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There are 2 variables you have to know the values of in order to understand what is happening to the battery. One is voltage and the other is amperes or current. I have a charger that will send over 15 volts at several amps into a drained battery to fast charge it then it will start reducing the amps and the voltage as the battery charge increases. This charger has a digital readout showing the current and status of the charge process. I'm only posting this to show that 15.5 volts by itself will not fry a battery. But I think you're right that 15.5 volts above .1 amps sent into a charged battery can't be good for it.
I have a digital voltmeter that plugs into the cigarette lighter socket. On my Mustang, as well as my other cars, I always see the voltage number changing when the engine is running. I see the highest voltages after the car has been sitting for several days and just after starting the car. Even then the highest voltage I see generated by the alternators is about 14.7 volts. That value usually bounces around like 14.1, 14.4, 13.8, 14.7, 14.4, etc. while the engine is running. Soon the values settle into the 13.x range. Are you seeing these sorts of numbers from your voltage regulator when you are driving the car normally? If you're not, I'd try a new voltage regulator.
This is the style of voltmeter that plugs into a 12v accessory socket:
https://www.amazon.com/Zeltauto-Cigarette-Lighter-Digital-Voltmeter/dp/B00VL9JZ0K
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I have a digital voltmeter that plugs into the cigarette lighter socket. On my Mustang, as well as my other cars, I always see the voltage number changing when the engine is running. I see the highest voltages after the car has been sitting for several days and just after starting the car. Even then the highest voltage I see generated by the alternators is about 14.7 volts. That value usually bounces around like 14.1, 14.4, 13.8, 14.7, 14.4, etc. while the engine is running. Soon the values settle into the 13.x range. Are you seeing these sorts of numbers from your voltage regulator when you are driving the car normally? If you're not, I'd try a new voltage regulator.
This is the style of voltmeter that plugs into a 12v accessory socket:
https://www.amazon.com/Zeltauto-Cigarette-Lighter-Digital-Voltmeter/dp/B00VL9JZ0K
