Getting more from your Alternator.


Most small 1 or 2 cylinder diesel engines (less than c16 hp continuous, as much as you will have in an Achilles yacht) are modified versions of industrial engines, the land version of the Yanmar 2GM20 for instance is extensively used in mini-diggers and Beta marine engines are based on Kubota engines used in their diggers. As such the alternator only needs to recharge a modest sized battery after engine start and support a few lights for night and road work, they may be upgraded for marine use often by simply increasing the alternators speed by changing diameter of the pulleys. 

An 8 or 9hp Yanmar has a 35 Amp alternator, a 2GM20 and the modern 2YM15  has 55 and 60 Amp amp respectively but at least the older models do not have regulators optimised for bulk charging of large battery banks.

As previously noted, another problem to be aware of is the load of a large alternator on the engine, even a small alternator under high load can significantly reduce the available power for propulsion, Sterling Power notes that speed reductions of 1.5 knots have been reported and on their external alternators provide a facility to switch off the external alternator to protect boat speed when required - its not a major issue with my 2GM20 which produces twice the power of the engine originally specified for the Achilles 9 metre, but the "power grab" by the alternator it is noticeable at low revs, for instance when at fast idle engine revs reduce significantly when raising the anchor. 

To enable long term high current charging third parties have taken two approaches, over-riding (usually) or replacing the alternators internal controller with an intelligent micro-processer controlled external regulator or by manipulating the internal regulator to produce more.

In either case the high load will likely trash a standard alternator belt, on mine these would be last a few days after needing to be tightened several times and leaving rubber dust all over the engine compartment. Fitting a premium toothed belt solvers this although early on in its life it will need tightening fairly frequently, mine has now done several hundred hours and is quite stable.

The toothed belt by Gates on my 2GM20.

External regulators

An external regulator takes over from the internal regulator, the down side apart from cost (£100 - c £200) is that the alternator usually has to be taken to pieces and a wire (or two) soldered on to the field brushes, not a difficult process but intimidating and some might want this to be done by a pro adding to the cost.

The normally micro-processor controlled units should: 

  • Work with the existing alternator regulator.
  • Support multiple battery technologies.
  • Support positive and negative field alternators (in Europe c 90% are negative).
  • Use multi step (usually 4, sometimes 5) profiles for efficient, normally constant current, charging whilst protecting the battery.
  • Automatically calculate the battery bank size, charge state, and alternator output, then set the timing sequence (between steps) every time you start the engine.
  • Be fail safe allowing the internal regulator to take over in the event of failure or when disabled.

and preferably:
  • Monitor battery temperature to modify the in use profile to stop the batteries overheating.
  • Monitor alternator temperature and protect it from overheating.
  • Have a built in delay (a few seconds) to allow the engine to stabilise after starting before loading the engine with high output currents.

This will often be the optimal option although it is trickier to fit.

"Manipulating" the existing regulator

A different approach is to  "fool" or manipulate the existing alternator into generating more power them applying it more effectively.

From the Sterling Power "Alternator-to-Battery Charger" installation instructions:

 "In order to maximize the alternator output current, the alternator-to-battery charger pulls the alternator output voltage down to about 13V. Then this low voltage is amplified to a higher voltage suitable for effective battery charging, i.e. 14.1V to 14.8V."

The main benefit is that the alternator does not have to be modified although some cable changes will be required, devices of this type are likely not to be as effective as an external regulator but with a good one the difference may not be significant

They are expensive compared to an external regulator plus diodes or DVSR but can be a viable option compared to a smart splitter + external regulator. 

Continue to Battery Management.

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