There appears to be a clinker in yer thinker. Just because you can turn on all the accessories doesn't mean the generator is okay. If it can't supply the needed current, the battery will just make up the difference until it slowly runs down over hours or days.
To me, 13.6 volts is too low, but not by much. I use 13.75 to 14.75 as the acceptable range. You'll typically find around 14.5 volts in properly-working charging systems. Regardless, that is only the first part of the test. The second part requires a professional load tester to measure the maximum output current the generator can develop. That will either be very close to its maximum rated current or it will be exactly one third of that value. That testing has to be done at 2,000 rpm because all generators are relatively inefficient at low speeds. That same is true when you're running the accessory circuits and finding 13.6 volts. To be accurate, you have to raise engine speed when you do that.
Even when you find the charging voltage to be okay, one failed diode of the six cuts the current capacity to one third of the maximum rating. That is not enough to run the entire electrical system under all conditions, and the battery makes up the difference. Charging voltage can still be perfectly fine or even just a little high because the voltage regulator is responding to the dips in output voltage during the time one of the three phases is missing. Those dips cause "ripple" voltage, and that is also measured by professional load testers. Ripple voltage will be very high when one diode has failed.
Where I would start is by having the charging system tested for maximum output current and ripple voltage. Those are part of all standard charging system tests, and take less than a few minutes.
Monday, February 23rd, 2015 AT 12:30 PM