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Voltage changing?

Tiny
GARY SORENSEN
  • MEMBER
  • 2005 JAGUAR X-TYPE
  • 3.0L
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 103,000 MILES
I just got this car last week.
The old owner changed the alt and went from a lead acid to an AGM battery. When started the voltage is 14.5 then after a few miles it goes to 13.1 and if I put on the lights, it goes down to 12.9 or 12.8
I don't keep the light on very long, so I won't get stuck somewhere. The old owner just put a trickle charger on it each night as he got tired of thinking about it and went and got a new car and sold it to me. I have not charged it and I have driven about 130 miles so far. Runs great and a nice ride. What do you think? The engine compartment is very clean.
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Thursday, February 9th, 2023 AT 8:29 AM

6 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

The voltages sound normal. When you turn on the headlamps, you are still close to 13v which is fine.

When you are checking this, are you doing it at the battery? Does the battery go dead overnight or within a short period of time?

Based on what you provided, there should not be an issue with it failing or requiring it to be charged each night.

If possible, do this. I want you to follow the directions in this link and let me know the results. Additionally, I need to know if the battery is failing at any point.

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Let me know.

Joe

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Thursday, February 9th, 2023 AT 2:55 PM
Tiny
GARY SORENSEN
  • MEMBER
  • 4 POSTS
Joe, I brought the car home on a trailer from MA. I started using the car lightly the next day.
I have more than 150 miles on it sense. It has never failed or has been charged sense I have owned it. The old owner put a digital voltmeter in the lighter socket. Before starting the meter shows 12.4 v. When it starts it reads 14.5-14.8v after a few miles it drops to 13.1-13.0v if I turn on the light and air it drops to 12.8. No battery light has been seen so far but the old owner said his wife had it shut off on her.
I am of vintage age and have had 3 other jags when I was in the 20's
I am good with electrical systems and a hydraulic expert and still working.
My son works for CAT and lives 2 hours away. I hope to see him this weekend and has a Snap on scanner. I will forward your suggestion to him and hopefully weather permitting work on this on Saturday. I will let you know what happens. I may drive it to him and that's 115 miles it may rain or snow so wipers and lights and heater would have to be on.
Thanks,
Gary
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Friday, February 10th, 2023 AT 5:06 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

If the voltages are staying where you indicated, you shouldn't have any issues with the battery.

Let me know how things turn out for you. I'm interested in knowing.

Take care,

Joe
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Friday, February 10th, 2023 AT 9:10 PM
Tiny
GARY SORENSEN
  • MEMBER
  • 4 POSTS
Joe, you may be right. My son connected his Snap on computer to the car and did not find the BMS to change it. I drove the car to his house and back a trip of over 220 miles I did use the lights on the way back for the most part the voltage said 13 volts. When I stepped on the brake it would go to 12.9 or 8 but would recover. My son and I have never seen voltage change in a car or truck. They have always been one voltage reading. I still have not had to charge the battery. It is an Optima AGM and it is only a few months old.
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Monday, February 13th, 2023 AT 5:17 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Gary,

Thanks for the update and I'm glad to hear it is still going. I feel that it will continue running. Remember, anytime you turn something electrical on, it draws from the battery. That will lower the voltage, but the alternator is supposed to keep it within a specific voltage range. Yours is doing that. Even at 12.8v with the lights on and other components, which is not a concern. If the alternator wasn't doing its job, you would have had a dead battery on that long of a trip.

Let me know if anything changes. I'm interested in knowing.

Take care of yourself. If I don't hear back from you, please feel free to come back anytime in the future.

Joe
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Monday, February 13th, 2023 AT 3:14 PM
Tiny
GARY SORENSEN
  • MEMBER
  • 4 POSTS
Thanks for your help.
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Tuesday, February 14th, 2023 AT 4:29 AM

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