Coming home on the interstate last night I noticed that the battery voltage display that I have set up in the DIC had dropped to 12.8-12.9 volts and stayed there. Occasionally at higher RPMs it would momentarily go up to the normal range of 14.4 - 15.0 volts, but would then return to the lower reading. I've never encountered this type of behavior unless there was a charging system problem. When I got home I checked the battery with a VOM and it showed fully charged. I hooked up a CTEK and it went into float mode after about an hour, which would confirm the fully charged indication.
The fact that the voltage reading never showed less than 12.8 volts would indicate the battery was not discharging while running. I would expect with the load of the engine ignition system, headlights, ventilation system, etc., if the alternator was not working, the reading would be in the lower range of 12 volts. But neither have I ever had a car that didn't maintain about 14v +/- 5% while running. I suppose it is possible that GM is doing something different these days and could be varying the voltage output of the alternator to match the needs of the battery as well as the current output. It just doesn't seem likely though.

I'm wondering whether this is common or unique to me. If common, it could explain some of the complaints that have been posted regarding dead or weak batteries. Most people probably don't monitor battery voltage as I do.

Would some of the more inquisitive members of the forum keep an eye on their charging voltage and report back with any unusual behavior? Thanks.

Ray

 

At idle, the alternator doesn't charge the battery....

Also, it takes more than a few minutes of running time (above idle) to replenish what was discharged by cranking.

 

My Silverado does this also. It is an economy thing. When the battery is charged up enough the pcm tells the alternator to stop charging fully. This is good for the battery and the load on the engine.

 

? Wrong.

 

Let me rephrase.

At idle, the alternator doesn't output enough to overcome the total current draw of the vehicle. It might keep up with the draw but generally doesn't provide substantial additional current (i.e. "charge the battery").

Then again, if the voltage is over 12.5 volts, the battery is receiving *some* charge but not much....

 

That may well be, I can't be sure because I don't monitor charging current - only electrical system voltage. That voltage, under normal circumstances, is always greater than 13 volts, even at idle. As I said, I'm not used to seeing under 13 volts especially when traveling at interstate speeds.

The previous post explaining that GM designs the system to shutdown the alternator output to lower levels, when appropriate, makes sense if indeed the case. Most systems do that, but it doesn't affect the alternator voltage, only the current output. Producing the current to charge the battery is what puts a load on the engine, and the current can be reduced without reducing the voltage. That is what I am familiar with.

Ray

 

I have been having battery issues so have been checking the voltage often. When driving mine always reads ~14 volts. Even at idle it is over 13 to 13.5. My battery however drops to 12.3 to 12.4 volts after about 1 day. It stays at that voltage. I have checked my meter with a calibrated Fluke and it matched perfectly. I even purchased the Chevy type charge, a CTEK 3300 and it shows fully charged but after 1 day it is back to the 12.4 volts. I am concerned it may be a bad or low cell. I will have the dealer check it out.

 

I have noticed mine as low as 12.1 at highway speeds. It got my attention too, but then at times it is between 13.5 to 14.1 an area we are all accustomed to. As soon as I have info available through All Data I will study the description and operation of the C7 charging system....

 

Mine does it too. Sometimes reads in the mid 12's other times as high as 15.
The software control of the alternator field for economy makes sense.

 

So here's a little empirical data for you to compare with. I put the CTEK on until it went into float mode. Removed it, and read 13.09 volts. Let the car sit for 24 hours and reread 12.67 volts. Now this is important; I read the voltage under the hood so that there was nothing but the normal parasitic draw on the battery. If you read it with the doors or hatch open, the draw increases because of courtesy lights being on and whatever else happens when systems are woken up. Under such circumstances the battery voltage will read lower. Since your reading stabilizes at 12.4 volts, I doubt anything is wrong.

Ray

 

Hi,
The batteries that are put in cars from the factory are marginal for capacity= cheap as they can get away with.
Lets say the battery has 550 or 600 cold cranking amps=CCA.
That is not enough for the vast electronics in the new car or for sitting around for weeks not being driven.
I install the best die hard battery that will physically fit in the car, my cobra has an 800 CCA rated battery and 7 year guarantee and it sits for long periods without driving. I charge it once a month for 8 hrs and it will last for at least 8 years doing this.
The voltage on a good battery should be 12.6 to 12.9 volts at rest.
If it drops to 12.3 there is a problem, high resistance or chemical buildup and should be replaced. If the battery has run down more than once it will never be trustworthy again and should be replaced, there are too many things to power in a new car. Some things like the ignition won't even get power from the computer if the voltage drops below 12 volts.
Then there is the alternator, I would not stress the 1000 dollar alternator trying to charge a low battery and may overheat it.
Bottom line is, replace the battery with a higher capacity one for peace of mind.
Perry.

 

I agree 100 pct...Honda is really bad - they put a 410cca batt in a big car like an accord!

I'm looking at a marine pc1500 odyssey and seeing if it will fit in the C7... It has std group 24 size specs....880 CCA......

 

Thanks Ray. Great data. I think you are correct, I measured while in the car and the door was open! I made an adapter to plug into the auxiliary plug in the console so I could measure voltage. I pressed the start button, without pressing the clutch to get power to the plug, so I was drawing some current. I believe you’re correct the voltage dropped because of that. It’s tough being so cold and rainy and not being able to drive the Vette, too much time to think! Dispite the ice and snow we had in eastern SC, it will be 65 this weekend so I'll forget the voltage and drive!!

 

Perry

Cold Cranking Amps are not a measure of the reserve capacity of the battery. The Amp-Hour rating is the measure of that. Typically, these batteries have a greater than 70 ah rating. That means that you can draw 1 amp for 70 hours, 70 amps for 1 hour, or any combination thereof up to the max amount the battery can chemically and physically deliver. CCA is important and must be greater than the engine needs to turn the starter and start the car in the winter and in the summer. A hot engine can need more power to start than a cold engine, and the CCA decreases with temperature. All of which I would expect the engineers to take into consideration when a battery is specified during the design process. The quality of the battery that is installed is a different matter, but the specification of the battery should be sufficient to meet the needs of the electrical system in the market where the car is sold. You probably know this already, but some readers may not.

Ray

 

Jerry

Enjoy the car, the battery will be fine. Now that I know you also had the ignition on, I'm quite certain the reading you got was normal.

Ray

 

Thanks Ray, Will do. I am going into the dealer next week to have them check the diff fluid, at GM's request (although with 1300 miles and no problem, doubt they will find any issue.) I'll also have them update the dash dimmer and HVAC software. I was going to ask them to check the battery but now will not bother. I really don’t want them to remove the battery and possibly mess something up in the process! Since I have no problems now, why ask for some! I even am concerned that they won’t use the jack pads I made so will ask that they use a drive on ramp. But in the past if one wasn’t available they will use whatever is!

 

Ray,
You cannot draw 1 amp for 70 hours on a battery.
Leave the inside light on overnight for 9 hours and most batteries will not crank the engine, even though the light draws milliamps. The reserve is just not there in lower CCA batteries, I use CCA as that is the only practical way to reference a car battery when you look at the longevity of a battery in the great white north
I worked in the telephone industry for 33 years, it runs off DC voltage and we serviced the rows of batteries and V16 twin turbo generators. You would be surprised how little resistance or corrosion it takes to stop a device from working.
Most of the troubles are actually caused by a bad ground connection. Funny thing EH, an aluminum frame that corrodes an can act as an electrode to slowly drain the battery voltage if any insulation isn't perfect.
P