Well had to take a ride today. Snow is gone , it rained and washed down the salt/CC on the roads so out we go. I turned on the Torque Pro and started watching my water temp and my voltage. I usually have both the ADJ and the CM voltage just to see the comparison. Took the car off of the smart charger , started it up and lowered the top. Voltage stayed at 12.7 (adj) and 12.9(CM). Started driving and the (adj) ran at 12.5 , 12.6 , 12.7 back and forth during the early part of the drive. The CM volts would jump from 12.9 , 13.1 , 13. 0 and 12.8 . There was a time where the ADJ would run at 12.6 for a few miles. I stopped to top off the gas and with the engine off I saw the ADJ go to 11.5 while I pumped the gas. Started it up and it went to 13.4 ADJ and after a minute or two it went back to 12.6 , 12.7. Just the info I saw while driving. 40 minute drive town/highway 30-75MPH. Oh water temp went to 200*F and dropped to 195.8 during the trip. Fun drive and put it back up on the lift until next time. Happy New Year to all.

 

I'm guessing your alternator is starting to go out. Mine never drops below 13 when the engine is running. At 12.x you really are barely keeping up with the electric draw of the car.

 

I would have maybe considered that but if you are seeing voltage over 13 for long periods of time that means your alternator is "working" to keep current supplied to the battery. If the battery is fully charged the alt. doesn't need to work hard to keep it charged. I won't expect the alternator with 14K miles on it to be bad. Battery is 2 years old and load tests good.

 

Since the actual Full Voltage of a car battery is 13.2 volts, the alternator shouldn't put out below that, ever. I've done the same Track on both ECM and OBD concurrently, and never did they get below 13.4 volts.

 

"resting voltage" is what you should be looking at.

The chart below is for convential lead-acid, ca-ca as found in the
Jaguar are slightly higher:

State of Charge Specific Gravity Voltage
12V 6V
100% 1.265 12.7 6.3
75% 1.225 12.4 6.2
50% 1.190 12.2 6.1
25% 1.155 12.0 6.0
Discharged 1.120 11.9 6.0

 

That is the important detail for the explanation of your voltages.

Your car was on a smart charger that kept the battery at a likely 13.6 V (approximate) level before you started the car. The voltages you observed in the high 12 VDC ranges tell me that the computers saw that the battery was fully charged and the smart charging system in your car "decided" not to "overcharge" the battery beyond those fully charged levels. After you stopped and restarted, you saw a brief charge to replenish what was pulled out of the battery by the starting motor.

Let me suggest that you do not put the smart charger back for a couple of weeks, allowing for a partial discharge, then start and drive your car. If the charging system works properly you should see at least mid-13 - low-14 V charging levels initially.

I have no access to the design of the Jag charging system but, I think that it is rather smart. After I had my XKR on trickle charger for weeks, I started and drove the car and my voltages actually dropped into the high 11 V range, indicating that the computers figured that the battery was overcharged and actually went into some discharge mode. Once I made a couple of stops and starts, pulling a lot of starting current from the battery, the system resumed its normal charge cycle at voltages near 14VDC.

 

This happened to me in August, battery not charging warning followed quite quickly by all computers shutting down at 70mph. With 13k on the clock even the Jag tech said it was unheard of.
One brand new alternator later and all is well again, so it can happen, if you are unlucky.

 

The more I read/hear the more interesting this gets. I did read about some alternators actually will go into a mode where they discharge the battery because it has to great of a charge. It reduces the battery to 75% and then recharges it. This would show a large swing in voltage down into the 11V range . Also some have start delay before the alternator starts charging. Then there is the overrunning pulleys that decouple the alternator at various speeds (I have this on my Lexus). So with the newer technology and the smart control modules there are a lot of differences on how these alternators work. Seems like the days of the 13.8 to 14.2 V constant charging have changed. Also throughout all of this there was no warning light on the dash so all appeared normal. Live and learn.

 

Today it was 40*F in the garage and I removed the smart charger. Started the Torque Pro App and resting voltage was at 12.6V. Started the car and adjusted voltage went to 12.9 and the Control voltage was at 13.4. Ran the car for maybe 5 minutes as I moved it so I could work on my wife's Volvo. I didn't lock the car so the computers were running until they shut down. Came back maybe 45minutes later and read 12.2 volts on the Torque. Started the car and the Controlled Module voltage went to 13.9 and the Adjusted voltage went to 12.9. Went for a 5 minute ride and the CM went to 13.1 while the ADJ. went to 12.9 . Just a little data here FYI

 

12.2 is low after such a short time. If left unused & unconnected overnight what is it?

Hint: probably near dead!

 

Even after an hour drive my ScanGauge still shows high 13's to 14. The battery (brand new AGM) is clearly fully charged by then, and it's still pulling well over 12 volts. Of course it depends if you are running the headlights, fan, seat heaters, etc.

I still think you have a weak alternator if you consistently see voltage under 13 while the car is running. What's the difference between Control Voltage and Adjusted Voltage? I've never heard those terms before.

Govtslug

 

Maybe Steve can chime in here and add to the conversation. Somehow I think there was a change made in the 2010 and newer cars. If we recall there was specific instructions on how to jump start and where to make the connections on the 2010 and newer cars. As to a weak alternator I have found it is either charging or not or overcharging. The newer systems with the control module in the circuit may need a little more investigation. I also see voltage changes on my Lexus depending on speed but that is explained by the overrunning pulley which decouples the shaft. We'll have to see and pending the investigation into the WP I'll have the dealer check out the system.

 

If I saw voltages in the 12's while driving my car, I'd be pretty nervous...

I'm with Cee Jay and GovtSlug on this one - seems like a weak alternator, based on what you've reported.

 

Another interesting discovery is the cost difference between the 2007-2009 alternators vs. the 2010-2015 alternators. It's about 40-50% more for the newer MY. So something has changed and it would be interesting to see what.
Oh i unplugged the charger and left the car overnight and this morning when I checked the voltage it was at 12.5 .

 

Then there is nothing wrong with your battery.

 

If you really want to get a feel for the state of your battery get a voltmeter that can record minimum and maximum, connect it directly to the battery and record the voltage when starting the car.

A "good" battery with 12.6V resting should not drop below 11V when cranking the engine, 10V is "hmmm" and 9V is a bad sign.

 

+1

Below 10V cranking - think about a new battery now

 

I think the OPs 'technical problems' would disappear if he stopped measuring the voltage.

 

Mikey I don't consider this to be technical problems but more of a data gathering exercise. It is just interesting to see how these things have evolved and continue to improve. My first car had a generator so going from 10-20 amp devices to todays 150 amp units is pretty interesting. Just like seeing HP going from 80-100 to todays 500-1000 super charged and turbo cars. I also find it interesting that a lot of high end cars/trucks are providing less information to the drivers. For instance 3 of 4 vehicles I own only have water temp and fuel gauges. The 4th being the Jag only has fuel. All of my previous owned cars have had WT , OP , Volt , Fuel and some with oil temp and transmission temp. For a turbo or SC motor I would think there would be others like boost pressures. I assume the auto mfg's. think we don't need to see this stuff and maybe for the new generation of drivers it isn't needed. The last bastion to evolve was the dieing out of the manual transmission. The dual clutch paddle shifter will be replaced as the auto box paddle shifters start to be competitive. Just my observations. Happy New Year

 

It's more that the majority of owners don't know how to interpret the data and what action to take when. There's not a week goes by where we don't see a post where despite all the flashing red warnings, pegged temp gauges and in some cases cars going into self protect mode, owners keep driving them to get home or to the garage.

If those clues are too complex for some drivers, the subtleties of charging system voltages under varying condtions would be seen as rocket surgery.

No offence, but if you'd not measured and posted your observations, there would have been no posts suggesting either a bad battery or alternator. Your car is operating perfectly, is it not?