Piper Boat Owners' Club

A club open to anyone who owns a Piper-built boat



This two part series is intended to create an awareness of the various forms of battery charging options that are available to boaters and to compare the relative costs of installation, maintenance, running and the flexibility (portability) of each option. The article is not intended to be definitive or prescriptive but Part 2 will hopefully enable boaters to perhaps consider some alternative forms of charging the boat’s domestic batteries that can be the source of considerable frustration when the lights go dim.


The first and obvious consideration is to ensure that the domestic batteries are in good condition because no matter how efficient or reliable the charging system is, no amount of charging will compensate for damaged or worn out battery cells.


Part 1 will consider the relative merits of using the boat’s main engine alternator with a battery management system and a portable generator. Part 2 will explore the use of ‘free’ energy using a wind turbine or solar panels and will include a comparison of the likely costs (at January ’05) for all the options considered. This article does not seek to imply that one method is better than the other but attempts to examine the various options in a balanced and objective manner and present the results accordingly. I have drawn on personal experience for some of the information and have spoken to other boaters for their experiences with other options.





All boat engines are fitted with a battery charging alternator that is primarily intended to charge the engine starting battery and when this has reached it’s full capacity it will then charge the domestic batteries; the separation of charging priorities is achieved either by use of a changeover relay or by the use of splitter diodes. This system is fine and reliable whilst the boat is cruising but what happens when you are away from your home mooring, the engine fails or your home mooring does not have a mains electricity supply?


You will all have seen at various festivals, and popular mooring places, boats with their engines running either in gear (BW say not allowed - but boat hire companies have not yet heard about this) or at fast no-load revs just to charge batteries.  This engine could be anything up to 45hp plus and is only being used to power an alternator that absorbs a small fraction of the available power.  For example a nominal 12Volt 70Amp alternator will require around 1.3hp (assuming an alternator efficiency of around 95%) to drive it at maximum output, but batteries do not require the full alternator output as their charge and terminal voltage increases towards full value.  So what, you may say – the fuel to drive the engine is readily available and not too expensive – yet, true but what about the hidden costs?  The long term hidden costs of regularly using this method of battery charging can be considerable as diesel engines are intended to be worked hard to ensure that the pistons and rings fully expand thereby producing a good seal to prevent excessive contamination and fuel dilution of the lubricating oil leading to premature engine wear. The other detrimental effects are fuel injection nozzle contamination and glazed cylinder bores producing excessive smoke and carbon in the exhaust.  Diesel oil (gas oil) is just that – a light oil, and this requires a high engine cylinder temperature and good fuel atomisation to ensure complete combustion.  The downside to this method of battery charging is a certain increase in maintenance costs and replacement parts.


In August 1997 (before we bought our Piper boat) we hired Maple from Middlewich Narrowboats (Lister Engine) and the engine exhaust was so full of carbon and smoke you would have thought that we were burning coal.  This I suspect was due to excessive no-load, or light load, running of the engine over the hire season.



For this method of charging the batteries we will consider the popular air cooled petrol engine driven units such as Honda, Yamaha, Kawasaki etc producing up to 1kW @ 

230 Volts at the mains power terminals and with a separate 12 Volt output for charging ‘small’ batteries.  There are other ‘portable’ generators with a greater output but these are not really necessary for the uses described here and would take up too much room.


To take full advantage of this option for battery charging you will need to install a mains battery charger on the boat so that it is only a matter of plugging in the charger to the generator’s 230 Volt socket, starting the generator, switching on and away you go.  Sounds simple, but we have all seen some poor soul pulling away at the starter cord of the generator engine as if they were using a saw, and questioning the parentage of the engine.  This can be a problem with portable generators but can be minimised with good maintenance, clean fuel, regular running and by storing the generator in dry conditions to avoid problems with the ignition electrics. The storage of a spare petrol container on a boat can be problematical in order to avoid conflict with the BSS rules which state that petrol has to be treated the same as bottled gas due to the volatility, heavy vapours and it’s highly combustible nature. The fuel tank on the generator that contains around 1-2 litres of petrol is another consideration in relation to the BSS.


Assuming that the fuel storage problems can be overcome, the advantages of the portable generator are that it can also be used to power other small items such as power tools, portable lights etc – especially useful on illuminated boat processions.  The selection of the battery charger and the generator have to be carefully considered as the portable generator mains waveform may not be compatible with some modern electronic battery chargers.  One disadvantage of this option is its high initial cost especially if a separate battery charger has also to be purchased. The usual 12 Volt output of the generator is around 8 Amps and that would take too long to charge a big bank of domestic batteries after a heavy discharge. A modern, mains powered, electronically controlled 12 Volt battery charger of 40/50 Amps output (with a battery management system) would cope with this duty easily and require around 510/630 Watts continuous output (allowing for efficiency losses) from the portable generator. Again the output from the generator will decrease as the battery terminal voltage reaches its full value; but as the engine is only developing around 1.6 -1.8hp with the generator at full output, this gradual reduction in power output is not so significant as it is with a larger engine.


For short term battery charging use the running costs of a portable generator are not too great (even though the fuel costs are rapacious) as it will only be used occasionally whilst away from the home mooring. This again can count against the portable generator as it represents a significant amount of money lying around doing nothing most of the time.  However, as previously noted, the generator also has other possible uses.


As a running generator needs to be sited outside of the boat, wet weather is another consideration in relation to mains voltage plugs and sockets. In wet weather when it is necessary to use the generator you will need to provide a shelter covering the electrical side of the generator without interfering with the engine air inlet or exhaust outlet.  Those people with a cruiser stern are fortunate in this respect as they can have a purpose designed enclosure incorporated on the back deck that will serve to house the generator and can be designed as part of the seating.


Peter Fairhurst - Spring 2005

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