The starter motor converts electrical energy into the mechanical torque needed to turn the engine. It draws a very large current for a short period, so the battery, cables, connections and ground path all matter. A starter that turns slowly is often limited by the circuit rather than by the motor itself.
Starter current estimate
Problem. A starter is rated at 1.2 kW on a 12 V system. Estimate its current draw.
Solution. I = P ÷ V = 1200 ÷ 12 = 100 A. Real cranking current is higher during the initial inrush and lower once the engine turns, so this is an order-of-magnitude estimate.
Cable voltage drop
At cranking current, even a small cable resistance produces a significant voltage drop. The voltage available at the starter is the battery voltage minus the drops in the positive cable, the ground cable and the connections.
Cranking voltage at the starter
Problem. A starter draws 100 A through a circuit with a total resistance of 0.02 Ω. What voltage is lost in the circuit?
Solution. V_drop = 100 × 0.02 = 2 V. If the battery holds 12 V during the attempt, the starter sees about 10 V.
- The solenoid engages the pinion and closes the high-current starter contact.
- The control circuit may include a relay and a clutch or neutral safety switch.
- Cable and connection resistance matter most at cranking current.
- A slow crank can be caused by the battery, cables, connections, ground or starter.
- Starter current is high and brief.
- I = P/V gives an order-of-magnitude estimate.
- V_drop = I × R explains slow cranking.
- The solenoid and control circuit must also be checked.
Worked example
A 1.5 kW starter on 12 V draws roughly 125 A. Through a 0.03 Ω path the drop is 3.75 V, so the starter may see only about 8.25 V from a 12 V battery.