You're right, I posted a lot of generalizations, many of which likely don't apply to your situation. Other people will come across this conversation while researching similar problems, so we want to be as thorough as possible and include as much related information as possible..
First let me go into more detail about batteries. When they are new, they do not need and do not benefit from chemical treatments around their posts and cables. That includes spray-on chemicals and the "juicy rings" that go around those posts. There's nothing for them to do, so they represent money wasted.
As batteries age, the lead in the plates gradually flakes off over time, floats to the bottom of the case where it collects and builds up. Every battery manufacturer knows that is going to happen, how fast it happens, and that it can't be avoided. Once that lead builds up high enough, it will short the plates in that cell together, then you'll have that dreaded slow cranking or the loud rapid buzzing sound with a failure to crank. Yesterday the battery worked okay, and today it has to be replaced. Most battery manufacturers provide a warranty that's just a couple of months longer than when they expect that battery to fail. Longer warranties can command a higher selling price.
As one of the electrical experts at the dealership I worked at, I noticed that a battery that was about to fail within the next six months did have corrosion build-up around the posts and cables. What I learned later was as that lead flakes off the plates, the generator and charging system is still pumping the same amount of current into the battery, but there's less and less lead to absorb those electrons. With that current going through what is in effect, a smaller and smaller battery, what's left of the plates gets hotter than normal. That leads to bubbling of the acid, and boiling the water out of it. As these bubbles float up to the top of the plates, they pop there, then spray acid onto the bottom side of the top cover. From there, the liquid migrates around until it seeps out the cover next to the posts. That is what causes the corrosion. By the time you see that corrosion, so much lead has flaked off the plates that we know failure is going to happen very soon. Some people such as myself will just keep on going to get every last ounce of life out of it. Some of our customers, especially a couple of elderly gentlemen, don't want to risk their wives becoming stranded, so they insist on having a new battery installed long before we recommend it. Most people fall in between those two extremes, and many listen to when we warn them of the imminent failure, but they don't necessarily buy the new battery that day. I just make sure I always have jumper cables, and I try to always be near where someone else can be asked to give me a jump-start if needed. I also watch for open hoods in parking lots so I don't drive off and leave someone stranded. I carry a small jumper pack with me too.
That corrosion is one huge clue that tells us to inform you that you're going to need a new battery soon, so if it fails, it doesn't catch you totally by surprise. I suspect that might be what your mechanic saw just before he sold you a new battery, but he knows there's more to the story. We also use a professional charging / starting / battery tester to analyze those systems. You won't be tested on this later, but for information purposes, a 12-volt battery, which is the standard voltage, (electrical pressure), in 99 percent of the world's cars, must pass a standardized test that only takes 15 seconds to perform. They all have a standardized rating so any two or more can be compared, called "cold cranking amps", (CCA). This would be similar to comparing a number of engines by looking at their horsepower ratings.
A common battery might have a rating of 700 CCA. With the majority of professional testers, we turn a large dial until the tester draws exactly half of that, in this case, 350 CCA. That will result in smoke coming out of the tester, which is normal and expected. To pass that test, the battery must be able to supply that 350 amps for 15 seconds while maintaining at least 9.6 volts. A fairly new battery will do that with ease, and may only drop to around 11 volts. By comparison, a mid-size V-8 engine's starter might draw as much as 300 amps at first, but that drops down significantly once the starter motor is spinning up to its normal speed. To say that more clearly, batteries get tested to tougher standards than what they'll encounter in the car. That brings up the second observation your mechanic may have found. Your car has a relatively small engine that is easy for a battery to get started. An older, tired battery might still get your engine started, but it is too old with too much lead flaked off to pass the battery load test I just described. That can only get worse as time goes on. Your mechanic knows that, and if he has your best interest at heart, he doesn't want you to be stranded a few weeks from now, so he is going to recommend a new battery.
I will never tolerate a mechanic or a salesman in any other profession selling products or services that aren't really needed. We already have a bad reputation for doing that. On the other hand, I hear too often about car owners who refuse to listen when we find serious safety issues. I've been involved with a few of these in my other specialty, suspension, steering, and alignment. I shudder when I think of those cars coming toward me on the highways. Every day tow truck drivers are picking up cars that went into the ditch or into oncoming traffic when a steering part broke. Some car models even have parts that cause us to shake our heads when we look at the designs that we know are going to fall apart. Based on what you provided so far, I would find it interesting to know what tests or observations he used to recommend a new battery, but I don't have any reason to think it wasn't warranted.
That finally beings me back to my original concern. That's the "Battery" warning light that told you something was wrong. I think I spelled out a clear explanation of the potential causes in my first reply, but if I left you with more confusion or questions, please ask me to try to explain it better. There is no warning light to report on the condition of the battery, at least not directly. I wish they would use a less-confusing word or symbol for their warning indicators. This light actually refers to the entire charging system. Its job is to recharge the battery after it was run down from starting the engine, and it supplies all of the electrical power for the car. If it were to fail completely, at most you could still drive the car for as much as an hour. The battery would be supplying the electrical power for the fuel pump, injectors, and ignition systems. Driving time will be cut to much less if you have the heater fan and head lights turned on. Even a brand new battery might only hold up for 15 - 20 minutes under those conditions. So, . . . the "Battery" warning light means the battery isn't being recharged, not that it is bad.
The generator is the heart of every charging system, but it is controlled by the voltage regulator, and that regulator needs to be turned on by some other circuit. That other circuit is usually through the dash warning light. That means the dash light does double-duty. It gets the system up and running once the engine is running, and it indicates when there's a problem. There's also fuses, wires, and connector terminals in the system. A blown fuse is a permanent failure, meaning it can never fix itself and start the system working again until that fuse is replaced. You can't do that while you're driving. A pair of mating connector terminals can become corroded and cause an intermittent connection. A few quick voltage tests can pinpoint that, but only while the problem is occurring. There's no point in searching for the cause of a problem when that problem isn't there. Some people just can't understand the nature of intermittent problems. This would be similar to being angry with your doctor because he won't tell you why you had a headache three months ago. You have to come and see him while you're having the headache.
So fuses can't be intermittent, but they could develop loose or corroded terminals that can cause an intermittent connection. Connector terminals can cause intermittent connections, especially when the engine is vibrating or rocking back and forth. I mentioned the brushes inside the generator previously. When the system is working at its hardest, those brushes only have to pass three amps, which is very little and is why they easily last well over 100,000 miles. When they do fail, it is because they wear down, then their springs can't push them out far enough to make contact with the rotating part inside the generator. One minute they'll stop passing that three amps, then a little later there's enough vibration to get them to work again, for a little while. As that wear continues to occur, the failure-to-charge will occur more often and for longer and longer periods of time. The warning light will turn on when the problem occurs, and more importantly, it will turn off while you're driving if and when the problem stops occurring. That is what you described, and I'm pretty sure is why your mechanic went looking further than the battery. The battery may have been ready to need to be replaced, but we both know there is something else causing the warning light to turn on. That hasn't been diagnosed yet.
I must also clarify my comment about replacing those intermittent brushes on a number of my vehicles. That's relatively easy on Chrysler products, and on the older models as far back as the early '70s, it could be done in a few minutes without taking their alternators off the engines. Except in very rare instances, we do not repair customers' cars that way. Same with starters and power steering gears. We always replace the assembly, either with a new one or a professionally-rebuilt one from an auto parts store. The reason is there's two possible outcomes if we take your generator apart and try to replace individual parts. We are either going to be successful, which will save you money on parts but cost you a lot more for labor, or there will be some other defect we overlooked or even caused, and that might not show up right away. The way we look at it is if the new parts cost a lot of money, we feel bad for you, but it wasn't our fault. If we try to save you a few dollars, and your car comes back on a tow truck, we are to blame, and we understand why you're angry. Better you be angry at the parts cost than with us.
In addition, when parts like generators and starters are sent back as "cores" to the rebuilder companies, they know which parts commonly fail or wear out and those always get replaced, even those that haven't failed yet. They also know the correct service procedures and special techniques that result in an individually-inspected and tested part to guarantee your satisfaction. You'll remember our failed attempt at saving you a few dollars long after you forget the cost of the proper repair.
Here's a list of some dandy related articles that might be of interest:
Please be aware the last article is representative of many car models. Other brands and models, and often depending on which engine they have, can be easier than what is shown here, and many are a real lot more miserable, frustrating, and time-consuming. Here in the U.S., most shops use a "flat-rate" guide when writing up a repair estimate. Those are really huge books that spell out the time required to do a procedure for a very specific car model and year with a list of optional equipment. That way every shop charges you the same amount of time regardless if an experienced mechanic with lots of expensive specialty tools gets the job done faster, or an inexperienced mechanic takes a lot longer. The only variable is what each shop charges per hour.
Also understand in the second-to-last article about testing a generator, that is only the first preliminary step, and can be done in a few seconds by any competent do-it-yourselfer. Passing this initial test only means it is okay to have the professional load-test performed by your mechanic. There is a different, less-common type of generator failure that can't be identified with this quick test, but it will show up with the professional testers.
I had a sense that you have a reputable mechanic, but it was your comment about sticking with him for many years that really puts my mind at ease. The former owner of the dealership I used to work for always told us, "it takes more money for advertising to get one new customer than it takes to keep ten current repeat customers happy". Most mechanics and shop owners are well-intentioned, but we suffer from some of worst communication skills. With three words we can tell an entire story to another mechanic, but have a mechanic explain something to a service advisor who never was a mechanic, and hope that advisor translates it into something he thinks the car owner will understand; you know things are going to be misinterpreted and lost in translation. That just adds to the often-undeserved reputation for being dishonest.
You can find many more informative articles here:
but for questions specific to your car and problem, I'll be here waiting for your reply.
Wednesday, January 13th, 2021 AT 5:31 PM