Advantages of grounding to the start battery
Enhanced Reliability
Grounding to the start battery creates a dedicated pathway for the auxiliary battery system, isolating it from potential interference or disruptions in other vehicle systems. This isolation enhances the reliability and stability of both your auxiliary battery system and vehicle's electrical systems.
Minimized Resistance
Connecting the auxiliary battery system directly to the start battery significantly reduces resistance in the grounding circuit, resulting in improved performance and efficiency.
Why you shouldn't use the frame as a ground point
Increased Resistance
Vehicle frames are typically made from steel which is not an efficient conductor compared to copper. The electrical connection between the frame and body to the negative start battery terminal is not always sufficient to carry the large amount of current a typical DC-DC charger will draw from the vehicle's electrical system. This results in electrical resistance, voltage drop, and in severe cases, damage to the vehicle or electrical system. Newer vehicles may also use adhesives instead of welds which further reduces the frame's effectiveness as a ground point.
Potential Interference
The frame is shared with multiple electrical circuits, some of which are very sensitive to interference like sensors and the engine control unit. Grounding the auxiliary battery system to the frame may introduce additional electrical interference, affecting the reliability or performance of sensitive electronic devices such as sensors and radios.
Corrosion and Rust
Off-road conditions expose the vehicle's frame to harsh environments, including mud, water, and various contaminants. Over time, this can lead to corrosion and rust on the frame, negatively impacting the grounding connection and overall system performance.
Read your manual
Your vehicle's manual may have information on how to connect accessories to the vehicle's electrical system. If available, be sure to read and follow any manufacturer's recommendations or instructions.