I bought a 2015 C7 A8 Stingray a month ago. I knew the car had the original battery so to be on the safe side I installed a new battery over the weekend. I’m used to voltmeters registering about 12.5 to 12.7. Mine is now consistently at about 13.5. Do I have a problem or is that still within acceptable limits?

 

As I understand it, the voltmeter on a C7 reads the voltage coming from the alternator if the engine is running. It will show higher voltage when under higher loads, like recharging the battery after starting or powering a lot of headlights/AC fan/etc. Usually, the voltage reading will be higher just after engine start and then decrease over time as the batter charges up.

If you stay off the clutch/brake and then hold the start button for several seconds, the car will go into Accessory and then into a Diagnostic mode which powers everything as if the engine were running. That should show you the battery voltage under that particular combination of loads. But I don't think you can read battery voltage unloaded, unless you use a separate voltmeter.

Standing by for corrections from the experts...

 

Thanks for the info. I’m registering 13.5 after reaching normal operating temps and at both highway speeds and on city streets.

 

It will vary depending upon state of charge, operating conditions, ambient temperature, among other factors. Your new battery wouldn't have been fully charged so the car will provide increased voltage until it reaches a full charge then it will tail off. The volt meter is going to vary a LOT because the charge algorithm is designed to maximize fuel economy and battery life. Under high ambient temperature, the charge rate is decreased and when you are coasting the charge rate will increase to avoid wasting energy; this is sort of a mini version of the regenerative braking/charging done by electric vehicles.

Higher voltage indicates the system is trying to charge the battery at a higher rate, the greater the voltage differential between the alternator output voltage and the static (no charge) battery voltage, the greater the charge rate. Once it tails off to 13 volts the charge rate is effectively a trickle charge for this AGM battery and at 12.6 volts it is actually discharging a little as the ECM is maximizing economy. If it drops too much, you will see the voltage suddenly increase for a time as the ECM commands a higher charge rate.

Normal voltage will be between the low 12 volt range and the low to mid 15 volt range depending upon operating conditions and state of charge. I replaced the AGM battery in my 2014 ATS over the weekend after I noticed the original was staying at 15.3 volts charging for an extended period of time which was a good sign the battery was on its way out. The new one ramped up to 15.2 volts for the first 15 minutes of operation and after an hour of operation it is now acting normally and spends most of its time in the low 13 volt range. AGM batteries at full charge have slightly higher voltage than traditional "open" type batteries and will be around 12.9 volts.

On edit, I can't remember if the LT1 and LT4 have a required idle relearn procedure after a battery replacement, might check your manual. If so it is something easily done at home (ignition on for 3 minutes without starting, then off for 10 seconds, then start and operate engine normally for at least a few minutes). If an idle relearn procedure is needed and not followed, you will end up with more than the usual rolling LT1/LT4 idle and it may trip a CEL. The window indexing should also be checked, again a simple procedure and it is in the owner's manual. Enjoy your new car

 

Wow, thanks for the help and the education. I’m going to check my manual and if necessary follow your instructions for the relearning.

It’s nice to have a community like this for help and advice.

 

You are very welcome!

 

Idle relearn isn't a big deal. You can get the same thing by just driving the vehicle for a few miles if the power has been disconnected and reconnected.

Bill

 

I don't believe the LT1 and LT4 require the "special procedure" and like Bill said will relearn quickly. The 3.6L in my 2014 Cadillac ATS does require a relearn immediately after the power is removed and reapplied or it will set a CEL for intake air related performance within the next few drive cycles. The ATS specifies the requirement in the owner's manual while my Z06 manual only refers to re-indexing the windows after power is removed.

 

In reading the manual and researching online it appears you are both right and relearning is not necessary on my car.