I wasn’t aware the PCM increased RPM when the voltage dropped below a threshold. Interesting.
But yea, the voltage regulator doesn’t even know there’s a battery in the system. It’s simply a closed-loop controller and modulates the alternator’s field current (using PWM) to maintain a constant voltage regardless of current and RPM. As the load increases, the voltage will want to decrease (due to the stator’s internal resistance). The regulator will sense this and increase PWM duty cycle to increase the voltage back to the set point. If the load gets real heavy, the duty cycle may reach 100%. And if the load continues to increase, the alternator has no choice but to let the voltage decrease. (And hence the PCM boosts the RPM, if I understand correctly.)
Every day I learn there is far more stuff I don’t know.
Now I know why I haven’t noticed the traditional dim-headlights-at-idle for so long.
Now I know to use the battery charger (which I have) and not try charging a flat battery with the alternator.
What about a weak battery, one that tries but can’t quite turn over the engine, is that a terrible burden on the alternator?
And on a related note, consider how complex and delicate car batteries have become. A low quality stand-alone charger can kill a battery in a single attempt. Try to use a car battery as a stand-alone power source without understanding exactly how it works and you will end up with a very heavy paperweight. I have tried that.
There are at least 2 types of consumer 12 volt batteries available-quick charge and deep cycle. The former is what is used in cars and takes a charge quickly, but doesn’t like getting discharged very far. The deep cycle (often intended for small boats) takes longer to charge but handles deeper discharges. It has a physically different design-more space under the plates to cope with scale and corrosion from the plates that all batteries produce when charging, among other differences. The extra space means it takes longer for the crud to build up and short out the plates. But eventually they all go.
So the electrical system of a modern car is a very complex and delicate thing. Be careful making any home-brew changes/additions. Think of it like a computer network. Which it is. You would think twice about using your internal computer circuits to power lights or a heater.
Older cars will have a much simpler charging system and a dead battery will put a strain on the alternator. What is worse though is that quickly charging a dead battery will often lead to permanent sulfate crystals in the battery plates, reducing the battery’s capacity. It’s better to charge the plates slowly so that the sulfate crystals will be more likely to break up and separate back into lead and lead oxide on the plates.
Many newer cars have much more sophisticated charging systems, and will charge the battery more like a smart charger than just running current through it like an old fashioned dumb battery charger. They won’t overburden the alternator like the simpler dumb charging systems in older cars, but they will tend to at least attempt to charge the battery fairly quickly. If the battery is very weak, it will be much better for the battery if you plug in a slow charger and let it charge the battery slowly over several hours.
In any event, discharging a modern battery to the point where it can’t quite crank over the engine is very likely to cause some permanent damage to the battery. The longer you leave the battery that deeply discharged, the more likely the loss in capacity will be permanent.
Throughout this thread I had assumed all modern charging systems were essentially the same: a voltage regulator inside the alternator controlled the output voltage to a (fixed) set point voltage. The regulator is a simple, closed-loop controller, and controls the output voltage by varying the duty cycle of the field current using a PWM controller. At most, this simple regulator will increase the set point voltage a little bit when the temperature is low, and decrease it a little bit when the temperature is high.
Well it looks like I am “behind the times.” The above describes how almost all systems worked up until about 15 or so years ago. But as **engineer_comp_geek ** and a few others pointed out, modern charging systems are more sophisticated. The alternator’s output voltage is no longer fixed; a computer determines what the “proper” voltage should be based on a number of factors, including the type of load that is being powered (e.g. windshield wipers).
What I stupidly did was leave the key in the ignition half turned on for a good 24 hours. I get from all the replies that I should use my plug-in battery charger to charge the battery. I can drag the herkin big thing in to the garage tomorrow. To be honest I have always disliked that thing because I am befuddled by the dial. Is the needle on red a good thing? Green good? Bad? What? I never can remember what my son has tried to beat into my head. I worry it will overcharge and explode acid all over so I will put it on trickle charge. For those of you who may be wondering, I’m a live-alone woman on a small acreage who will be 79 in three weeks. So I’m allowed some absent-mindedness, right? I’ve had to learn a lot of mechanical stuff but it doesn’t always stick if I haven’t had to deal with it recently. I appreciate my ignorance fought here!
You don’t have to get it down to stone cold dead to do damage to the battery.
A typical lead-acid car battery will be somewhere around 12.6 volts when fully charged and down around 12 or so when discharged (as far as you want to discharge it in normal use). If you discharge it down below about 11 volts, you’ll damage the battery.
Lead acid batteries have two sets of plates. For each cell, one plate is lead, and the other is lead oxide, and there is a mixture of water and sulfuric acid in between them. As the battery discharges, the sulfuric acid turns into water and the plates both turn into lead sulfate. When you charge the battery, this gets reversed, and you end up with lead and lead oxide plates and the sulfuric acid restored in the acid/water mix.
Lead sulfate is kind of a soft and spongy material, but if the battery discharges too far, the lead sulfate starts to form hard crystals. These crystals won’t break up and turn back into lead and lead oxide, so once the crystals form, you lose battery capacity permanently (there are ways to break up the sulfate crystals and remove the sulfation from batteries, but it’s generally not something that you can do at home).
Discharge the battery so that it’s stone cold dead, and both plates are going to be pretty well sulfated with hard crystals, and the battery will no longer charge at all. But you can have hard sulfate crystals and permanent loss in battery capacity long before then. The car might still crank and turn over with a certain amount of battery damage, so it might not be as noticeable unless you get the battery’s capacity tested, but the damage will be there if the battery is discharged too far.
