Charging Today's Car Batteries

Have you ever heard that driving around to recharge your battery is ok, but it will not “fully charge” the battery? The AAA recommends putting the battery on an amp charger (I believe it’s called) to “fully charge” the battery. (Note: My battery and alternator are both 1 yr old, replaced together.)

First, no battery can be “fully charged” as charging is asymptotic. And, is driving around all that different from using an amp charger??? What’s the SD on this?

Specifically, what’s the risks as I do not have access to an amp charger.

I’ve never heard of this.
If the alternator wasn’t capable of replacing used charge, then eventually the battery wouldn’t be able to start the car.

It is true that for battery arrays (like solar energy installations) a high-current “equalization” charge routine is recommended, but I’ve never heard of this for a single-battery system.

You have probably mis-read or mis-understood something but given your habit of not returning to threads I am not even going to try to guess or explain. In short, your information is wrong.

For the sake of others. If your battery is run down to the point you need a jump, it does take a whole of driving to fully recharge your battery. It’s easier to be sure you have a full charge by taking the low battery and put it on an eternal charger

Those things take forever

Your alternator is a battery charger, it charges high amps when the battery is low and then tapers off the amperage as the voltage comes up. Yes it will fully charge in the car and if you have a good battery it will happen in a reasonable amount of time.

Many years ago was told the same thing by an old guy who had spent much of his life reconditioning alternators. He’d rig his car’s battery up to a charger once a week.

If it’s a decent alternator system, and the car doesn’t do too many short stop-start journeys (cranking the engine sucks out a lot of charge), then you can get away without an additional charging regime, at the expense of reduced (but acceptable) battery life. However, an alternator won’t provide the long float charge that the battery needs to approach 100% capacity, and a partially charged battery will tend towards sulphation.
Here’s the science bit:

Aah, no.

The alternator is designed to keep a fully charged battery fully charged, or to recharge a mostly charged battery back to full.
The alternator is not designed to recharge a fully dead or mostly dead battery. Most alternators will trickle charge the battery under those conditions and bringing the battery up to fully charged this way would take hours and hours of driving.

From personal experience, I can attest that driving will re-charge a battery adequately. I don’t know if it is “fully charged”, and I don’t care. The car starts the next time.

I have been draining car batteries for one reason or another for decades now, since cars had carburetors, and the only difference I have noticed is that the batteries seem a lot easier to jump.

But people are funny about car batteries. My mechanic has been telling me for years I need a new one, but it keeps lasting the winter, a little more tired maybe, but the car does start.

I do not think this is correct if plainly interpreted nor do I think it flatly contradicts HoneyBadgerDC

I have some experience with batteries, alternators and regulators. The usual way it works is that the regulator keeps a constant voltage at which the battery “floats”. This means a rather slow “trickle” charge at the end of the charge.

On the other hand neither the battery nor the alternator nor the regulator can handle more than X Amps so if the battery is too discharged the system will limit the current but this is high current, not trickle.

So a car regulator has two stages: Constant (high) current until a certain voltage is reached and then constant float voltage is maintained and the battery accepts what it wants.

The problem with this is that this is not efficient when running the motor solely to charge batteries (as is done in sailboats). It can take forever to top off the battery and it is not worth it. In any case, running a car motor (or boat) solely to charge the battery is a terrible waste because only a mimimal part of the energy goes into the battery and the rest is wasted. A charger is always more efficient.

To overcome this problem you have three stage chargers which maintain higher current above float voltage until the battery is charged and then they drop the voltage to float level. See the diagram and explanation half way down Solar Power Charge Controllers

In my boat I had manual control of the alternator field current so I could bypass the regulator and increase the current for bulk/absorption charging. These days you have computerized charging controllers but manual control was the common way of doing it just some decades ago.

As a policy we like all batteries fully charged before leaving the shop anytime we have replaced alternators or batteries, it doesn’t always happen and for some odd reason when they come back a few days later the batteries are fully charged.
I can tel you from a lot of experiece that the alt might start off just a little slow but will quickly amp up as the battery voltage rises and will tail off as needed. I do agree as proper procedure batteries should be fully charged.

You will prematurely wear an alternator out if you use it to charge a low battery over and over. Alternators are generally not cheap nor is the labor even if you do it yourself.

Not so. Not true at all. Why would this be so? The alternator in your car is turning whether it is charging ir not and whether more or less current is flowing through the wires the difference in mechanical wear is going to be negligible.

Running on alternator at high amp output has the same affect as running any electric motor on a heavy load, it will wear them out sooner but for the amount of time the alternator is at high output and the number of times it will be asked to run like this I would say the extra wear is minimal. If an alternator is too small to keep up with the amount of accessories it is asked to keep up with it will wear out much sooner.

In my experience (owning an F-350 Powerstroke), alternators often fail because the solder connections to the diodes on the rectifier plate crystalize. This is 100% due to poor design / manufacture, but it’s probably not helped by large current flow through the diodes, which will cause expansion and contraction cycles due to ∆TCE. I’ve rebuilt several alternators, and re-soldered the diodes using stronger (silver-bearing) solder, which helps.

Vibration is a huge enemy of soldered connection, specially for heavy components. High frequency transformers in power supplies, monitors, etc. often fail due to the ultrasonic vibration. Anything in a car (or boat) has to be very well designed to withstand vibration.

Effect, the word you are looking for is “effect”.

I think we can all agree a discharged battery can put a heavy load on the alternator. How much of a load? I don’t know, as I have not seen a study on current measurements recorded while an alternator is trying to charge a depleted battery. But if it can be assumed the current produced by the alternator is “quite high” for a certain amount of time when it is charging a very depleted battery, then it means the temperatures of various components inside the alternator are also high. Specifically, the temperature of the diodes in the three-phase rectifier will be high, and the temperature of the stator windings will be high. High temperatures in the windings and diodes may not cause immediate failure, but it wouldn’t be good for them either, as high temperatures will cause materials and components to degrade over time.

As an interesting side note… a few weeks ago we were testing a 28 V aircraft alternator in our lab. The alternator was spun using a large electric motor, and we loaded the output using a 4000 W programmable load at various RPMs. We were monitoring the stator winding temperature using a thermographic imaging camera. At 110 A, the temperature of the stator windings was around 115 °C. I do not know the NEMA temperature class for the magnet wire used for the stator windings, but I’m guessing it’s over 115 °C. But as mentioned above, operating at high temperatures can and will degrade insulation over time, and will thus cause a failure sooner vs. operating at lower temperatures.

I have seen alternators come into the shop so hot you couldn’t touch them and smelling of melting insulation. This is usually when a bad battery is involved. They seem to be ok once they cool down but I have to believe they have a shortened life after a hard episode.

Yes, that characteristic smell of burning copper wire insulation is well known to anyone who has repaired electric motors, transformers, etc.

And I am always amazed at how hot some of those things work without damage. Many are designed to work really hot although it obviously means losses in the form of heat.

A regulator controls the output current by adjusting the field current. Some alternators have an external regulator and others have the regulator included. In any case a good regulator should limit the current to safe levels in any case, whether it’s due to a discharged battery or just excessive load. In my experience with boats the problem was the opposite: the alternator could give more current if you tinkered with the regulator.