DC Distribution Fault on a Small Vessel
A 12 m vessel has two 12 V lead-acid batteries in parallel, a 60 A alternator, and a distribution panel with blade fuses. The owner adds a 600 W inverter and a 40 A DC-DC charger. After the upgrade, the cabin lights dim when the inverter runs, the 100 A main fuse at the battery feels warm, and the voltmeter at the panel reads 11.8 V while a multimeter at the battery reads 12.6 V. The inverter is connected with 16 mm2 cable about 3 m long, and the negative return goes to a nearby hull bonding point rather than the battery negative. The DC-DC charger is wired to the same busbar as the inverter.
1. Interpret the voltage difference between the panel and the battery and explain its likely causes.
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A 0.8 V drop between battery and panel indicates resistance in the supply path or a return-path problem. At inverter currents of tens of amps, even small resistances produce significant drop. Likely causes include undersized or long cable, corroded or loose terminals, a fuse holder with poor contact, and a negative return that runs through the hull bonding system instead of a dedicated cable to the battery negative. The warm 100 A main fuse suggests it is carrying high current and may have a poor connection or be marginal for the combined load. The dimming lights are consistent with a shared supply suffering voltage sag. The panel voltmeter may also be inaccurate, so the multimeter readings should be trusted after verifying the meter. The difference must be diagnosed under load, not at rest, because a high-resistance joint may show near-normal voltage with no current.
2. Assess the inverter cable and return arrangement against good practice, using the given numbers.
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A 600 W inverter at 12 V draws about 50 A continuous, with higher surge. Over 3 m of 16 mm2 cable, the round-trip resistance is roughly 0.0021 ohm, giving about 0.1 V drop at 50 A, which is not the whole 0.8 V but adds to other joints. The bigger concern is the return path. Using a hull bonding point as the inverter negative return is poor practice: it puts high current through bonding conductors, can cause electrolytic corrosion, and creates a path that may not be rated for the current. The inverter negative should run directly to the battery negative or to a dedicated negative busbar with a cable sized for the current and protected appropriately. The 100 A fuse should be checked against the combined maximum demand and the cable rating; it must protect the cable, not merely the inverter. The DC-DC charger sharing the busbar is acceptable only if the busbar and fuse are rated for the total and each branch is individually protected.
3. Describe a load-test sequence to locate the high-resistance point without damaging equipment.
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First, inspect and tighten all terminations with the system isolated. Then reconnect and measure voltage at the battery, at the main fuse, at the busbar, at the panel, and at the inverter input while a known load is applied, such as the inverter driving a resistive load. Measure the voltage difference across each joint and cable segment; a joint showing more than a small fraction of a volt at rated current is suspect. Measure the negative return from inverter case to battery negative under load. Use a clamp meter to confirm current split. Do not use a multimeter in current mode across a supply. If a fuse holder or terminal heats, replace it rather than polishing. After repair, repeat the test and record readings. Confirm the alternator and charger are not exceeding cable ratings during combined charging and inverter load.