What's the typical Boxster voltage while driving?
 

I've got some battery charging questions.

First one in the thread title - what should a voltmeter read on a properly running Boxster while driving? I get 14.2 for about 1 mile then tapering off to 13.75 after about 15 minutes. The readings didn't change when I put on a new voltage regulator. I'm used to seeing 14.3 - 14.5 on my GM vehicles and with no variations during the drive.

Second - which measurement is more meaningful - battery voltage or hydrometer reading? My 2000 S is showing 12.6 volts with the motor off, measured with a digital gauge throught the cigarette lighter. This measurement was taken after holding back the flash-to-pass headlight switch for 10 seconds, to burn off the "flash charge" whatever that is, per the voltmeter instructions. I think this is considered a good reading. Yet the hydrometer reading is in the fair to low range - about 1.175. The battery is a conventional unit from O'Reilly and is only about 2 months old. When I put it on a 6 amp charger it briefly starts out reading low on the charger gauge but moves into the green 60% charged region within 30 minutes. The digital voltmeter reads 14.6 while charging.

Basically I want to know whether I should replace the alternator and if that will give me the 14+ volts that I see on other vehicles. Or whether Porsches are different somehow and my Boxster readings are normal for the car.

Your alternator is weak, and most likely candidate would be the voltage regulator (which you have indicated you already changed), so the next suspect would be one or more of the diodes are weak. You should be in the low to mid 14 volt range, either at idle or running.

Hydrometer reading is a measure of the battery charge condition, not a measure of the charging system. To find a somewhat weak battery on a car with a below par voltage output is not unusual. Replace the alternator, or the diodes if you are so inclined, and you battery will most likely come back up to snuff, assuming it has no organic issues itself.

Thank you. I replaced the alternator with a remanufactured unit from O'Reilly and I'm pretty sure it's going to totally fix my charging problems. The voltage while running is now 14.3-14.4 and remains steady while driving. Before it used to drop off after a while and the battery would get weaker over time.

I've been pissing around with this issue for a couple of months. The old alternator was barely good enough to make the diagnosis confusing. O'Reilly put their tester on the car and told me the charging system passed but I got a replacement alternator anyway. The only trouble was I forgot to plug on the small wire and had to take out the alternator a second time. I'm actually pretty good at it now, lol.

Thanks again.