An engine starter motor tester applies power to a motor under test at the level normally applied while that motor is installed in a vehicle and monitors the ripple in the driving current. The ripple rate, scaled by the number of windings used in the construction of the motor, provides a direct reading...http://www.google.es/patents/US7640795?utm_source=gb-gplus-sharePatente US7640795 - Apparatus and method for starter RPM detection
