Bill,
Nick had pointed out that my previous reply focused on only the output of the alternator and he was emphasizing the fact that the voltmeter is always measuring the battery voltage instead. The truth is that, with the rather low resistance of the wiring, the voltmeter is actually connected to the combination of the battery, the alternator, and the load. The voltmeter can only display the net result of this combination, It cannot tell you what any one part - Alt , Bat, or Load, is doing. But by understanding how each of these elements interract you can usually tell most of what you need to know.
Gordonra early in this thread showed us what the battery voltage will be on a typical unconnected battery at different levels of charge:
The open circuit voltage, measured when the engine is off. It can be approximately related to the charge of the battery by:
Open Circuit Voltage ~ State-of-charge
12.65 V 100 %
12.45 V 75 %
12.24 V 50 %
12.06 V 25 %
11.89 V 0 %
Note that this is a small change from full charge to zero. In addition, due to the chemistry of a battery the biggest factor is that the internal resistance of a battery goes up as it loses charge. Take that battery at 0% as noted above. But gee, it still has voltage! Yeah, but put even a small load on it, like one lamp, and its voltage will go to just about zero. (that big drop when loaded is what we call "Internal resistance" ) Add in the temperature variability and you can see that it's pretty hard to use a voltmeter as an absolute and accurate "state of charge" meter. This is true for just about any kind of battery. To really know well the state of charge of a battery you need to measure its voltage with no load and with some load. You mention the value of looking at the voltmeter before you turn the key, I would suggest you look at the meter, and then briefly turn on the parking lights and see what it reads. If it drops a bunch, you don't have much charge.
So that's about a unconnected battery. Now consider an alternator. It puts out about 14-15 volts at up to ,say 35 Amps. Remember the amps is what flows (like water) and the volts is what pushes the amps (like pressure) . The output voltage of an alternator would normally vary almost totally with the load- high voltage with no load and lower voltage with high current load. But the regulator controls the output voltage to be 14 to 15 volts, no matter what the load current is- unless the load current exceeds 35 Amps. Then it can drop below the regulated level simply because it does not have that great an output capacity.
Now connect the Alt to the Bat. Consider the empty battery noted above. Its voltage is 11.89. But when we connect the running alternator to it, its voltage goes immediately up. Not quite all the way to 14.5, but nearly. Does that mean the battery is now charged ? No. The voltage we measure at the battery terminals or at the alternator output will be about the same - not much difference - due to low resistance of the interconnecting wires. But that voltage we measure is mostly the result of the output of the alternator. But you say, how can the alternator put out 14 volts and be connected to a battery at 11.89 volts. Where is or what happened to the 2 volt difference? Remember what happened when we put a load on the empty battery? The battery voltage dropped to near zero, due to the battery's "internal resistance" With a load the internal resistance took away all the voltage. But now with an alternator SUPPLYING current instead of a load taking current, that same internal resistance ADDS voltage to the battery. That's how the battery voltage goes up to 14 volts, matching the alternator output voltage. That 2 volt increase is the direct result of the charging current flowing into the battery. But if you stop or disconnect the alternator shortly after starting it, the battery voltage will drop back to 11 volts.
But if you maintain the charging current for some time, gradually the battery will build up its charge and its internal voltage goes up, and the difference between the internal voltage and the alt voltage will decrease and the charging current will reduce.
So before starting the car the voltmeter can tell you if the battery has some charge, especially if you put a small load on it by turning on the lights while reading it.
Then when you start the car you can see, if the voltage goes up immediately, if the charging system is working.
And if you see the voltage go up a bit more over the next 5-10 minutes of running, it tells you that the battery is nearly fully re-charged, replacing most of the charge you lost during starting.
And if you turn on all the electrical devices and the voltage stays about the same, you know the alternator is not only working but can handle the full electrical load.
If you are driving along and you notice that the voltage has dropped below 14 volts you know that the charging system has failed.
What else do you need to know from your meter (ammeter or Voltmeter)? Not much.
Wow, this is long. But it will take you a lot less time to read than it took me to write!
Hope it helps. Kinda fun trying to explain stuff.
Tom H
