Any alternator will deliver a very high voltage spike when it first comes on line. I have measured these to be as high as 90 volts, using an oscilloscope. It only lasts a few milliseconds, but that is enough to fry electronics.
The battery will absorb a lot of the spike, if the cables from the alternator to the battery are large diameter and short. Circuit breakers can take up to half a second to open when there is an overload, because they are a thermal device and have to heat up before they open. If you are using fuses to protect your electronics, make sure you are using the quick blow type. Some fuses are time delayed so they won't blow on a spike, but open when the average current goes high. The best approach to limit the spike is to put an 18 volt, 5 watt or larger zener diode from the alternator output connection directly to ground. It doesn't take a large one to swallow the spike. Any electronics supply store should have them in stock. When the voltage regulator goes belly up, there's little that can be done, since most electronics will go before the fuse or circuit breaker blows. I'm breadboarding up a low voltage indicator (with a flashing light to get my attention), since I'm using the standard VW ignition system and want maximum time to get on the ground before my battery quits. I will probably play with a high voltage warning circuit, too, but that is a good winter project. Jim Vance va...@hbcomm.net
