Starter Current
Posted: Wed Apr 23, 2025 8:49 pm
Ever wonder how much current the starter actually draws when you crank the engine?
Below is a chart I made shortly after installing an EarthX ETX900 battery.
The recording was done at 1000 Hz. At this speed many interesting details are revealed
The initial ripple in current is the pulsating of the Facet fuel pump. The next jump to about 45A is the solenoid on the starter that drives the gear forward. This is followed by a 728A peak once the contactor on the starter essentially creates a short. Note the battery voltage of 7.9V and the starter voltage of 4.27V!
As the engine starts to turn, current drops quickly and one can see the compression strokes go by.
An interesting byproduct of using permanent magnet motors is that they become a generator when spinning. After the starter is disengaged one can see the starter motor spinning down by looking at the output voltage it is producing.
Below is a chart I made shortly after installing an EarthX ETX900 battery.
The recording was done at 1000 Hz. At this speed many interesting details are revealed
The initial ripple in current is the pulsating of the Facet fuel pump. The next jump to about 45A is the solenoid on the starter that drives the gear forward. This is followed by a 728A peak once the contactor on the starter essentially creates a short. Note the battery voltage of 7.9V and the starter voltage of 4.27V!
As the engine starts to turn, current drops quickly and one can see the compression strokes go by.
An interesting byproduct of using permanent magnet motors is that they become a generator when spinning. After the starter is disengaged one can see the starter motor spinning down by looking at the output voltage it is producing.