I have been searching under the topic for battery voltage, but can't quite find a definitive answer on this, so here goes. I have a new battery that starts the car fine and has no obvious issues, but the following characteristics at 60 to 70F ambient.

• Fully charged = 12.7V (looks good)
• After car has sat for 24 - 48 hrs = 12.3V (probably OK ?)
• Key on, A/C off, fan off, radio off,trunk lights on = 11.8V (is this OK ?)
• At OBDII as read onscreen by Autoenginuity key on = 11.07V (this seems too low and Autenginuity lights up a red ball icon for low voltage).
• Engine running = 14.1V at battery and also at OBDII port as displayed by Autoenginuity. (looks good)
• Voltage reads the same at the terminal at the main fuse box in the engine compartment, so that would seem to indicate main battery connections are good.

I know from other comments and my experience with BMW a battery that is low by only a few tenths can cause problems with the electronics, but establishing what is normal has been hard to find. I am going to pull the battery out and have a load test done next.

 

Glad you got your Autoenginuity running now.

On Jaguars, you should ignore the voltage reading taken at the front of the car, and take readings with a multimeter directly across the battery in the boot. With the battery in the distant rear, the front readings are constantly changing due to voltage drops in the wiring due to various loads being switched on and off. Front voltage readings can lead to erroneous conclusions.

 

Thanks, yet again. My Autoenginuity is actually not working fully yet but will do some limited things including reading voltage. I will be talking to them on Monday to get the proper (pre 2003) Jag adapter you so helpfully alerted me to in my other post.


I think after I get the load test done I will have more confidence in this battery.

 

Battery load testers have really dropped in price, handy to keep in your garage.


I got this one for less then £20.

 

Sorry to be off topic but I see you have a '55 Silver Dawn. I owned a '52 Silver Dawn for about 20 years. One of the few cars I wished I'd kept but I no longer had an indoor place to store it when I wasn't using it.





Dave

 

Off topic is fine with me anyway, as I have received good advice that might be useful to others in the first two responses above.


Yes, we enjoy driving the Silver Dawn and it gets priority parking spots when we go to events or out to dinner which is nice (and somewhat amusing to me). It is actually a fun car to work on. I recently tackled a rebuild of the differential and updated modern rear axle bearings along with new axles (worn splines). As you may know these have the GM Hydramatic 4 speed and it still shifts fine and drives smoothly with apparently only minor adjustments over all these years. Ashley James' website Home | KDA132 is an extremely useful resource for technical information and provided all the information needed for the updated axle bearings for example. Flying Spares and Albers Bentley Zionville can supply almost any OEM part needed. (take that Jaguar who has apparently decided that pre 2003 Jaguars no longer deserve 100% parts and software support).

 

Mine had the 4 speed manual trans. I got to know Tony Handler there in CA real well over the years for parts and owned a few other Rolls and Bentleys over the years. I sold my Silver Dawn about 5 years ago to a guy in GA because a friend had been let me store it in his warehouse but he sold it and I lost my indoor storage and I have no garage. After keeping it in the driveway under cover for a while I got tires of seeing it deteriorate and put it for sale. I do miss it. It was funny that that tank got 20 mpg. Drove really nice.


Dave

 

Possibly not so good. Was the 12.7 V measured immediately after charging the battery? If so, it has to be taken with a pretty large grain of salt. 12.3 V suggests a not a very good state of charge. Unless there's a parasitic loss somewhere with the car turned off, charge (which can be inferred from measured voltage) should be better after 48 hours.

 

After sitting on the Battery Tender overnight it was at 12.9V, then 2 hours later it is 12.8V. Previously this same scenario yielded 12.7V. I'll try it in 48 hours, but bet that it will be 12.3 V.


My wife's BMW battery after 7 years would stiil start the car fine and showed about 12.3V, and was having very odd electrical issues (windows rolling down on their own, CD player randomly spitting out CD, and more) all gremlins went away with a new battery.

 

Makes sense. A weak battery can have its voltage pulled down well under 10 volts while starting the car. Many electronic bits go a little nuts in the presence of this low voltage. Our cars are among the most sensitive in this regard.