So I drove my 2000 Ranger (4.0) to an event yesterday afternoon. I had left my lights on for about 3.5 hours and when I came back to start the truck it was completely dead- no solenoid click even. I was able to get a jump and the truck started right up. The dashboard voltage indicator was at about 1/3 as I started the drive home. After about 40 minutes on the road, I noticed that the voltage reading was getting lower. A few minutes later all the dashboard lights and headlights started to go dim. I had been in several cars with bad alternators so I knew what this meant. When the engine started to chug I made my way over to the side of the interstate and called a tow truck before the lights completely died.

So what happened here? How could the truck have run for so long on a bad alternate and a dead battery? Or was there some reserve power left in the battery when it was jumped and I just ran on that for 40 minutes?

 

Its a which came first, "the chicken or the egg?"

A car battery lasts for 5 to 6 years, then its plates and chemicals are depleted, so its a known lifespan, all things being equal
They can last a year longer or fail a few years earlier if things are not equal, lol
Car batteries are built with thin plates, so they can discharge high AMPs quickly, even in cold weather, for starter motors.
That's the only time the battery is used, once engine is started ALL voltage comes from alternator, battery is not used at all
Car batteries do not tolerate slow steady drains very well, i.e. leaving lights on, this shortens their life span every time it happens
There are Deep Cycle batteries, that are made with thicker plates, they are not as good at discharging AMPs fast, but do handle slow steady drains quite well

A good car battery will show 12.3v to 12.9v after vehicle has set for a few hours, at rest voltage
12.8v to 13v is a brand new battery
12.5v is a 3 year old battery
12.3v is a 5 year old battery and time to watch for battery sales
12.2v or lower is a failing battery
If you test a battery with voltage meter and its voltage is dropping while volt meter is connected then battery has a shorted cell, so is self draining


Alternators, with internal voltage regulators, can last 20+ years, normal failure at that age is brushes, bearings or Field
Voltage regulators can fail earlier, because they are electronic now voltage surges can cause issues, i.e. disconnecting a battery cable when engine is running, or Jump Starting, either can cause voltage regulator to fail

There are also 3 fuses, One in the cab for the Battery Light, this fuse is the ON/OFF switch for voltage regulator, if you turn on the key and don't see the Battery Light come on, the alternator won't work, check this fuse, in 2000 Ranger its fuse #15 in cab fuse box, 7.5amp, this is the light green wire on alternator
Monitor or Pushback voltage uses engine bay fuse, ALT SYS FUSE, 30amp, this is the Yellow wire on alternator
On the back of alternator is the B+ terminal, it is connected to the 175amp MEGA FUSE, usually located on the outside of the engine bay fuse box, larger black/orange stripe wire

When working an alternator will produce a MINIMUM voltage of 13.5v and a MAXIMUM of 14.9v
A car battery has maximum of 13v, so battery has no part in vehicles electric system when engine is running, alternator voltage flows TO the battery, to keep it charged

To test alternator, key off, remove the 2 or 3 wire connector on alternator
Use volt meter, grounded on alternators metal case, and test Yellow wire, you should see Battery Voltage, so test battery FIRST, if not the same voltage then you have a bad wire or fuse
Then test B+ voltage, you don't need to disconnect it's wire, should be battery voltage, if not the same then you have a bad wire or fuse

Turn key on test green wire, should be battery voltage, if not the same then you have a bad wire or fuse

Put connector back on alternator
Start engine
Test voltage at battery
Should be above 14volts just after startup
After a few minutes of idling voltage should drop to under 14volts, 13.5v-13.8v is expected
If it stays above 14volts then battery is bad or voltage regulator is and that will "cook" a good battery

Voltage regulator works using electromotive force, best described as "push back" force or voltage
Just after start up the battery is drained, so has little push back voltage, so voltage regulator sets higher voltage
As battery gets recharged it starts to push back, doesn't need as much voltage, so voltage regulator lowers the voltage, if battery is failing/bad, then no push back, so voltage stays high
As you turn on head lights or heater fan to high, the push back goes down so voltage regulator turns up voltage until push back returns, so you would see a drop in voltage then a return to 13.5v at the time the lights or fan are activated.

Pretty simple setup, if everything is working.

And just a heads up, dimming lights at idle is not normal, lol, it means one of the 3 Fields in the alternator has failed, so engine RPMs must be above 1,100-1,200 for alternator to produce enough voltage, not a big deal unless you idle alot, but it does mean parts are failing in alternotor

"The chicken and the egg"
A bad battery can kill an alternator
And a bad alternator can kill a battery
So you need to TEST both before replacing either