The consumer is running weaker than it should, while the battery voltage appears good. The difference between the two is the voltage drop – and it's not evenly distributed, but accumulates where there's contact resistance. It's found by measuring under load, section by section.
The one rule without which nothing works
WarningNo current flows without a load, and no voltage drop occurs without current. A wire that is three-quarters severed shows the same value at idle as a perfect one. Every measurement for troubleshooting must be taken under operating current.
The physical background is a single line: the drop across a point is the current flowing there multiplied by its resistance. A corroded terminal with 20 milliohms will drop by 0.02 volts at 1 ampere – imperceptible. At 30 amperes, it's 0.6 volts, and the pump runs audibly slower.
This also determines which consumer is suitable for measurement: the one that draws the highest current. Anchor winch, bow thruster, water pump, inverter under load. The greater the current, the clearer the fault location will be.
What is still normal
An onboard electrical system is designed so that the total drop from the power source to the consumer remains within a defined range. Typically, three percent of the nominal voltage is considered for sensitive circuits and a maximum of ten percent for non-critical ones.
| Onboard Electrical System | 3% — Target for electronics, lighting, charging lines | 10% — Upper limit for non-critical consumers |
|---|---|---|
| 12 V | 0.36 V | 1.2 V |
| 24 V | 0.72 V | 2.4 V |
These values apply to the entire circuit, supply and return lines combined. Mathematically, this means almost nothing for a single terminal point — more than about 50 millivolts across a clean connection is noticeable.
The procedure in six steps
- Take a reference. Measure the voltage directly at the battery terminals while the consumer is running. This is the baseline value.
- Determine total drop. Measure the voltage directly at the consumer's terminals, under load. The difference from the reference is the total drop.
- Halve. Find a point approximately in the middle of the circuit – main switch, busbar, fuse holder – and measure there. This clarifies which half the fault is in.
- Separate positive and negative. Measure each against the battery terminal on the same side. This shows whether the drop is in the supply or return line.
- Measure across connection points. One probe before, one after each terminal, stud, switch, fuse. This reveals the exact spot.
- Confirm. Repair the spot, measure again under load, record the value.
Halving in step 3 is the core of the procedure. Instead of checking twenty points individually, each measurement halves the remaining distance – for twenty connections, that's five measurements instead of twenty.
Suspect the negative side first
Experience shows that a large proportion of all voltage drops occur in the return line. The reason is historical: the positive side is deliberately routed and fused, while the negative side is often connected "somewhere" - to an existing bolt, to a sheet metal, to the nearest consumer.
Two patterns occur particularly frequently. First, the ground as a chain: consumer A passes its ground to B, and B to C. The current of all three then flows through the first connection, and whatever drops there is missing from all subsequent ones. Second, the common ground point in a damp environment – bilge, engine compartment, stern area – which corrodes silently over years.
NoteThe clean design is the star point: a central busbar from which each consumer gets its own return line. Anyone working on the distribution during a refit will solve a whole class of future problems this way.
Common locations
- Battery terminals. Too loose, oxidized, or too many cable lugs stacked on one post.
- Main switch. Internal contacts burn out over years – nothing visible from the outside.
- Fuse holders. Spring contacts weaken, blade fuses sit loosely.
- Crimped cable lugs. Made with pliers instead of a crimping tool, they are mechanically and electrically unsound.
- Clamping points under tension. Where a wire pulls, it moves out of the clamp.
- Connectors in humid environments. Especially those without seals.
- Too small a cross-section over a long distance. Not a fault, but a design flaw – the drop is then distributed evenly rather than locally.
This last distinction is useful: a drop distributed evenly over the length indicates too small a cross-section, while a sudden drop indicates a poor connection point. Both require different measures.
Heat as a second indicator
Where voltage drops, heat is generated. After ten minutes under load, terminal points can be checked by hand or with an infrared thermometer – a noticeably warmer spot is a hit, even without a measuring device.
This applies in both directions: a connection that gets hot during operation is not only a performance problem, but also a fire hazard. It should be repaired, not merely observed.
After the repair
Finally, the same measurement should be taken again, under the same load, with the result recorded. Two reasons: Firstly, it proves that the measure was effective and not just a shift of the problem. Secondly, it provides a reference value that can be used for comparison years later.
If you want to keep a close eye on the measured values, you can't avoid a permanently installed current measurement — a shunt in the negative line of the battery bank makes currents visible that would otherwise only be noticed during targeted searching.
Typical errors
- Measured without load. The most common mistake of all — the measurement then says nothing.
- Only the positive side checked. The return line is at least as often the cause.
- Measured at the wrong point. Measurement is taken at the device's terminal, not at the cable lug before it.
- Everything replaced simultaneously. Afterwards, nobody knows what the cause was.
- Warm terminal only tightened. Where corrosion once existed, tightening rarely helps permanently.
- No reference value recorded. The next search will start from scratch.
