Why must the chassis (negative) connection be run separately and thick?
In a DC circuit the return current comes back through the negative line; that is, the negative cable carries as much current as the positive and must be of the same cross-section. Using the sheet-metal body as chassis creates resistance at the contact points. At 12V, half of the total 5% voltage drop budget is often spent on the return path that everyone ignores.
The common mistake is to run the positive line in 6 mm² and connect the negative to the nearest sheet-metal point with a thin cable. Current flows in a loop: if the motor draws 30 A the return path carries 30 A as well. In the voltage drop calculation the formula is ΔV = 2 × L × I × ρ / A, and the leading factor of 2 expresses exactly the sum of the outgoing and returning conductor.
Practical rules:
- Run the negative cable in the same cross-section as the positive and, if possible, along the same route, twisted together.
- Connect the chassis terminal of the driver directly to the battery negative terminal or the main chassis bar; do not use an intermediate point, and do not jump across a hinge or painted sheet metal.
- Clean paint/oxide at the connection point, use a serrated washer and tighten the cable lug.
- Do not share the ground return of the motor power and of the signal/RF circuit over the same thin wire; ripple in the motor current creates a voltage on that wire and shifts the signal reference.
On modules such as the KS250 (15 A continuous) and PT500 (30 A continuous) a weak return path produces both power loss and measurement error: the module sees a low voltage at its supply terminals, the motor runs slowly, the current rises, and the current-threshold AntiSwitch function may trip at the wrong point.
The full topic: DC Motor Driver Selection Guide