Thurston Car Talk: Alternator And Battery Basics

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Brons Automotive is located in the heart of West Olympia. Photo credit: Bron's Automotive.

Submitted by Bron’s Automotive

What’s that funny squealing noise that I hear for a few seconds after I start my car in the morning and why do I care?  It’s like this:  The starter motor and alternator are electrical mirror images of each other.  The starter motor takes a large amount of electrical energy to produce mechanical energy.  The mechanical energy produced is of course the car’s heavy motor being cranked over so it can start.

Conversely, an alternator takes a large amount of physical energy from the motor driven belt to produce electrical energy.   Initially this electrical energy is used to recharge the battery and replace the electrical energy it took to start the engine.  After the battery is full again the alternator’s task is simply to supply the normally smaller amount of electricity needed to run the ignition, lights, radio, etc. on the running vehicle.   Here’s the important point:  the more electrical amperage an alternator is putting out, the more the alternator is resisting being turned.

Let’s take an alternator that is capable of putting out 100 amps. It will only be able to put out this 100 amps at about 1200 RPM or higher.  At idle speeds of about 650 RPM it can only produce 50 amps or so.  This is why when you have a loose alternator belt or a defective belt tensioner, you only hear the squeal when you hit the gas initially after you start the engine, not with the engine idling. (When you hit the gas is when the alternator is trying to put out a full 100 amps.)  Remember, it resists being turned in proportion to how much electricity it’s trying to produce.  As soon as the battery is recharged the squeal goes away because the electrical demand is less.

So again, why do I care if the noise goes away within 30 seconds?  Here’s why:  The carmakers look at how much electricity the car will ever need and then install an alternator that can provide 10-15% more energy than that number just to be safe.  So let’s say that when you’re coming home from Seattle on a dark rainy night, this is your time of maximum electrical need.  At this time of maximum need, to run your defroster, wipers, lights, radio, ignition system, fuel pump, on-board computers, and more it takes 88 amps.  This is why your car came equipped with a 100 amp alternator.  If you have a slipping belt that announced itself by that squealing noise that you only heard for the first 30 seconds or so after you started your engine, your alternator will only be able to produce 70-80 amps.  This slight slippage will not be enough to make a squealing noise, but it will result in the battery being chronically undercharged since the battery will supply the rest of the electrical energy needed to get you home.  The slippage also produces heat that can harm the front alternator bearing and damage the drive belt.

The next time you start your car the alternator will have to produce more energy than ever to make up the electrical loss of the slipping belt.  A chronically undercharged battery will also go bad before a fully charged battery, a process called sulphation.  So here is another important point: A battery that is defective or chronically undercharged from a slipping belt causes the alternator to work overtime, and will result in premature alternator failure.  It’s also true that a weak alternator will result in an undercharged battery which can lead to early failure of the battery, which stresses the alternator further.

The bottom line is this:  When you hear a belt squeal, get it fixed even if the noise goes away right away. The condition is still hurting the longevity of your battery and alternator. When retensioning a slipping belt, the battery should always be tested. When replacing an alternator, the battery should always be tested. If you’ve been told your battery is weak, replace it even though it may still start your car for a few months.  Your alternator will thank you.  At Bron’s Automotive we always make sure all these items are considered and tested when doing electrical work. 

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