Truck is a 2014 Silverado 1500 LTZ, with a new battery, & 71k miles. I noticed my ammeter reads 14v at start up, but while driving it comes down to 12ish. Then comes back to 14 for a time, et cetera, et cetera.

I left it running when I got home & popped the hood to check it with a multi-meter. While running it reads 12.6 at the battery. Then I shut it off & again 12.6 at the battery.

Start it back up & it reads 14.6 at the battery.

Is this normal & I just never noticed it? Or is my alternator (voltage regulator) taking a crap?

Thanks in advance for any insights.

Totally normal. Modern automotive electrical systems draw from the battery which is charged by the alternator. The days of running with a dead battery, but functional alternator are almost over. The battery provides more consistent power without the constant fluctuations of an engine-driven alternator. Modern electric systems are also capable of load shedding for greater efficiency and decreased parasitic loss.

Thanks for easing my mind. I guess I never really paid attention to it before. A premature battery failure this week at 18 months (thankfully in Sam's parking lot) got me wondering about the charging system.

Totally normal. I have a 2010 Silverado and depending on the battery needs, It charges over 14 volts and later will drop below 12 volts. It save the battery from too much charging.

I'm almost certain your alternator has a decoupler pulley. Those pulleys will fail. My daughters camaro (2016) was showing different voltages like you discribe. About a month later alternator died, so I thought. Turned out to be the "decoupling pulley" failed.

Had to buy a new alternator because no pulleys were available local. Have to order online and you need to rent a special tool set to change those pulleys.

Most auto manufacturers are using "decoupling alternator pulleys now."

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I don't think its normal at all. I know nothing of the tech in a 2010 Silverado and there are always special cases in life, but I have never met any charging system that lets the voltage drop to point the battery is actually providing output while running. You should see a minimum of 13 volts at all times. I'd vote your voltage regulator is on its way out.

^^^ Thanks. I'll have a guy look at it. I can't take a chance it will take a shit when I'm up at the cabin.

running should show close to 14v.

the alternator light goes out when voltage is 1/2v above batt voltage.

As a side note, I'm sure you got all the Texas members' attention with the proper use of "fixin'"...

I do not own Chevys, but have rented a few, including a couple of new suburbans. They BOTH did this.

I think it is a feature, and I believe its purpose is to save energy. Periodically, when there is a light electrical load, the voltmeter goes to 12 Volts when it was normally close to 14. I think the electrical system "removes" the load from the alternator periodically, and lets everything run on battery only. After doing this for awhile, the system "reconnects" the load to the alternator so the battery does not get too far discharged. During the time the alternator is "disconnected", it presents very little load on the engine. When it "reconnects" the alternator, it does not take long for the battery to charge back up. Overall, I think this saves gas.

Perhaps someone else can explain this more eloquently than I, but I think this is what's happening.

Most "ammeters" are sampling two sources, one is the battery state, and the other is the alternator output and demand on electrical system.

The difference "swings the needle", unlike the old days an "ammeter" was in the path between battery and generator. Alternators do not "read" the same way as an ammeter is only reading the charge rate from alternator to battery and typically indicates high.

In some cases, a "sensitive" mechanical regulator will cycle rapidly and you get flickering lights, fluttering needle, based on the timing of the relay contacts cycling.

Most modern cars have Voltmeter and that is preferred to monitoring an alternator than an ammeter.

Some people (older folks) "want to see" and ammeter and think they know how to understand what they are reading, but most have no idea of the "why" and ammeter and what the readings indicate.

And why generators went away with curb feelers.

The rate required to properly charge and maintain a battery as well as power accessories precludes any "disconnect" of alternator and battery. The battery is a large capacitor and also acts as an "electrical noise filter" and critical to the electronics proper functioning. Lower charge rate, by regulator is normal, but too high a discharge rate on non-deep state batteries is not a good practice.

There may be exceptions. New technologies everyday, and all.

First, it's normal. A chip controlled voltage regulator jumps in and out like that.

Idling the vehicle does very little, it actually runs the battery down. The alternator needs to spin over 1800 rpm to charge and idle is 1000 rpm too low. A low battery may need to charge quite some time at 1800 - alternators aren't the best choice for it. If it's a 800 CCA battery at 25% charge it will need 600 amps put back in, and a 100 amp alternator at max would take 6 hours. Most don't tolerate high amp charging - max is 40 amps. They need the same treatment we do - prevention rather than intervention.

A battery charger is preferred, lower amp rates necessary.

As for "will it finally bite me?" the better answer is "how many miles on it?" Most get replaced from 90 to 150k - the lifespan improves over the years with newer ones in better makes. Therefore - how old is the alternator, and secondly, why do so many of us wait for catastrophic failure rather than a planned replacement cooperative with a budget and work schedule?

It's called preventive maintenance. IE jerk the junk out at your convenience not some worn bushing or blown diode. If it's over 120k consider doing it sooner than later at your convenience. Or, wait until you are halfway up in the hills, raining, 30 miles from a parts house or mechanic.

First option is always better. Last option is the most practiced and it really doesn't save money at all. Tow truck fees, inconvenience, etc.

My 2013 Chevy Silverado always read around 14 v.

quote:

Originally posted by Tirod:
First, it's normal. A chip controlled voltage regulator jumps in and out like that.

Idling the vehicle does very little, it actually runs the battery down. The alternator needs to spin over 1800 rpm to charge and idle is 1000 rpm too low. A low battery may need to charge quite some time at 1800 - alternators aren't the best choice for it. If it's a 800 CCA battery at 25% charge it will need 600 amps put back in, and a 100 amp alternator at max would take 6 hours. Most don't tolerate high amp charging - max is 40 amps. They need the same treatment we do - prevention rather than intervention.

Battery charging rates are based on a battery’s amp/hour capacity and unrelated to the battery’s CCA. There’s not a single, non-EV car battery that requires 600 amps to charge. The pair of batteries in my truck are rated at over 1,800CCA. They don’t need 180 hours on a 10 amp charger to charge. That is just plain ridiculous. On the other hand, they are rated at 150 amp/hours. If discharged to 25%, then they require a little over 13 hours on a 10 amp charger to be fully charged.

What you said about alternators is sort of true. There’s a minimum speed they need to turn to make electricity and it is about 2,000rpm. But the engine does not need to turn 2,000rpm because you’ve neglected the difference the relative sizes of the alternator’s pulley and the crankshaft’s pulley. With a gas engine spinning 750rpm at idle, the alternator is spinning about 1,500rpm. That’s only 500rpm low, so while it’s true that a car won’t charge it’s battery at idle, it will under normal driving conditions.

The charging voltage is dependent on electrical load, not maintaining constant voltage, on these trucks. When the electrical demand is low (for example, idling without A/C or lights on), the alternator will charge at a lower rate. There should be a ring with wires connected to it encircling the battery negative cable. This is an inductive current sensor. If there is an actual failure in the charging system, a message will appear on the dash to tell you.

I appreciate the info all y'all. I guess I'ma gonna ride it out.

quote:

Battery charging rates are based on a battery’s amp/hour capacity and unrelated to the battery’s CCA. There’s not a single, non-EV car battery that requires 600 amps to charge. The pair of batteries in my truck are rated at over 1,800CCA. They don’t need 180 hours on a 10 amp charger to charge. That is just plain ridiculous. On the other hand, they are rated at 150 amp/hours. If discharged to 25%, then they require a little over 13 hours on a 10 amp charger to be fully charged.

Its common working with batteries at auto parts stores to take the CCA, divide by the charger amp rate, to get the number of hours needed to charge from dead flat to full.

The battery factories use the same procedure. A 850 CCA battery at ten amp charge rate takes 85 hours. Most go slower and in the 70's alloted five days charging after manufacture, per the tour I took at the plant in KC. That formula hasn't changed for me - dead flat to charged takes days, not hours.

Asc Auto Tech, 12 years selling parts and batteries. Ive done hundreds over the years. Im simply explaining what industry standards are. having worked in the industry. Since your example is a truck battery, and each one is 900 CCA, 25% of that is 675, at ten amps a full charge would take about 67 hours max. The common practice is to get them to 85% and leave the system to finish. They could be pushed to 20 or 30 - a computerized charger will do that, but if they start exceeding a certain temp level the infrared sensor will throttle it back. The last few hours on a charger are what does most of the work, not the initial, as driving the chemical reaction into the deeper recesses of the plates is where the long term power is stored, not the flash charge on the exterior.

I've sold my share of truck batteries and batteries in general, and the most abusive practice is to hot charge them with a high amp charger, which will shorten their life by half or less if done just half a dozen times. The industry constantly discusses this issue, and why commercial warranties are for 90 days, not 6 years. Getting a load delivered in time forces some expedient measures and since a lot of equipment isn't owner operated nobody bothers to handle it correctly. Telling a supervisor "It needs longer to charge" isn't within their wheelhouse - delivering on time is the goal, not proper maintenance.

Its VERY common for some to just buy new batteries and get the load moving. Those old batts are taken to the charger, and once up, resold as used, for profit. Some shops and stores make a nice bit of change off of it. This time of year a lot of construction equipment is being fired up over the long winter, no maintainer or even a solar cell on the batts. It's diesel battery season with strong sales right now. Current prices are about 25% higher than what they were two years ago. I expect battery maintenance to increase and abuse to taper off.

My GP 65 is now 5 years old. The clock is ticking . . . do I buy a replacement before next winter or just suffer when it won't start? How much hassle do I want to put up with?

OP: Your choice to not worry over the alternator yet, keep in mind doing it when you schedule it is a lot less expense than being inconvenienced when it does fail. It will all fail eventually.

quote:

Originally posted by Tirod:
Idling the vehicle does very little, it actually runs the battery down. The alternator needs to spin over 1800 rpm to charge and idle is 1000 rpm too low.

I recall alternators working that way, back in the day. In this century, I have always monitored alternator output with an in-dash digital voltmeter (currently using a Blue Sea 1733 with OLED display). I have never seen a modern alternator behave that way, except on my Kubota tractor. Granted my newest vehicle is a 2010, but the alternator jumps up to 14+ volts as soon as I start it and let it idle.