Battery Tester's Voltage Rollercoaster Decoding Fluctuations While Driving

On the way home on a 150 mile drive we noticed the volt meter fluctuate between 12 and 14 volts (I had a fluke meter with me). It was warm ... No it will NOT immediately go to 14 volts while the grid heaters are cycling...they pull a huge amount of current, more than the alternator can ... Noticed today my volts are fluctuating while RPM's are near or above 2000 on highway trip. At stop light had battery light come on. This is completely normal with modern charging systems from GM and other manufacturers. Output voltage is computer controlled to provide only what is necessary ... It sounds like the voltage regulator inside the alternator is bad. Just an FYI most all auto parts stores will test your alternator, battery, ... It's normal. Modern cars will fluctuate voltage depending on load and battery charge state. Hope you got the answers you were looking for, but ... This is a battery maintenance function, not a electronics operation function. When batt needs more charging, ECU commands (regulates) alternator to meet that ... They will intentionally let the battery charge drop while driving - battery voltage will drop to around 12.6v or so when this is occurring. ... and coming back on, no trouble codes and not all the time. I was in the power room today when it happened, the battery voltage drops ... Some of the battery voltage fluctuation might be normal. Check your owners manual about the special feature your truck has that was designed to increase ...