Battery charge problems

More failure in the alternator. Replace it with a new or rebuilt unit

Roy

Low voltage is not the cause, it's the symptom. The generator should be professionally load-tested for current output and "ripple". What you're describing is typically caused by one bad diode of the six. That will reduce the current to exactly one third of its design rating which is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference. For the common 90 amp generator all you'll get is 30 amps.

When the generator can't deliver the needed current it can't build up system pressure, which means voltage. The voltage regulator won't even kick in yet and do anything until system voltage gets to around 14.5 to 14.75 volts.

Think of a pressure relief valve on an air compressor or hot water heater. They open to release pressure only if the controls fail and pressure gets too high. If the pump can't develop the needed pressure you don't fix that by replacing the pressure-relief valve. You have to fix the pump. Your mechanic replaced the pressure-relief valve, so to speak, and expected the voltage to be higher. Voltage regulators can fail that way, but it is extremely uncommon. The weak pump, (generator), is almost always the cause of the low system voltage. The diodes can be replaced in a block of six or two blocks of three, but it's very time-consuming so it's always less expensive to just replace the entire generator.