Monitor electrical system: Readings, sensors, displays

An electrical system that cannot be measured cannot be designed. This section explains what parameters can be meaningfully recorded, at what effort – and how to tell when a display is not showing reality. You can start anywhere.

What can be meaningfully measured

Five parameters drive decisions in the electrical system: voltage, current, state of charge, tank level, and temperature. They differ significantly in how easy they are to acquire and how reliable they are.

Voltage is trivial to measure and yet the most frequently overestimated parameter. It tells something about the state, but only at rest, only at a known temperature, and only for lead-acid batteries. Current is the most honest parameter: it shows in real time what the electrical system is doing. State of charge is not a measured parameter but a calculated result – which is why it needs maintenance. And tank level is mechanically the most complex because every tank is built differently.

Rule of thumb: Anything derived from a single voltage is an estimate. Anything that runs through a shunt is a measurement.

Measuring Current: The Shunt

A shunt is a calibrated resistor with a very small value, inserted into the current path. The voltage drop across it is proportional to the current. In analog versions, this drop is fed via thin measurement leads to the display device; common designs drop 50 or 60 millivolts at their nominal current. Digital shunts evaluate directly at the measurement point and transmit finished values via a bus system – without interference-prone measurement leads and with galvanically isolated connections, allowing a separate battery bank to be monitored.

In both cases, what matters is not the shunt itself, but its position. It must be placed so that every current flows through it: both the current drawn by consumers and the current supplied by the alternator, charger, and solar regulator. Therefore, exactly one cable should be connected to the battery's negative terminal – the one to the shunt. Everything else connects to the load side. If even one ground wire is still connected directly to the battery, this current will be permanently missing from the balance, and the counter will slowly but inevitably become inaccurate.

The second design question is the lower measurement limit. If you want to find the quiescent current of the electrical system – i.e., the value that drains a battery bank over weeks – you need a resolution down to the milliampere range. Devices that only start evaluating above almost one ampere will simply show zero in this case.

NoteFor analog shunts, the measurement circuit is tapped separately from the load circuit, at its own small terminals. This four-wire technique ensures that contact resistances of the thick connections do not falsify the measurement result. If the measurement tap is connected to the load terminals, contact resistance will also be measured.

From Current to State of Charge

A battery monitor does not measure the state of charge; it calculates it. It continuously counts the ampere-hours flowing in and out and compares the result with the set capacity. This is an integral over time – and every integral accumulates errors.

Three settings determine the quality of this calculation. The capacity must correspond to the actually installed battery bank, not the planned one. The Peukert exponent reflects that lead-acid batteries deliver less capacity at high currents; for lead-acid, it's typically between 1.1 and 1.25 depending on the type, and for LiFePO4, it's close to 1.0. And the charge efficiency accounts for the fact that more goes in during charging than comes out later – about 90 percent for lead-acid, and about 99 percent for LiFePO4.

Only one thing helps against drift: synchronization. If the monitor detects a full charge – charging voltage reached, charging current fallen below a small residual value, both stable for a minimum time – it resets the counter to 100 percent. A battery bank that is not fully charged for weeks will therefore inevitably have an inaccurate display. This is not a device fault.

Measuring Tank Levels

For tanks, it's not the electronics but the design that matters. A flat, irregularly shaped tank behaves completely differently from a tall cylindrical one, and every measurement method has a weakness that becomes apparent precisely there.

Two points are regularly overlooked: ultrasonic sensors have a dead zone of a few centimeters directly below them where they cannot measure – a spacer ring creates clearance here. And pressure probes need to know whether they are measuring water or diesel, because diesel is significantly lighter and the same fill height generates lower pressure.

Display and sensor must match. Several resistance ranges are common in the market, and a sensor connected to the wrong instrument will show something, but nothing accurate. If multiple tanks with different methods are to be shown on one display, an interface solves this more cleanly than trying to install the same sensor type everywhere. For holding tanks, there is an additional legal requirement: the three-quarters full level must be monitored and reported.

Leakage Current: The Current That Doesn't Return

All previously mentioned parameters describe an electrical system that works. Leakage current describes one that is "leaking" somewhere: part of the current does not return via the ground wire, but via the hull and the surrounding water.

This goes unnoticed in the electrical system – nothing switches off, nothing gets warm. It becomes noticeable on the anodes, which are consumed after one season instead of three, and later on components that no anode protects. The underlying process is electrolytic corrosion, and it occurs orders of magnitude faster than galvanic corrosion, for which anodes are designed at all.

Measurement is done with a dedicated shunt in the connection between the electrical system's negative terminal and the hull. More important than the instantaneous value is the stored peak value: most leakage currents occur only temporarily – when a specific consumer is running, or when moisture gets to a certain spot. If you only look at it occasionally, you usually see nothing.

WarningLeakage current in the DC network, galvanic corrosion via shore power connection, and an insulation fault in the 230 V network are three different problems with three different solutions. Leakage current monitoring in the DC electrical system does not replace either the circuit breaker in the AC circuit or a galvanic isolator.

Alarms and Thresholds

An alarm is only valuable if it leads to action. Anything else will be dismissed after two weeks and then ignored. Therefore, the rule is: few thresholds, but set correctly.

For state of charge, the threshold is based on technology. Lead-acid should not be discharged below half, which is where the warning belongs. LiFePO4 can tolerate significantly more; here, a threshold at about 20 percent is reasonable – early enough to calmly start a charging source. Voltage alarms are the second level, not the first: they act if the meter is inaccurate.

Where a notification alone is not enough, a shutdown takes its place. Deep discharge protection disconnects the electrical system or the DC input of an inverter before the battery bank is damaged, and automatically reconnects after recharging. This is the right answer wherever no one can react to an alarm.

WarningA temperature alarm on a LiFePO4 battery bank is not a convenience feature. If the cell temperature falls below the manufacturer's specified limit, charging must stop – even from solar or alternator. Without a sensor and a threshold, no one notices, and the damage is not externally visible.

Display and Transmission of Values

Where the values appear is a question of use. A round instrument on the switch panel can be read in passing and does not require a start-up process. A touch monitor bundles batteries and tanks on one surface and displays trends. And for those who want the values on a mobile device or an existing display, they are transmitted via a bus system instead of installing a second display.

For design purposes, it is important that the measurement point exists only once. A second shunt for a second display device is superfluous and creates two truths that diverge. It is more sensible to have one measurement point whose values are forwarded – to a secondary monitor, to an existing chart plotter or vehicle display, or to a mobile device.

The same idea applies to additional measurement points. If you want to know what a single charging source delivers or what a single large consumer draws, you place another measurement point in its negative line there. This is not a second battery monitor, but an additional line in the same energy balance.

Typical Errors

• Ground at the wrong point. A single wire still connected directly to the battery negative makes the entire ampere-hour balance unusable.
• Capacity set incorrectly. The monitor calculates with the entered value, not the installed one. Reset after every battery change.
• Never synchronized. Without regular full charging, the display drifts. Those who rarely fully charge need a conscious schedule for it.
• Voltage read as state of charge. With lithium, the discharge curve is so flat that voltage carries practically no information.
• Sensor and display do not match. Different resistance ranges result in a display that looks plausible but is still wrong.
• Anodes enlarged instead of measuring leakage current. Treats the symptom and lets the cause continue.
• Too many alarms. Warning for every event trains the crew to disregard warnings.

What the Standard Requires

ISO 13297:2020 makes no specifications for measurement technology itself – it regulates the electrical installation, not the display. However, it becomes relevant in two places: the shunt is in the main current path and must be secured, protected, and fused accordingly like any other point in this line. And measurement leads running from the battery to the display are separate circuits and require their own small fuse as close as possible to the tap point. A thin, unfused measurement cable connected to battery voltage is a fire hazard.

For holding tanks, monitoring the three-quarters full level is mandatory. This is not a comfort issue but a requirement that is checked during inspection.