With an alternator, you have a voltage regulator and this voltage regulator is responsible for keeping the voltage on the vehicle from exceeding a limit. This limit is usually 13.8 to 14.2 volts, which is enough voltage to allow for proper battery charging, but not enough to burn out light bulbs or damage delicate computer circuits. The best way to test a voltage regulator is with a multimeter. What you do is you put your multimeter clamps directly on the battery terminals, positive red depositive and black to negative. With the car off you should have a little over 12 volts. That’s a healthy battery. Once you’re hooked up, you want to start the vehicle and what you’ll see is that at idle, the voltage will increase.This means that the alternator is charging the battery. We’re making about 13.8 – 13.9 volts. At this point, what you want to do is to slowly start increasing the speed of the engine. Once you hit between 1500 and 2000 Rpm, the voltage should reach a point where it no longer increases. What we’re noticing is that it’s maxing out right around 13.98, nine seven or so, which as well within that 13.8 to 14.8. It needs to be replaced if the voltage is too low, which causes the alternator to not charge properly and it needs to be rebuilt or replaced. Lastly, if the voltage with the vehicle off is below 12 volts then the battery is most likely in need of being replaced.