Electrical/Charging Issue

[JoeZ78]

Hi everyone, I need some help. My issue started on 8/19 when my wife told me that her 2016 Tahoe struggled to start when leaving work. When she got home, I tried to start it to troubleshoot, but it was completely dead. The battery was almost 5 yrs old, so I purchased a new battery and installed it. Battery cable terminals looked good and I cleaned them before installing new battery. It fired right up, no drag, and everything operated normally (no messages or lights). On 9/17 (just under a month with new battery), my wife texted me and said it struggled to start again when leaving work. When she got home, I got about 3 starts and on the next start attempt it was dead. Put my charger on the battery and it read 14%. I charged the battery and now it starts every time (7 times, then drove it around the neighborhood) with no issues. I am thinking bad alternator, but volt gage on instrument cluster reads just over 14 volts (even when I put a load on- A/C, radio, lights, etc.). Here’s where it gets weird, I put a voltmeter at the positive battery cable and it reads just over 6 volts??? Here are some other notes:

-when battery was low, I would get a “Service Trailer Brake System” message. I believe that is a nuisance message that pops when battery is low and it cleared when battery was at full strength (have not towed anything in a while)

-I did not check the new battery charge before installing it but was driving around long distances for almost a month before issue started again.

-I did not get a chance to check what the other end of the battery cables looks like yet (maybe loose or corrosion would cause this?)

-I know this Tahoe has a Regulated Voltage Control (RVC) system that is separate from the alternator. Do these go bad with no indication?

Any help on this issue would be appreciated. I have a new OEM GM alternator and serpentine belt coming from Amazon this weekend.

 

[Doubeleive]

most auto parts stores will check the alternator for free, that would be the next step unless you are able to check it yourself.
you should have 13.4v at the battery terminals with the engine running.
if the Alternator is possibly being intermittent then you should check it multiples times
like check it with a cold start then go drive it and warm it up and check again, shut it off and restart it and check again, drive some more check it again. etc.

 

[Rygrego]

These trucks use an alternator that has decoupling belt pulley and will only charge when the ECM commands different rates of charge due to needs.

 

[OR VietVet]

Does sound like intermittent charging issue but would also check the other ends of cables for tight and cleanliness.

 

[B-train]

These trucks use an alternator that has decoupling belt pulley and will only charge when the ECM commands different rates of charge due to needs.

The belt is always spinning, its a standard alternator, no decoupling takes place. If that were the case, there would be an electro-mechanical clutch (like A/C compressor) to activate, deactivate, which it does not have.

The voltage output is controlled via the field excitation commanded to the alternator. The ECM will let the battery float down in voltage to save fuel. When voltage gets to a predetermined threshold the ECM then provides the alternator field voltage to "excite" the windings to make an electro-magnet, thus generating AC power that is then filtered through a diode bridge (rectifier) to spit out the desired DC voltage.

 

[Doubeleive]

These trucks use an alternator that has decoupling belt pulley and will only charge when the ECM commands different rates of charge due to needs.

eh, no
it charges anytime the engine is running for the most part, how much it charges (amperage) is determined by the commanded state, the RVC system has different settings determined by battery temperature and other factors like if the windshield wipers are on, hvac running, heated seats, headlights, defroster, etc
for instance on a long drive cruising at 55 with no accessories on it will or MAY go into economy mode, but it is still maintaining I believe around 12.6v.
if in doubt connect a scanner and monitor voltage & commanded RVC state while driving
if you are driving around and see it showing something like 11.5v (on a meter or scanner) then something is wrong, bad cell in battery, bad alternator, poor ground, etc.

 

[Rygrego]

eh, no
it charges anytime the engine is running for the most part, how much it charges (amperage) is determined by the commanded state, the RVC system has different settings determined by battery temperature and other factors like if the windshield wipers are on, hvac running, heated seats, headlights, defroster, etc
for instance on a long drive cruising at 55 with no accessories on it will or MAY go into economy mode, but it is still maintaining I believe around 12.6v.
if in doubt connect a scanner and monitor voltage & commanded RVC state while driving
if you are driving around and see it showing something like 11.5v (on a meter or scanner) then something is wrong, bad cell in battery, bad alternator, poor ground, etc.

eh, no
it charges anytime the engine is running for the most part, how much it charges (amperage) is determined by the commanded state, the RVC system has different settings determined by battery temperature and other factors like if the windshield wipers are on, hvac running, heated seats, headlights, defroster, etc
for instance on a long drive cruising at 55 with no accessories on it will or MAY go into economy mode, but it is still maintaining I believe around 12.6v.
if in doubt connect a scanner and monitor voltage & commanded RVC state while driving
if you are driving around and see it showing something like 11.5v (on a meter or scanner) then something is wrong, bad cell in battery, bad alternator, poor ground, etc.

Did I not say different rates of charge due to ECM commands? If you feel better with your answer then enjoy. I believe simple is better.

 

[West 1]

I will look at this differently, new battery, when running the alternator is showing 14v. Sounds normal, starting slow after sitting makes me think you may have a battery drain happening?

I do not like the battery charge system that allows voltage to drop down to save Gas but it is part of these trucks. I plugged in a volt meter to my cigarette lighter just to observe voltage more accurately. It is normally 13.7 to 14.2V but in certain conditions the battery voltage drops down to 12.7, I have not seen it dip below that but if you are looking at the dash gauge it drops low enough to make you think the alternator has failed. But it has not, if you have no warning lights it is working normally. At 12.7v you still have a fully charged battery in the car but it might charge you iPhone a little slower. HA. Mine only drops the voltage on long trips I have not noticed this on short trips yet.

 

[Doubeleive]

when you start the vehicle for the first 30 seconds afterwards it commands the alternator to give a full charge around 14.5v in order to replenish the power used to start the engine, then it goes to normal charging 13.4v+, the other various modes depend on the battery temperature & what is turned on or not turned on.
basically unless it is in economy mode you should have 13.4v at the battery terminals
GM considers the electrical system flat lined @10.4v. 10.5v is the bare minimum voltage required
so if a person is troubleshooting the charging system those are your electrical guidelines.
if you take it to a auto parts store and have them test the alternator and it is not putting out at least 13.4v there going to tell you the alternator is bad.
if it is not charging at startup or anytime afterwards for a good period of time then the alternator is bad or the battery is bad or there is a poor electrical connection, rust/corrosion/loose wire, etc between the alternator and battery and ground point.
if there is a short bad enough to make the battery drain in a short period of time then it's possible a wire will be hot (if bad enough). In which case a draw test should be done between the negative battery terminal and negative cable with a multimeter, doing that forces the meter to complete the circuit and you can see the voltage change when you start pulling fuses. once the meter is connected set the meter to dc amps. normal drain is typically 50 milliamps or less.
if you have a phone charger, dashcam, etc be sure it is unplugged. also wait a little while for the vehicle to go into sleep mode, typically that is 10-15 minutes if you haven't messed with anything, even opening the door can wake systems up.

 

[PG01]

Did I not say different rates of charge due to ECM commands? If you feel better with your answer then enjoy. I believe simple is better.

So you’re 1/2 right, thats not simpler, its wrong. You get 1/2 credit.

 

[ac0bl]

I've also had 3, count them, 3 of Walmart best batteries go bad in the last 2 years in my 03 z71. Check all your grounds are clean and tight as well.

 

[West 1]

Batteries die because they are being drained and then rapidly charged far more than normal. As mentioned test your system to see how much your amp drain is with the car fully shut down. As mentioned it takes anywhere from 10 to 30 minutes for all the cars modules to fully shut down. Once shut down your drain should be no more than 50 ma. On my 03 Yukon I had a drain of almost 200 ma fully shut down, ended up finding a problem with the HVAC control in the dash.

On my 07 Escalade it had a 200 + ma drain, ended up needing to replace the underhood fuse box assembly. The drain was found in the lower fuse box itself.

These can be hard to find but you won’t have a long lasting battery with a drain in your system.

With a 200 ma drain it might take 4-8 days before your car would be hard to start if not driven. Park for a week or more and maybe you need jumped to start. A larger drain of course would happen faster.

If your drain is normal and under 50 ma you can park for 2 months and the car will fire right up.

Find your drain and the battery will quit failing.

 

[Fless]

Batteries die because they are being drained and then rapidly charged far more than normal. As mentioned test your system to see how much your amp drain is with the car fully shut down. As mentioned it takes anywhere from 10 to 30 minutes for all the cars modules to fully shut down. Once shut down your drain should be no more than 50 mv. On my 03 Yukon I had a drain of almost 200 mv fully shut down, ended up finding a problem with the HVAC control in the dash.

On my 07 Escalade it had a 200 + mv drain, ended up needing to replace the underhood fuse box assembly. The drain was found in the lower fuse box itself.

These can be hard to find but you won’t have a long lasting battery with a drain in your system.

With a 200 mv drain it might take 4-8 days before your car would be hard to start if not driven. Park for a week or more and maybe you need jumped to start. A larger drain of course would happen faster.

If your drain is normal and under 50 mv you can park for 2 months and the car will fire right up.

Find your drain and the battery will quit failing.

Note that the parasitic drain measurement would be in milliamps ("mA") or if extremely large, in amps. 1 milliamp = 0.001 A.

 

[West 1]

Thanks Fless, I will go back and clean up the post.