I think your assesment was totally correct and complete from jump. You don't have to clarify anything as it was spot on: If you see 15V with the car running and 10V with the car off, it most likely is the battery. Just like when you drive the car and hear "whompa-whompa" it is most likely a flat tire. Can it be something else? Sure but to first assume it isn't is just silly :)
Good call.
As Remco details, Elle got it right.
Any battery that is charging at 15 volts and after charging reads 10 volts under load is bad... poor connections or not. As Remco pointed out in an earlier post, the caveat is that there really could be a significant load, and that slim possibility had to be ruled out to be iron-clad certain.
Mike
Bad batteries take a lot of current trying to pretend they are okay. As they get weaker the battery voltage deteriorates, from failure to hold a charge and sometimes from shorted cells. Either way, failing batteries (or dead batteries) cause the alternator to put out maximum current for progressively longer times in order to bring the voltage up to the regulator level.
Alternators are odd things - by design, they will only put out so much current for a given field excitation. The ratings you see on alternators are those maximum ratings for about 14 volts on the field. The way that works is that the voltage they produce is proportional to rpm, and the frequency at which they operate internally is proportional to rpm. Since the stator windings are inductors, their reactance is proportional to the frequency (in turn proportional to rpm) so the maximum current the alternator will put out, even into a short circuit, is limited.
In the '60s, the alternator could put out the full rating (usually 35 amps) indefinitely. They were big, Tim Allen devices that scarcely ran warm at full output. Those days are long gone. Modern alternators are much smaller and are called on to put out much more current - often 60 to 100 amps. With a good battery, the electrical systems are carefully designed not to burn up and still keep up with demand. With a bad battery, the alternator runs too hot and cooks the insulation and the diodes. Eventually too much damage is done and the alternator fails.
Mike
"Michael Pardee" wrote in news:1-mdnaKB-sX9-VnenZ2dnUVZ snipped-for-privacy@sedona.net:
Nice writeup. It's going into the FAQ.
Jumping in here... if you're reading 15+ volts across the battery with the car running, it's very possible that your voltage regulator is bad and the overvoltage is killing your batteries over time - alternator output should normally not be much over 14.5V with engine running and no load (no lights, wipers, blower, etc.).
The regulator is, I believe, internal to the alternator, so you're looking at needing to remove and rebuild or replace the alternator. Have Autozone do a proper alternator test to make sure there's no other problems, before you end up needing to replace your new battery again.
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