When I purchased this car two months ago, it came with a brand new battery. In using my Torque app, the battery voltage is always at 12.7 to 12.8 with the engine running. In my experience with multiple vehicles (and a boat), the initial charge voltage when the motor is first started is at least 13.6 volts and pretty much stays there while the engine is running. I also own a 2000 XK8 and am well-aware of how sensitive the onboard computers are when it comes to voltage. No problems of any sort now, but I'm wondering if there's problem with the alternator regulator. Anyone else have a similar situation?

 

Forgot to mention that this is an AGM battery which might account for or answer my question.

 

https://www.jaguarforums.com/forum/x...nation-202162/

 

Max, what you are seeing can be caused by many things. A bad alternator is one, bad battery is one (regardless of how old or new it may be). It could be something as simple as a battery terminal with some corrosion on it. Granted, from our dealings with the car, what you are experiencing is most likely because of a bad battery cable.

In an ideal world, what you should see is with the car off and a multimeter across the battery, it should read 12.6 VDC. If you are getting something higher, your multimeter either is not properly calibrated or there is an issue with the multimeter (this voltage is a function of how a battery is designed and cannot go above 12.6 VDC). If the voltage is down at 12.5 VDC or lower, then your battery is not at 100% charge. Put on charger. If the voltage is still reading low, then your multimeter is not properly calibrated. With the multimeter still connected across the battery, start the car with all electrical loads turned off. You should see the multimeter jump up to around 14.4 VDC (possibly up to around 15.0 VDC depending on how your multimeter reads the output of the alternator). It should stay at this level for a period of time (this is calculated by the car based on how much it thinks has been pulled out of the battery). Normally about 20 seconds or so after starting the car, you should see the voltage ramp down to around 13.7 VDC (may read a little higher depending on your multimeter). Now, if you turn on your headlights and seat heaters, the voltage that you saw after it ramped down should still be indicated. if it is down around 12.5 VDC or lower, then your alternator has an issue. If you are finding that just after starting the car that the voltage is up around 13.7 VDC and then after say 5 minutes of idling with some electrical loads it is now down around 12.7, then this would most likely indicate a bad battery cable (cup your hand OVER - DO NOT TOUCH - the battery terminal to see if it is extremely hot or if it is at the same temp as the rest of the engine bay components in the area, the hot terminal has the bad cable).

I have seen where people have questioned the output of the alternator because of having a high reading multimeter. There are 2 kinds of multimeters out there (RMS and average). This is buried deep in the literature and for 99% of the time, it doesn't make a difference. But, this is one of those 1% times. If you really want to know more about the differences, let me know and I will explain it. But, it is beyond what is needed for this.

 

So I took my trust multi-meter and went out to the car; now the @#%& hood won't release! Researched it on the forum and found it to be a common problem and will attempt to remedy the situation tomorrow and get back to addressing the original problem. Unfortunately, there have been a few problems in the last week or two and the wife has lost confidence in the Jaguar (and me for purchasing it). Anyone have a fix for that lol?

 

 

Okay, got the @#$% hood open. Put my multi-meter on the battery; car sitting for a few days, and got a reading of 12.32 volts. Started the car and the alternator charged the battery at 14.8 volts before tapering down to 13.2 Volts, which appears to be normal in my experience. The battery is a Duracell AGM, a type of battery I've never owned before. Anyone know what the static voltage should be for this type of battery? Should it be different than a conventional flooded battery?

Thanks.

 

max224, the AGM stands for Acid Glass Mat. In short, there is a pourous glass mat inbetween the lead plates of the battery. A lot of batteries that are kept at 100% charge all the time (pretty much, any starting battery) will develop what is called "mossing". In short, the plates will develop little fingers that will extend out perpendicular to the plate. If you let these grow enough, they will touch the next plate. Since the next plate is the opposite polarity, this will cause a cell in the battery to go dead and in short, makes the battery completely useless. The glass mat prevents these "fingers" from touch the next plate, extending the life of the battery.

A little trick that you can do is by discharging the battery, you can burn off these fingers. The easiest way I have found is to every spring, pull the car into the driveway/grass and give the car a wash. Then you can roll down the windows, turn on the radio at a decent volume and play the radio for an hour or so. This will be a high enough discharge that it will burn off the fingers, extending the life of the battery. But, the time it was discharging is not enough to cause any damage to the battery and should leave you with enough power to start the car (granted, should turn over slower than normal). Then you take the car for say a half hour drive to get the battery back to 100%. You do this religiously once a year, this should help you get out to 7-9 years of battery life. The oldest battery I had made it to 11 years.

As for the expected voltage, any lead acid battery should be at 12.6 VDC. Each cell of a lead acid battery makes 2.1 volts. Since there are 6 cells in each lead acid battery, that should leave you with a fully charged battery of 12.6 VDC. Having a lower voltage says that part of the battery is discharged and not at 100%.