Need Answer Fast...Car Charging Question!

Factual Questions
21

Concur. A crappy one, like almost all modern gauges. It should have a happy face on one side and a sad face on the other.

The car running badly likely had nothing to do with the battery condition, unless in trying to charge it the alternator was remaining in full-output mode and thus pulling off a maximum horsepower drain while the engine’s computerized electricals were disturbed enough to run poorly. It might have been in limp-home mode.

The charging system will create and try to maintain about 13V no matter what the condition of the battery. If you know how to read a voltmeter, it will tell you everything there is to know about your charging and electrical system.

Both, besides not having to run high-current and potentially dangerous wiring into the dash tangle.

But the real reason is that an ammeter (which was easier to make, and make robust, in the early days) is a very limited instrument. A voltmeter tells you standing voltage, starting voltage and cranking voltage… and by knowing the norms for those, you can be alerted to almost any electrical system problem and diagnose fuzzier ones. An ammeter tells you little more than whether or not your alternator is working.

For most modern cars:

[ul]
[li]Standing voltage (key off) should be right at 12.0V. If you turn on a heavy load like the headlights, it shouldn’t drop more than maybe half a volt. More, and your battery is likely going bad. If headlights make the battery decline visibly in 2-3 minutes, you definitely have a failing battery.[/li][li]Cranking voltage (starter engaged) shouldn’t drop below about 8-9V. It might, on big engines and in cold weather. Lower, and your battery charge is down.[/li][li]Running voltage should stay within a short range of about 13.5-13.8V. Turning on headlights and the AC blower and other high-draw items might pull it down a little, but any lower and you probably have a bad alternator, regulator or wiring. Any higher, you have a bad regulator and it’s going to burn up your battery.[/li][/ul]
An ammeter tells you whether current is going into the battery or coming out, which is about an RCH from useless info.

22

Your numbers are totally 100% wrong.
More later when I’m not on my phone.

23

Next time you buy a car, get one with a manual transmission.

You’ll never get stuck in the middle of nowhere (or anywhere) with a dead battery.

24

unless it’s uphill both ways from where your car died.

25

Yeah, you came at me on this before, Rick. Sorry I’m only speaking from about 30 years of automotive experience, much of it spent in electrical/electronic design for mobile purposes and the rest spent with wrenches.

How about you put up or shut up this time?

26

Reminds me of my final semester in school when I couldn’t afford to replace my dead battery so I always parked with the car pointed down an incline if possible. A few times I had to park on relatively level ground and push start it. Good times.

27

I used to know every hilly parking lot around. My test hill (to answer the question, “Is the damn battery charged up enough yet?”) was a church with what must have been a 300-foot downslope. I knew I could swing in there, position at the top of the hill, and safely shut the engine off. Fire up? No problem. No start? Much cursing in the Lord’s driveway, but one clutch-pop later I was on my way home.

28

OK, you asked for it.
Since you want to have a dick measuring contest, before I show you just how and why you are wrong, excuse me while I whip this out.
46 years in the business as a wrench, shop foreman, parts monkey, technical instructor for both Sun Electric and a car company, service writer, and currently a service manager. ASE mater Automotive technician. While working for the car company as an instructor, I edited the Starting and Charging Systems textbook for use in teaching technicians.
Yeah, I’m the guy that wrote the book. :smiley:
Remember you asked for it…

Tell you what, why don’t you wander out to your garage grab a voltmeter and measure the voltage on the battery of your daily driver, and report back? I have a crispy $5 bill in my pocket that is above 12.0V
Go ahead, I’ll wait.

Let’s start with your statements:

how about all modern cars with a 12V system and an ICE engine.

A wet cell in a battery generates 2.12 volts open circuit when fully charged. There are six such cells in an automotive battery 6 X 2.12V = 12.72V. It does not take an EE to realize that 12.0V does not equal 12.72 V. It is generally accepted that anything above 12.6 (ignoring surface charge) is considered fully charged. cite Here is a consumer site that says “the voltage should be about 12.7V.”
Furthermore open circuit voltage drops as the state of charge drops. it looks something like this:
12.6V+ = fully charged
12.40V-12.45V = 75% charged (depends on the cite you read)
12.20V-12.25V = 50% charged (depends on the cite you read)
12.00V = 25% charged.
11.80V and below = 0% charged.
Cite for the above chart Here is a Wiki link that says that 11.7V is fully discharged. ::: shrug::: if you want to call 11.8V not dead, that is fine by me, but trust me at 11.8V you are in Reebok mode. (you are walking)

The computers on modern cars start shutting down if the supply voltage drops to around the 9V range. Different modules will shut down at different voltages even in the same vehicle. By the time you hit 8V supply I am unaware of any ECM that will still be up and functional. A short read of the Bosch Blue Automotive book will verify this for Bosch control modules. Mine is out on loan so I can’t quote you exact numbers.
As far real world numbers go I would like to see 10.5V or above, but anything above 10.0V I’m happy with. Here is a video of a guy torture testing some Motorcycle batteries with meters attached. Look at the left hand meter, the second line minimum voltage measured. at the beginning it is around 10.5V but starts to go down after repeated start attempts. As it approaches 9V watch the electronic gauges on the bike, they start to freak out due a lack of voltage. (BTW, note the voltage before he starts to crank the engine, it is NOT 12.0V)

So close, yet so wrong.
Batteries have an internal resistance. That works out to .2V/cell or 1.2V for the entire battery. Your generator/alternator/battery charger has to run 1.2V above the open circuit voltage of the battery before any electrons will flow into the battery.*
So 2.12V X 6 = 12.72 + 1.2V = 13.92V. (the wiki link above gives a float charge number of 2.32V which would be 2.12V + .2V)
Pop quiz is 13.92 any higher than 13.8V?
Furthermore there is a thing called an equalizing charge where the charging system goes above 13.92V in order to equalize the charge between the cells of the battery. to do the equalization the system runs at 14.5-15.0V for a short period of time. Again look at the Wiki link for verification.

::: Rick wanders away singing.:::

29

Jesus Christ on a pogo stick, Rick - for one thing, I could have written every word you did. This is not an ASE class nor a theoretical engineering class or even a specific approach to a 2012 Chevy Cruze with the 1.4T… yes, you can always go into endless layers of specific detail for an audience that needs the info or when you’re testing a specific vehicle.

This thread and my post is for a guy who isn’t even quite sure what gauge his car has. My answer is for the vast sweep of people with cars with nominally 12V electrical systems. And however far you want to drag the numbers into added decimal places and special conditions and the nuances a trained mechanic can infer from 0.02V variations, my numbers are as correct, accurate and useful as anyone reading this post who is going to go out and stick a multimeter across their car battery terminal needs.

So I flunk your ASE exam, Professor… but you are way the fuck out of line to drop in from your phone to tell me I am “100% absolutely wrong.” I am 100% absolutely right as far as it matters for 99% of the people who are ever going to look at their car electrical system.

It’s all about context, cap.

30

See that is the neat thing about technical stuff, either the answer is right, or it isn’t. Mine are, yours aren’t. This message board is dedicated to fighting ignorance, your posts on the other hand seem to dedicated to preserving it.

Don’t forget you are the one that said

You asked, no make that begged me to prove you wrong. No problem, I am happy to oblige. If you don’t want to get smacked around for posting incorrect bullshit in GQ, it is really easy. Don’t post incorrect bullshit and try to pass it off as facts.
Misstating the open circuit voltage by 75% is NOT 100% accuracy in any automotive subject I am aware of. That is like setting the timing on an engine at 75 degrees BTDC instead of 10. Or the bolt broke when I tried to torque it to 75 ft. Lbs instead of the 10 the book called for. No shit.
Picture this:
Guy posts that his car won’t start. You post that incorrect crap you posted above.
He checks his battery it reads a tick above 12.0V. According to you, good. Car won’t start. He checks the voltage at crank 8.5V. Again according to you good. He discovers he has no spark. He buys a brand new ECM for $$$ because according to you his battery is good. Car still won’t start. The actual problem is a dead cell in his battery causing low voltage during crank and the ECM shut down to due to low voltage.
On the other hand I would tell him 12.6+, 12.4,12.2,12.0 for the various states of charge and he would be able to see that it is probably a battery.
When J. Q. Public can buy a DVOM for about $12. at Harbor Freight there is no excuse not to be accurate with voltage readings.
It’s not about context, it is about accuracy.

31

No, they’re not. Not at all.

Standing voltage (key off) should be right at 12.0V.
Wrong, just wrong. 12.0 is significantly low. 12.6 is the almost universally stated nominal rest voltage. Why you think it’s helpful to say 12.0 (which is wrong) in place of 12.6 (which is right) is just unfathomable.

Cranking voltage (starter engaged) shouldn’t drop below about 8-9V.
That’s just way too low. Any less than 9.5 represents a failure, and less than 10.5 is suspect.

Running voltage should stay within a short range of about 13.5-13.8V.
Again too low. 14.0-14.5 is real world ideal. If it’s in that 13.5-13.8 range, that’s a problem.

You seem to think you’ve simplified things for the layman, but you haven’t simplified anything at all. You’ve simply provided wrong information.

I don’t know what your 30 years of experience entailed, but it doesn’t sound like it’s worth much if you think rest voltage of 12.0 and charging voltage of 13.5 are acceptable.

32

Jesus Christ guys, take it easy. I wasn’t trying to inspire a war here.

Can anyone answer the quoted question in my post please? How this scenario I describe is possible, etc?

33

Most cars these days rely on the battery to do the bulk of the filtering and voltage regulation. If the battery is shot, then not only does it not start the car, but it also doesn’t regulate the voltage very well either. The voltage coming off of the alternator isn’t a very “clean” DC voltage. Each coil in the alternator produces alternating current (that’s why it’s called an alternator and not a generator) which gets rectified. This produces a very bumpy DC voltage. The battery filters out all of the bumps and makes it smooth.

Anything designed to work in a car has to be able to handle a certain amount of voltage fluctuations and noise or it won’t last very long, but if the voltage gets too far out of whack then things like the engine computer aren’t going to operate properly, especially since not only is the voltage that powers the computer varying too much but also the voltage that goes through the sensors is varying wildly and making all of the sensors ready funny. It’s no surprise that your car wasn’t running very well.

The battery meter is just a voltmeter, and it doesn’t move very quickly so it completely filters out the rapid changes from the alternator voltage. It gives you more of an average voltage reading, which is why in your case it looked ok on the meter even though the electrical power in your car was definitely misbehaving.

34

I suspect that on startup the voltage dropped to the point that the ECM did not have an adequate supply and the car went into a pre-programmed limp mode, set a code and lit the check engine light.
Replacing the battery did 2 things

  1. Erased the code when the battery was disconnected
  2. Cured the cause with the new battery.