Sterling Power 12V Alternator Protection Device

The APD works when the spike back voltage to the alternator shoots above 16.5V -> infinity (potentially) as the current goes to 0A (to conserve P = I x V).

The APD turns on with a small resistive load of mA to reduce this escalation of voltage as a result of the break of cable that has led to 0A.

If the spike is excessively rough, it can lead to APD damage (which can’t be fixed) but, your alternator/regulator has been protected.