An old system usually has a voltmeter and nothing else. This allows you to see that something is wrong, but not what. The first and most important step in retrofitting is therefore always the same: a shunt in the negative line of the house battery bank.
Why voltage alone is not enough
The resting voltage of a lead-acid battery gives a rough indication of the state of charge – but only when at rest, i.e., after hours without load and without charging. In operation, it fluctuates with every consumer that switches on and off.
With lithium, the informative value is even lower: the discharge curve is almost flat over long stretches. Between 80 and 20 percent state of charge, there are only a few tenths of a volt. Anyone who tries to deduce the remaining runtime from this is guessing.
The alternative is balancing: a shunt measures the current flowing into and out of the battery bank, and a calculator counts amp-hours. This provides the values that are truly helpful in operation – state of charge in percent, amp-hours consumed, remaining runtime at current consumption.
The order of retrofitting
| Stage | What is added | What you know afterwards |
|---|---|---|
| 1 | Shunt and monitor on the house battery bank | State of charge, consumption, remaining runtime |
| 2 | Measurement point on the starter battery | whether the second battery bank is also being charged |
| 3 | Measurement points at individual sources | what alternator, solar, charger actually deliver |
| 4 | Measurement points at large consumers | where the consumption goes |
| 5 | Residual current monitoring | whether current takes unintended paths |
| 6 | Tanks, temperatures, remote access | Comfort and monitoring during downtime |
Stage 1 provides the greatest gain in knowledge and costs the least. Everything else answers questions that only arise once Stage 1 is in place.
Where the shunt must be located
The shunt belongs in the negative line of the battery bank, and in such a way that all current flows through it – all consumers and all charging sources. Nothing must be connected between battery negative and the shunt.
This is where most retrofits fail. In an established system, several cables are often connected to the negative side of the battery: the main ground, the engine, a single consumer that someone has directly connected. Everything that remains on the battery terminal instead of moving behind the shunt is missing from the balance – and the display drifts.
WarningA single consumer that remains connected past the battery negative makes the entire state of charge display unusable. During installation, every negative cable must be individually checked and reconnected.
Practically, this means: only one connection remains at the battery terminal – the one to the shunt. On the other side of the shunt is the bus bar, to which everything else is connected. The starter current usually does not run through the consumer shunt, because the starter battery has its own ground.
Setup and synchronization
After installation, the device needs three pieces of information: nominal capacity of the battery bank, battery type or charge termination voltage, and the value of the shunt used. After that, it needs to be synchronized once – the counting needs a known starting point.
Synchronization is done with a full battery bank: charge until the charge termination voltage is reached and the charging current has dropped to the typical residual current, then set the counter to 100 percent. If this never happens, the display drifts – not because the device measures poorly, but because it lacks a reference point.
This is also the first useful insight from the new measurement technology: many systems do not even reach full charge in normal operation. Anyone who sees this knows that something needs to be done about the charging sources – before, it was just a suspicion.
Retrofitting a system without a bus
Not every retrofit requires a bus system. For a manageable system, a standalone battery monitor with shunt is sufficient: one device, one display, two wires. Where tank levels also need to be displayed, there are combined battery and tank monitors that consolidate both on one display.
Switching to a bus is worthwhile as soon as multiple battery banks, multiple measurement points, displays in different locations, or connection to a chartplotter are involved. However, the decision should not be made at the beginning: a shunt on the house battery bank is the first step in both worlds and never wrong.
Leakage current: the monitoring no one plans for
A current that does not flow back via the intended return line, but via the hull, water, or casing, is not noticed in any normal balance. It only becomes apparent over years – as corrosion on metal parts, as a slowly draining battery bank, as an anode that is used up remarkably quickly.
A leakage current monitor compares forward and return current and reports the difference. It is not necessary in every system, but in any where corrosion or inexplicable consumption has been an issue.
Practical installation tips
- Timing. The shunt should be installed while the battery bank is accessible anyway – doing it later means dismantling everything again.
- Location. Dry, accessible, mechanically secure. The shunt carries the entire current of the system.
- Fusing. The thin measurement line to the monitor needs its own small fuse near the tapping point.
- Labeling. Note what is connected where on the shunt and the bus bar.
- Record initial values. After commissioning, record a normal day once – this will be the reference for later.
Typical mistakes
- Consumers bypassing the shunt. A single forgotten negative cable makes the display worthless.
- Never synchronized. Without a full charge as a reference point, the state of charge drifts.
- Incorrect capacity set. The percentage display is then mathematically correct but practically wrong.
- Unfused measurement line. A thin wire directly at the positive pole is a fire hazard.
- Started with Stage 6. Tank display and app are pleasant, but do not answer the question of the state of charge.
- Shunt planned too late. After closing the battery box, installation costs many times more.
