Generating energy on board: Alternator, solar, shore power

On-board energy comes from three sources: the engine, the sun, and shore power. Each has different characteristics, different regulations, and different weaknesses. This section explains how to design them individually and then connect them together without them interfering with each other.

The three sources in comparison

The crucial difference lies not in power, but in availability. An alternator delivers a lot of current, but only as long as the engine is running. Solar delivers little current, but every day and without intervention. Shore power delivers unlimited current, but only at the berth.

The line most frequently missing in planning is the first: A standard alternator is built to power the engine — not to fill a large house battery bank.

The alternator

The standard alternator of a diesel engine often delivers only 30 to 40 amps at idle and operates with a simple, usually single-stage regulator. This maintains a fixed voltage and thus effectively stops charging at about 80 percent. If you run the engine for two hours daily and want to charge a 400 Ah battery bank, you will never get there.

A high-performance alternator changes two things simultaneously: It delivers significantly more current, even at low RPMs, and it can be controlled by an external multi-stage regulator. Only this regulator turns the alternator into a full-fledged charger with a characteristic curve, temperature compensation, and a shut-off that matches the battery type.

How large it can be is not determined by desire, but by the battery: Wet cell batteries accept up to about 25 percent of their capacity as charging current, Gel up to 35, standard AGM up to 40, TPPL and carbon foam AGM up to 100 percent. A 300 Ah AGM battery bank is thus suitable for about 120 amps.

Three mechanical points determine whether the conversion will last: the mounting geometry on the engine, the pulley, and the belt type. Above about 100 amps at 12 volts, a single V-belt is no longer sufficient — then double V-belts or serpentine belts are necessary, possibly with a conversion kit.

WarningIf you convert to LiFePO4, you must protect the alternator against load shedding. If the battery management disconnects under load, the battery suddenly disappears from the electrical system, and the resulting voltage spike destroys diodes and the regulator. A protection module between B+ and B- is the simplest safeguard against this; a buffer battery or a charge converter is a more thorough solution.

Connecting two battery circuits

As soon as there is a starter and a house battery bank, the question arises of how both can be charged from the same source without one draining the other. There are three types of construction, and they are not interchangeable.

A battery isolator relay connects both circuits as soon as a charging voltage is present, and disconnects them again when it drops. Simple, inexpensive, low-loss — but both battery banks see the same voltage. This requires that they can tolerate the same charging characteristic curve.

A charge converter electrically decouples the circuits. It takes energy from one circuit and supplies it to the other with its own, adjustable characteristic curve. This is the right answer if the battery banks have different technologies — lead starter, lithium service — or if the charging voltage is no longer sufficient at the end of a long cable.

A battery isolator diode is the oldest variant and costs voltage: It drops across the diode and is missing from the battery. Without compensation at the alternator, both battery banks remain undercharged.

Solar

Solar is the only source that runs without intervention, and therefore the only one that can maintain a battery bank for weeks. It is also the source where calculations are most frequently incorrect.

The nominal power of a module in watt-peak applies under laboratory conditions. In practice, the daily yield in Central European summer is roughly four to five times the nominal power in watt-hours, and a fraction of that in winter. If you plan with the datasheet values, you are planning for half the necessary area.

For the controller, there is only one real question: Is the system large enough or the module voltage high enough for an MPPT controller to be worthwhile? It continuously searches for the operating point with the highest power and extracts significantly more power compared to a simple controller, especially in cold conditions and partial shading.

Shore power

Shore power is the only source where AC voltage comes into play, and thus the only one where an error immediately endangers people. The requirements are accordingly different: residual current protection, defined isolation when switching between sources, and a way of dealing with the protective earth conductor that does not worsen corrosion.

Because that is exactly what happens when the protective earth conductor connects several boats on the same jetty: A path for galvanic corrosion through the water is created. A galvanic isolator or an isolation transformer interrupts this path without eliminating the protective function.

Combining multiple sources

As soon as two sources charge simultaneously, their regulations compete. Each measures the battery voltage and derives from it which charging phase it is in. If one source raises the voltage, the other concludes that the battery is fuller than it actually is, and reduces its output.

In practice, this means that the charge termination voltages of all sources must match each other and the battery type. A source with an outdated lead-acid characteristic curve in a lithium system not only slows down — it prevents the battery bank from ever reaching the synchronization threshold of the battery monitor. Where the regulations communicate with each other via a bus system, the problem solves itself, because then one instance leads the charging.

Typical errors

• Only the battery was replaced. A lithium battery bank behind an unchanged standard alternator will neither be fully nor safely charged.
• Alternator chosen without considering charge acceptance. A large alternator on a wet-cell battery bank does nothing.
• Belt not adjusted. Above 100 amps, a simple V-belt slips, dusts, and breaks.
• No temperature sensor on the alternator. An external regulator without a sensor regulates blindly and can overheat the alternator.
• No protection against load shedding. The failure does not happen gradually, but at the moment the BMS disconnects.
• Solar yield calculated based on nominal power. Regularly results in half the necessary area.
• Charge termination voltages not balanced. Two sources with different characteristic curves inhibit each other.
• Charge converter chosen too large. It then draws more than the alternator and wiring can provide.

Standards and regulations

ISO 13297:2020, in its current edition, covers both sides: DC systems up to 50 V nominal voltage and single-phase AC systems up to 250 V. For the charging side, this primarily means: Every charging source with its own cable to the battery needs protection for this cable, and that close to the battery — not at the device.

On the AC side, residual current protection and the question of switching are added. Two sources must never be switched on simultaneously; the switching must necessarily disconnect before connecting. In North America, ABYC additionally regulates the requirements for charging systems and explicitly requires a battery management system for lithium systems that keeps the battery within its safe operating range.