Technology: Alternator and lithium batteries – How the engine becomes a power station

​It’s a chilly spring morning in the Stockholm archipelago. The Linjett 36 offers all the comforts of home: fridges, freezers, heating, 230 volts and a whole host of navigation electronics. Over the last 24 hours, we’ve deliberately used a lot of power. The battery monitor shows a charge level of around 70 per cent. The electrical appliances have drawn a full 120 ampere-hours from the lithium battery. Ten minutes later, after a short stretch under engine power through the archipelago, the display already shows just under 40 ampere-hours recharged. That’s roughly the same amount the Linjett would consume in 24 hours without our wasteful ‘let’s switch everything on’ attitude.

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What’s at work down there in the engine compartment is no ordinary alternator. It’s a water-cooled high-performance system developed by the Swedish engineer Dan Kimblad, and it demonstrates what’s possible when you consistently optimise charging via the alternator. The alternator constantly feeds 250 amps into the battery whilst remaining barely more than lukewarm; the rectifier, which is also water-cooled, is housed separately, with the coolant diverted from the engine and passing through its own small heat exchanger. This system represents one end of a broad spectrum. Most owners are familiar with the other end: a standard alternator from the automotive industry, a new lithium battery pack and the quiet hope that it will all somehow work together. More often than not, it simply doesn’t.

Why large lithium deposits create new problems

​Large LiFePO4 battery banks make it possible to do many things on board that were previously only possible when connected to the shore power supply – such as induction cooking, using a washing machine or a dishwasher. Comfort increases, but so does energy consumption. A well-equipped yacht with 400 to 500 ampere-hours of lithium capacity easily consumes 100 to 150 ampere-hours per day whilst travelling. However, the energy must also be fed back into the battery, ideally without relying on shore power. The desire of many owners is obvious: to use the engine running time – which is necessary anyway – as efficiently as possible for charging. This is precisely where the crux of the matter lies. Because the alternator, which is supposed to do this job, is not designed for this specific task.

“Standard alternators in boat engines are almost without exception derived from the automotive sector,” explains Michael Kögel of Philippi, one of Germany’s most experienced manufacturers of on-board electrical components. In a car, the alternator performs a fairly simple task: once the engine has started, it replenishes the approximately five ampere-hours that the starter motor has drawn from the battery. It then supplies the on-board electrical system, which comprises a manageable number of electrical consumers. Everything is optimised for cost-effectiveness.

The ampere rating on the type plate refers to the peak output when the alternator is cold, not the continuous output. Kögel’s comparison hits the nail on the head: it’s like an engine where the redline is at 6,000 revolutions – in practice, you drive at 3,000 to 4,000. The main problem is waste heat. Standard alternators rarely achieve an efficiency of more than 60 per cent. This means that, for every mechanical kilowatt-hour transmitted to the alternator by the belt, at least 40 per cent is converted into heat before even a single ampere reaches the battery. This was already a problem with lead-acid batteries, even though their high internal resistance automatically limited the charging current and protected the alternator from itself.

Why lithium puts the alternator under a constant load

​Anyone switching to lithium batteries is fundamentally changing the operating conditions for the alternator. LiFePO4 batteries have an extremely low internal resistance, regardless of whether the battery pack is charged to ten, fifty or eighty per cent. They draw every last bit of power the system can provide – in some cases for hours on end – until they are almost 95 per cent full. The alternator runs at maximum power from the moment the engine starts. And it remains there for as long as the engine is running. A simple calculation illustrates what this means in thermal terms. An alternator delivering 80 amps at 14.4 volts generates an electrical output of around 1.2 kilowatts. With an efficiency of 60 per cent, the belt must therefore deliver two kilowatts of mechanical power.

The difference of 800 watts is converted into heat within the alternator, and this is precisely where the problem lies. In a standard alternator, a single fan blows air over the stator windings, the rotor and the rectifier diode block at the rear – the most thermally vulnerable part. Under continuous lithium load, the housing temperature quickly rises to over 100 degrees Celsius. In the best-case scenario, the regulator rapidly reduces the alternator’s output.

Of the 80 amps stated on the type plate, perhaps as many as 40 amps will reach the battery after a few minutes. In ten minutes’ running time, you can recharge just under six amp-hours.

For lead-acid batteries, an external high-performance regulator promised an improvement. However, when used with lithium batteries, it does not solve the problem. The external regulator detects that the lithium battery bank is empty and immediately switches the alternator to maximum output – briefly reaching 70 to 80 amps. As the standard alternator is not thermally rated for this sustained load, its temperature rises to over 130 degrees within a short time. The regulator is forced to throttle the output, often down to just 25 to 35 amps – significantly less than the lithium battery could handle.

​The B2B Charger: a simple and often effective solution

To get the temperature issue under control, it is therefore better to combine the standard controller with a battery-to-battery charger. Whilst the B2B charger ensures the correct charging curve for lead-acid batteries, thereby feeding more energy into the battery, it acts as an electronic gatekeeper for lithium batteries: no matter how flat or ‘hungry’ the battery is, it receives only the set current and no more. The alternator runs continuously at 50 to 60 per cent of its rated power and does not overheat. On short journeys of under 15 minutes, the external regulator has the edge. During longer periods of use, however, the advantage is reversed.

An external high-performance regulator requires an alternator designed for continuous operation. Such high-performance alternators from Balmar, Electromaax, Arco Marine or Mastervolt feature thicker copper windings with a higher fill factor, more heat-resistant insulation materials, more robust bearings and often two cooling fans, which significantly increase airflow. In addition, the rectifier is larger and is positioned in a thermally more favourable location. Many high-performance systems consistently relocate it: as an externally mounted remote rectifier, situated in a better-ventilated area of the engine compartment and capable of being cooled separately. The Swedish system from Kimpower takes this the furthest – the alternator and external rectifier share their own water-cooling circuit. The result: a surface temperature of 44 degrees during continuous operation, compared with over 100 degrees in air-cooled systems.

​The belt is often the next bottleneck

At very high power outputs, the belt drive quickly becomes a mechanical bottleneck. A standard V-belt is suitable for alternator outputs of up to around 100 amps. More powerful alternators require not only a sufficiently robust diesel engine, but also V-ribbed belts and new pulleys. For Yanmar, Volvo and a few other manufacturers, there are ready-made conversion kits available from Balmar or Electromaax, for which you should budget around 600 to 900 euros. An important pitfall: if the coolant circulation pump runs on the same belt path, the high belt tension can damage its bearings – a separate belt system for the high-output alternator is therefore the best solution. Newer engines usually come fitted with V-ribbed belts as standard, making the conversion easier.

BMS: A Death Trap

‘Load dump’ is the name given to the phenomenon that occurs when the BMS of a lithium battery pulls the emergency cord and abruptly shuts down the battery, whilst a high charging current is flowing at the same time – whether due to overvoltage, overtemperature or cell faults. The problem lies in the energy stored in the alternator’s magnetic field. This energy can no longer flow back into the battery and must go somewhere. The voltage in the vehicle’s electrical system rises abruptly to as much as 100 volts. The first components to be affected are almost always the alternator’s rectifier diodes, which are destroyed by this voltage spike.

​The cleanest solution is a lithium system with an externally communicating BMS: it notifies the alternator controller in good time before it switches off. Systems from Victron with an external BMS or Mastervolt offer this interface. Anyone opting for drop-in lithium batteries without external BMS communication must incorporate external load dump protection modules, such as those available from Balmar or Sterling Power. A load dump can also occur without lithium batteries, caused by loose cables or the engine’s electrical system switching off whilst the engine is running.

The market ranges from budget to high-end

For conversions to high-performance alternators, the market offers solutions in power ratings ranging from 100 to 250 amperes and at prices between 2,500 and around 8,000 euros. Balmar has been an established name for decades and supplies combinations of high-performance alternators and external regulators tailored to most engines. The same applies to Electromaax, Mastervolt and Arco Marine. Their Zeus and Omega regulators are among the most powerful models available and can also be combined with other alternators.

The ultimate solution – albeit an expensive one – is currently likely to be Kimpower’s small-batch production solution. Thanks to its unrivalled heat dissipation, it can deliver the highest continuous power output.

Solutions at a glance

​The B2B charger: low power, but reliable

The battery-to-battery charger is the simplest solution – and for many owners, the right one – to the problem of charging a lithium battery using the standard alternator. It does not increase the charging power, but it does protect the alternator. On the input side, it provides the alternator with a constant, regulated load – regardless of how depleted or power-hungry the lithium battery bank is. On the output side, it provides a charging curve optimised for LiFePO₄. To prevent the alternator from being overloaded, the input current should not exceed about half the alternator’s rated current. Various manufacturers; cost: 40 A, from 330 euros.

​​Auxiliary alternator: separate and powerful

​Instead of replacing the standard alternator, a second one – typically a high-performance unit – is fitted to the engine. The advantage lies in the complete separation of the systems: the engine’s electrical system remains unaffected. The load bank is charged directly via the auxiliary alternator using its own high-performance regulator. No B2B charger is required. For drop-in lithium batteries without a communicating BMS, an external load-dump protection module should be included in the design. The disadvantage is the additional space required and the necessary mounting bracket, which may need to be custom-made. Cost: 170 A, from 2,500 euros.

​Maximum power: the water-cooled alternator

​Anyone who wants more charging power than the standard alternator with B2B connection can provide should replace it with a high-performance unit. This is designed for continuous load. For common Yanmar and Volvo engines, there are tailored conversion kits available from Arco, Balmar, Electromaax and other manufacturers. Ideally, this alternator charges the service battery directly: the starter battery can be charged via a B2B charger. At the top end of the range is the water-cooled system from Kimpower: developed by the Swede Dan Kimblad, the system uses a specialised alternator and an external, water-cooled rectifier. The result: 220 to 250 amps of continuous charging current at a temperature of around 44 degrees. Kimpower is available as an option on Linjett yachts (surcharge of 7,900 euros); retrofit kits for Yanmar and Volvo engines are available on request. Further information: kimpower.se

​Book recommendations

Theory and Practice of Vehicle Electrical Systems by Jens Feddern covers the vehicle electrical system in its entirety. The focus is on power generation, batteries, charging methods and the distribution of electrical energy on board. This volume provides the technical foundation particularly for readers who wish to understand how the battery, alternator and charging technology interact as a system.

Perfect boat electrics Andy Johnson’s book places greater emphasis on practical on-board installation and the neat layout of electrical systems. The overall architecture of the power supply is also taken into account. Anyone wishing to move from a specific problem with the alternator to a broader understanding of a boat’s electrical system will find a more comprehensive practical framework here.

​A large lithium battery bank, plenty of comfort, high expectations: is the on-board alternator the most underestimated bottleneck in many conversion projects? Join the discussion in the comments.