The mechanical type relay as found on all 1938 thru 1954 45CI and Big Twins has a number of disadvantages. It was proven unreliable and there was no way the output voltage was regulated, so not very easy on the battery. This solid state full electronic item is very reliable and controls the voltage to your battery. Also available in a version for models that have been converted to a 12 Volt system. It fits under the stock relay cover, which we also have available in your choice of black and chrome.