First off, and may I suggest you write this down and commit it to memory.
Don’t ever disconnect a battery cable from a car with the engine running.
I am as serious as a heart attack about this. Anyone that tells you to do this does not know shit about cars that have been built in the last, oh say, 40 years. Back in the day when cars had DC generators you could do this and find out if the generator was working. The problem is if you do this with an alternator equipped car, you can damage the alternator (as in, it wasn’t bad before, but it is now) or you might just wipe out one or more electronic control units to the tune of several hundred dollars each.
OK now that we have that out of the way, if you have a digital multimeter we can probably find out where the problem is.
First off measure the voltage in the battery after the car has been sitting for at least 3 or 4 hours.
If the battery is fully charged you should have 12.6V or above
3/4 charge 12.4V
1/2 charge 12.2
1/4 charge 12.0
if the battery is below 12.4 recharge it.
If you have any doubts about the battery have it tested a an auto parts store.
Then start the car and measure the voltage at the battery cables (they are clean and tight aren’t they?) with everything turned off and the engine at a high idle (1500-2000)
You should see 13.5-14.5 (some cars might go a little higher but not much)
Below 13.5 suspect a voltage drop, or a bad alternator.
Much above 14.5 a bad voltage regulator.
If you see straight battery voltage you alternator is a dead puppy.
Now Turn on the headlights and the heater fan. measure the voltage again. Is it still between 13.5 and 14.5? Flip the meter to AC and measure voltage at the battery terminals. If there is more than 0.4V AC at the battery terminals, your alternator diodes are bad, and you need a new alternator.
if you have not found a problem by now, it is time to do a voltage drop test. Set the meter to DC, engine running, lights and heater fan on. Place the red lead on the big fat output lead of the alternator. Place the black lead on the + battery terminal The reading should be less than 0.2V. Absolutely no more than 0.4V. If it is above this figure you have excess resistance in the cable between the alternator and the battery. Clean, tighten, or replace as necessary to bring the value down. Voltage drops are like golf, low score wins.
Next place one lead on the alternator case, and the other on the battery negative terminal. Same numbers apply.
By now you should have found the problem if it is in the charging system. (we have covered about 90%+ of all charging system failures, there are a couple of oddballs, but they are outside the scope of a message board.)
If you have not found the problem you may have a draw. you will need a test light to test for this.
Hood open, key off and out, car locked. Remove the battery negative terminal. Place the test light between the battery and the cable. If the light lights, you have a draw, find it. Fix it.
If the test light does not light, you do not have a draw.
NOTE: You cannot use a voltmeter for this test.