Yes use the isolated charger, if it fails internally the current and voltage from the lithuim will be seen on the AGM.
DC Grounding
Should the DC system remain floating (ungrounded)?
Recommendation: On a steel hull, the DC negative should generally be bonded to the hull at a single point. This avoids accidental grounding through equipment, which can lead to corrosion. A floating system may inadvertently ground through components like the engine block, creating uncontrolled paths.
Use sacrificial anodes (e.g., zinc, aluminum, or magnesium depending on water type) on the hull to protect against galvanic corrosion.
Ensure that grounding is deliberate, with a single bonding point to the hull to minimize stray currents.
Where to establish the grounding point?
Preferred Option: Bond the DC negative bus to the hull at a single, intentional point. Ensure this point is clean, unpainted, and in a central location near the distribution panel.
If using a submerged plate, ensure it is not isolated from the hull but bonded properly to provide a consistent grounding path.
AC Grounding
Reconciling AC and DC ground systems:
Standard Practice: Tie the AC ground to the DC negative bus, which should also be bonded to the hull. This complies with ABYC regulations and ensures a fault path to water via the hull.
Use an isolation transformer or galvanic isolator in the AC system to prevent stray currents between the shore power ground and the boat’s electrical system.
Safety of marine-rated equipment:
Marine-rated chargers and inverters are designed to minimize risks, but proper grounding and bonding are still necessary. Equipment chassis should be tied to the DC negative bus to provide a fault path.
Bonding
Is bonding unnecessary in a floating system?
Bonding is critical on a steel hull. Even in a “floating” system, there’s a risk of stray currents finding unintended paths through bonded hardware (e.g., thru-hulls or prop shafts). Bond all metal components exposed to seawater (rudders, thru-hulls, etc.) to the common bonding system.
Bond the hull directly to the DC negative bus to ensure it serves as the primary ground plane. Coating the hull with epoxy and antifouling paint minimizes corrosion, but the hull still needs to be electrically bonded.
Sacrificial Anodes: Regularly inspect and replace anodes to ensure they are providing protection.
Galvanic Isolators: Install on the shore power connection to prevent stray AC currents.
Single Ground Point: Maintain one deliberate connection between the hull, DC negative, and bonding system to avoid ground loops.
Inspections and Maintenance: Periodically check all connections and bonding systems for wear or corrosion.
This approach ensures compliance with safety standards like ABYC while minimizing corrosion risks and protecting the boat’s electrical system. If doubts remain, consulting a marine electrician—even remotely—can provide additional assurance.
