When the DIC reads battery voltage while I'm driving, is it reading battery voltage or incoming voltage from alternator? And, if the alternator were bad what sort of reading would I expect?

Thanks

 

It reads battery voltage with ignition on, engine off. When engine is running, it reads generator voltage or battery voltage, whichever component has the higher potential difference (which is normaly the generator voltage).

 

So, if I'm seeing 14.4 - 14.5 while running I can assume alternator is working OK?

 

Yes, that's normal.

I believe the owner manual gives 14.5 as nominal expected reading. However, since mine was about a month old, it has read just slightly under 14.5 with the engine running. If the guage gets down around 13.9-14, and doesn't recover quickly, you have a problem developing.

 

The alternator wire and battery wire are directly connected, so the voltage is the same. Anything over 13 is fine, although <14 is normal for a healthy charging system and battery. Voltage will drop to around 12 when the charging system isn't working.

 

Just a FYI. The DIC displays the voltage reported to it by the ECM. So the voltage you see displayed is the voltage seen at the ECM, not the voltage at the dashboard. This may be useful to know when troubleshooting an electrical problem.

The alternator is connected in parallel with the battery, so the reading you see will be the higher of the two. When in the ignition on/engine off mode (hold accessory for 10 seconds), the voltage displayed will be battery voltage. A fully charged battery should read 12.6 volts, a completely discharged battery should read 10.5 volts. A dead or defective battery will read less, but the computers will go nuts at lower voltages so the DIC should not be trusted when a defective battery is suspected.

With the engine running, if the alternator is working properly, the voltage should be higher, up to 14.4 volts when it is charging heavily. When the battery is near full charge, the reading should be near 13.8 volts. The reading should always be greater than 12.6 volts. If it is lower, there's an electrical problem.

 

With the engine off and the accessory mode on the battery is being drawn down because the accessory mode is on. The normal votage at that time is around 11.9 volts. I tried it and someone else on the forum also tried it and we both had the same reading.

 

That's not normal. Battery voltage should not sag that much under just accessory load. Either there is a wiring problem (high resistance in the wiring between battery and ECM, most likely a ground), or the battery isn't fully charged, or is approaching end of life.

When a lead-acid battery is fresh off of a shop charger (or a long drive in a properly working car), and read with a high impedance voltmeter, it will read 13.8 volts. This is called the surface charge voltage. As soon as a load is placed on the battery, ie an amp or so draw, the surface charge effect disappears and the voltage should show the normal 12.6 volts of a fully charged battery. It should hold this voltage up to about a 20 amp draw. Beyond a 20 amp draw, ie under starting load for example, the voltage may sag a bit, but still shouldn't drop as low as 11.9 volts. A sharp voltage drop under a light load indicates the battery (or wiring) has a high resistance. A battery with high internal resistance is approaching failure. Bad wiring is just bad wiring.

PS, there is a third possibility. The calibration of the ECM may be off. Check this two ways. First read the battery voltage directly at the battery terminals with a good meter. The reading should match the DIC reading. If it doesn't, there may be a calibration problem. Second, turn on the headlights and see if the voltage reported by the DIC changes. It shouldn't. If it does, and the reading directly at the battery terminals doesn't, then you have a wiring problem. If the reading at the battery changes too, you have a battery with high internal resistance, ie near end of life.

 

I just checked it again only this time I used my volt meter across the battery at the same time. With engine cold the the battery read 12.6 volts (fully charged). I then turned on the accessory mode and turned off the radio, NAV, backup lights and head lights. The reading on the DIC was 12.0 volts and the volt meter was 12.1 volts. I consider this normal. I have driven the car 7500 miles with no problems, no DBS and never had a sign of a low battery. At least one other member reported the same readings.

 

Today i had problem if i will shift the gear to N the battery
drob to 12.5 and when i start driving it goes to 13.9 to 14.4
and some times it just goes crezy 12.9 to 14.0
i will take it to the dealer.

 

A tenth of a volt difference between what you read at the battery terminals and what the DIC reports is acceptable, digital displays are only accurate to +/- one count in the least significant digit. But a change in reading from 12.6 to 12.1 volts at the battery terminals with minimal accessory load is not acceptable.

As counterpoint, my battery, after 22,847 miles, shows a drop of only 0.18 volt at the battery terminals under the same test conditions (12.58 no load, 12.40 in ignition on/engine off mode). I just did this using a recently calibrated Beckman meter. DIC reports 12.5, but as I said, +/- one count in the least significant digit is to be expected.

A good lead-acid starting battery should have an internal resistance at least 3 or 4 times lower than could account for the voltage drop you're seeing. At a 100 amp cranking load, that much internal resistance would drop the voltage to around 9 volts, and that's near the threshold where the computers start to go nuts.

I'm going to go out on a limb and guess your car, and that of the other member, are not daily drivers (I drive 60 miles per day). I suspect your battery is partially sulfated. But it is possible that GM is putting in crappy batteries, and I was just lucky enough to get a good one.

 

Good information. Your right, I drive 3 miles to work and 3 back each day. I put the charger on once and awhile but it goes to green light (fully charged ) within 10 minutes. Sometimes less. I drove it home from the museum last April (2100 miles) and in June I will take it to Oregon.

 

Just keep in mind, just because you have voltage, doesn't mean you have amperage. So the alternator could be bad.

 

If you have voltage, the current through any given load R across which the voltage appears will be I=E/R, where I is current in amperes, E is voltage in volts, and R is load resistance in ohms. It isn't just a good idea, it is the law, Ohm's Law.

In addition to Ohm's famous Law, there are also Kirchhoff's laws which say that the current through any point in a series mesh (circuit) is the same as the current through any other point in the mesh. This is due to the physics requirement of conservation of charge, ie electrical charges can neither be created nor destroyed.

Kirchhoff's voltage law says that the sum of voltage drops around a circuit (paying close attention to sign) must sum to zero. In other words, if you have a voltage source (negative voltage drop) and a load (positive voltage drop) in series, the voltages must sum to zero. Applying Ohm's law, we then find that current through the circuit will be proportional to either the source voltage or the load voltage drop (which Kirchhoff just told us must be equal).

 

Thanks shopdog. My battery is a brand new red top, but car is not daily driver. But since I work at home, neither is my Range and it NEVER drops dead on me. I'll keep checking and see what I find after a week.

 

The issue isn't the battery it's the car and driving paterns of the owner.

The company that makes the Corvette battery makes over 70% of all batteries that go into vehicles made in North America.

Here is an expample of vehicle dynamics and there impact. The XLR is supposed to compete with the MB SL500. Both have retractable tops that strain the electrical system. Mercedes puts an 70 amp AGM battery in the trunk and a 35 amp starting battery under the hood. GM puts the same battery in the XLR as the Solstice. Do ya think that's an issue?

 

As I mentioned above, brand new Red Top, regardless how much I drive the car -- or even top up with charger -- Priority Start will have intervened by morning. Less than 24 hours. I noticed when recharging, if I have activated battery using lighter etc. with Priority Start, THEN charge, takes significantly longer to "top off" then if I charge with Priority Start keeping battery deactivated. This problem did not occur until Priority Start attached. Naturally there could be a new "draw" on my system, which I am checking for. But would have to be a heck of a draw.

 

We make the Red Top and it will sit for almost a year and still be at about 12.45, so if you are seeing a drop in 24 hours, there is something wrong. I did not buy an 05 because of "issues" but those issue should not happen on an 06.

I put a Yellow Top in my car only because it is a Yellow car. A yellow can replace a red, but a red top can n ot replacve a yellow.

 

Car in question is '05. What I'm suggesting, and is possibly supported by at least one other post, is that the problem is the Priority Start. Either kicking in way too soon, or even causing the draw. I had one possible DBS with factory battery, charged it and had no other trouble (charged very fast). Later determined battery was faulty. Replaced with Red Top and put in Priority Start as a precaution against valet parking, etc. Have had Priority Start kick in ever since.

 

If you keep discharging the OE battery, it will die becasue it wasn't designed to be deeply discharged and recharged repeatedly. The plates just fall apart when ypu take the battery apart. The Optima is more forgiving to that type of abuse.