under a load test?

 

I took my '13 XJL to the dealer for the low battery warning. I was convinced it was the secondary battery as the car cranked well, even after sitting a few days. Lo and behold, the main battery was found to be the culprit. New one is fitted and no more message.

 

I call BS on this. Yours is out of warranty so hence they found your battery bad. Mine was in warranty so it was good. I charged mine manually and the warning went away (for several months and came back, rinse and repeat). There is nothing wrong with my battery and there was nothing wrong with yours. A battery in a trunk like our XJs have can and should last 10 years.

I stand my initial conclusion; The engine does not turn fast enough to keep the alternator spinning at a speed to adequately provide a voltage above 14 volts to charge the battery properly if we do not drive the car enough.

Your battery will give you the same warning message in a few months once the charge is depleted enough again.

Best thing on this issue is to accept that Jaguar has a design flaw in the charging system and get yourself a battery charger to use when the message appears.

 

But wait, there's more! I picked the car up today and drove 300 round trip. Twice the car bucked, error messages started flashing on the dash so fast I couldn't read them, both displays strobed about 20 seconds, and headlights went out. Luckily I was on a well-lighted freeway or I would have been driving blind. The system righted itself and the car drove flawlessly the rest of the way.

Is this related to the the "battery" issue? I guess I'll put my nose into it tomorrow.

 

Being an electrical type in my line of work, I see a few misnomers about how the alternator works and when it should do certain things in the above posts. All alternators have 2 ratings on them: The voltage that they output and the maximum current that they can generate. Please note, I specifically call out "maximum" current. This current normally has a number next to it that refers to the minimum shaft RPM of the alternator that it takes to be able to reach that number. I will go into more detail about this shortly.

Now, on to what can drive some people nuts since it can get into a gray area of many variables and "what ifs". Keeping things simple, the voltage that an alternator outputs is a function of the field current, the speed of the alternator shaft, and the amount of current on the output. The more field current (can be simplified to simply think of it as the field voltage), the higher the voltage. The faster the alternator spins, the higher the voltage. The more output current that is needed, the lower the alternator voltage is.

With all this being said, the regulator for the alternator is always attempting to make the output of the alternator 13.7 VDC (there are exceptions to this that Jaguar has programmed into the car and if people want to learn about this, I will go into detail about it). So, if you start up the car with nothing turned on, the voltage drop due to output current is minimal, the engine RPM is relatively low (so, not much input to raising the voltage), so, the output voltage is almost solely based on the field current. Since there is no load, not much field current is needed. Sticking with the concept of output voltage = field current x RPM - the voltage drop from the output current, as you start turning on things (seat heaters, dash fans, etc), the voltage drop is going to start rising. But, assuming that the RPMs remain constant, the field current will have to go up to maintain things at 13.7 VDC. The issue comes, at some point, the field current will not be able to rise any more because it has reached a limit, therefore, if you keep raising the output current, the output voltage is going to drop. For most cars, the point of the output voltage starting to drop is around 50% of the rated maximum current of the alternator. At this point , additional current demands are handled by the battery.

To correct the above situation, you can either reduce the output current (not always an option) or you have to raise the RPMs. This is where a new equilibrium is reached as you have the RPMs trying to raise voltage (as the RPMs go up), the output current will be high, so the field current will drop some to maintain things at 13.7 VDC. As a general rule, most cars reach maximum alternator output at around 2000 engine RPM. So, between idle (700ish RPM) and 2000 RPM, where the alternator maxes out varies, but as a general rule is a linear rise between the idle point and 2000 RPMs.

Above 2000 RPM, you start running into limitations with the amount of current that the windings can handle and reaching a saturation point (beyond what I am really trying to cover here). But, in short, this is why the alternator doesn't really make any more current beyond the maximum current, regardless of the RPMs.

If you really want to know more, let me know and we can talk about it separately and not make others end up with glazed over eyes because of going beyond what they may want to know.

 

13.7V must be a US spec thing. Back in the day in England a 13.8V alternator was one with a diode popped somewhere.

You can bring all the techno you want, Thermo. I'm game.

 

agentorange, you bring up an interesting point. When it comes to multimeters, they are not all the same. In short, depending on how they filter the incoming voltage, you end up with either an averaged voltage or you end up with an RMS (root mean squared) voltage. There are slight differences and why some people see 13.7 VDC, while others will see 14.4 VDC, on the same circuit. The difference really comes into play when you are designing circuits. For the backyard mechanic/tech, the values are interchangeable.

So, in the case of your 13.8V alternator, the rectifier was taking multiple AC signals, rectifying them, and then laying them one over the other. When a diode failed, you ended up with a "dead area" where the voltage dropped a fair amount. So, when your averaging multimeter saw this signal, it resulted in a voltage drop of 0.6 VDC, where the RMS meter would have seen a smaller change.