Functioning,starter circuits & associated parts:
The starter motor is a powerful electrically operated motor. The starter motor may have to provide upwards of 1/2 to 1 horsepower. In cold weather more power from the starter motor will required. The starter motors in our Airheads, like all vehicle starters, are not very efficient motors. The Valeo is more efficient than the Bosch. Quite a lot of amperes might be needed under some rather common conditions. Starter motors are, at best, 60% efficient due to magnetic field losses, friction losses, etc.
745.7 watts is DEFINED as ONE horsepower. If the system is a nominal 12 volts during cranking (typical, with good battery, wires, and connections, voltage as read at the starter terminals), then 745.7 divided by 12 equals 62 amperes. Due to the efficiency losses, & the need for many more amperes to BEGIN engine rotation, it is NOT uncommon to require twice that number of amperes. The power rating of the most powerful of the three Bosch units used on the Airheads was 0.7 KW. 0.7Kw is 700 watts; divided by 12 volts is 58 amperes. Notice that the starter is also rated at 320 amperes. That is the supposed maximum drain under a severe load. That is equivalent to 3,840 watts….and is equivalent to a bit over FIVE horsepower. The battery, cold day, thick oil, so-so starter, ETC., may need to provide a LOT of power! If the battery is marginal, it may not have enough power to cause the starter motor to rotate the engine properly for starting ….possibly not rotate it at all.
There must be a means of switching on and off the large amount of electrical current to the starter. A heavy-duty solenoid-operated switch is physically located on the starter motor itself. It is a fairly large round cylinder with two electric terminals of the bolt/threads type and one small spade type which supplies a modest amount of electricity to the solenoid from the starter relay, located along the backbone of the motorcycle.