alternator charging- only above certain rpms?

[CBradio]

I recently got stuck with a dead battery. I charged it and it died again,
so I replaced the alternator. That was about two weeks ago, and I've had no problems since until today.

Today I was driving around, and the car sputtered and died.
After that, it would not turn over, the battery was dead.

I got a boost, it started right up, and I drove the car home.

Prior to the car dying, I was driving real slow around the city for an extended period of time, once I got the boost, I kept revving over 2,000 rpm anytime I was stuck at a light.

The reason I kept doing this is because I *think* I recall reading something about the alternator only charging above certain rpms---
DOES THIS MAKE ANY SENSE?

.............................Could my battery have died because I was just driving real slow for an hour or so?

[2002jm]

Unless you replaced your battery, it is most likely dead for good. Sometimes when you kill a battery they will not recharge to normal, and then maybe driving slowly in town, may have just made the little charge in the battery go away.
Have the battery 'load' checked to see if it is any good.
jm

[Blaise]

Have you tested the charging system?

You should be able to cruise around town and idle at lights without killing your battery.

Make sure your battery is still good and charged. Most autoparts stores can do a load test.

Then check the voltage when the car is running. You should read the charging voltage at 13+ volts. You can check the correlation to RPM then too. If the voltage remains the same as when the car is off, then you are not charging (obviously). Then you can track down the issue.

[T.Hanson]

If visiting a shop for a load test, beware there are quicky winky dinkers using a little table computer dealy, and real load test machines.

Interstate battery. MT-47 (?) Goofy green color but they work great, guarantee is real and they get rebranded, sold for a dab more under other labels.

Having fought a battery fight recently it turns out trickle chargers will take one up to 12.5, but a full charge can be 13.5 if in the car with a good alternator.

If your battery is much past three years old, check the warranty or just salute it and buy a new one.

[Falkenberg]

With engine running, the voltage can be up to 14.4 approximative. 14 is good, 13 is low.
Voltage should not vary much with neither RPM nor load (lights, defog, blower, AC, turn them all on).