In this chapter, the standard charger design will be discussed. This design is a `general purpose' design that will do for a wide variety of situations. To keep it simple, it is meant for charging one battery at a time. For more extreme situations or if a higher power output is needed (so that it can charge a number of batteries in one go), the design can be adapted, see annex B.

From a technical point of view, it would be best to make chargers `custom-built'. Then charger design can be adapted to the head at the site, the alternator type that is built into it and to the power output users want from it. However, sticking to one standard design has advantages:

• Then all runner and nozzle dimensions are fixed. In itself, calculating what would be the optimum dimensions is not difficult but there is always a risk of making errors and people who are capable of building the charger, might become discouraged by having to make calculations.
• If there is just one design, it is cheap to keep all parts in stock and one could even have some complete chargers in stock. This makes that broken chargers can be repaired or exchanged faster.
• The charger will have a higher second-hand value if it can be used at a variety of sites.
• There is no need to measure the head at a site before building the charger.

In designing the turbine for the charger, a compromise must be sought between two conflicting demands:

• The runner should be so big that it can accomodate the flow that is needed to generate the desired electrical power.
• The runner diameter should be so small that its speed will be high enough for the alternator.

The head at which the charger will operate, also plays a role in this:

• At a low head, an even larger flow is needed to generate the same electrical power.
• A low head also means that the water flows less fast through the turbine so an even smaller runner diameter is needed in order to reach the minimum speed of the alternator.

Below a certain head, no such compromise can be found any more. Then one could build a charger with a V-belt transmission so that the runner can be built big enough to accomodate the flow while the transmission makes that the speed of the alternator will be high enough. This is not an attractive option because the V-belt transmission will make the charger more expensive and more difficult to build.

Fig. 4.1: The standard charger: Not a big thing. The shield around the runner is not fitted yet.

See annex B.2 for a further explanation of the principles behind designing a firefly charger. Here only the standard charger design and its characteristics will be discussed.

Important dimensions for the standard charger are:

From these figures, the characteristics of the standard design charger can be calculated (see annex B.2.5 for how to make those calculations). The results are given in fig. 4.3 and some of them are summarized in table 4.1.

The table shows that the standard charger needs a head of at least 5 m to generate 100 W or more. Just 100 W electrical power is a bit low, but still high enough to charge even quite large batteries within daytime only. Also optimum runner speed is low but still well above 1000 RPM, which is the absolute minimum speed for most alternators. For 3 m head, the standard design will not do, since both electrical power and runner speed are too low, see annex B.3 for a design adapted to low head.

The table also shows that when head increases, electrical power increases rapidly. In fact, power output becomes too high for charging just one battery at a time once the head is higher than 7 m. At 7 m head, electrical power is 165 W, which means that the charging current will be 11.8 A (11.8 A times 14 V makes 165 W). This is already quite a high charging current for small car batteries, see annex C `more about batteries'.

In order to keep the power output at around 165 W even when head increases above 7 m, the flow through the charger must be reduced. This could be done by fitting a less wide nozzle, but one could also use one size nozzle for all conditions, and when necessary: Fit a piece of timber inside the nozzle that will partially block it: A blocking timber. Then only the blocking timber has to be made for the head of a specific site, with its width adapted to the head of that site. Or one could bring a set of blocking timbers with different widths and choose the one that is closest to the desired width.

Fig. 4.2: The charger seen from below. The amount of water coming through it is quite impressive.

The table shows that now flow decreases as the head increases further and the blocking timber is made wider and wider. This means that at a high head, the charger could do with a relatively small water source.

Another important characteristic of the runner design is the maximum allowable head. This is the maximum head at which the charger can operate without risking that eventually blades will break out: Blade Maximum thickness: allowable head: 1.25 mm 15.6 m 1.00 mm 12.5 m In fact the quality of the soldering of the blades is more important than blade thickness itself, see box 4.10. So if soldering is questionable, it is adviseable to stay well below these limits.

The above values for maximum allowable head are valid in case no blocking timber is installed. With a blocking timber that blocks at least half the nozzle, forces on the blades are reduced so much that a much higher head is acceptable. When head is 11 m or above, the blocking timber should be made half the nozzle width or wider. This means that with the correct blocking timber installed, the strengh of the blades poses virtually no limit to the head, even with the 1 mm blades.

However, the seal around the shaft does put a limit to the maximum head. When head is very high, the speed becomes so high that the alternator fan could suck air with water droplets through the seal. It is hard to predict until what head the seal will work satisfactorily since it depends on the shape of the pulley and fan of the alternator. For the time being, it seems safe to assume that this charger design can be used up to a head of 25 m when the correct blocking timber is installed and the seal is made carefully according to the design. The calculation of electrical power output given in the table and fig. 4.3 is based on an estimated overall efficiency of higher and consequently power 0.30. This is a safe estimate, so usually efficiency will be a bit output will be higher than expectedinefficient alternator, efficiencies of up . For a charger fitted with a rather to achieveable for chargers fitted with high 0.38 have been recorded and an efficiency of up to 0.45 seems capacity, efficient alternators running under favourable conditionsseems a pleasant surprise if the charger . It turns out to produce more power than expected, but it could also cause problems: If the turbine part is built neatly, efficiency might come close to efficient, high capacity alternator is used, 0.40 and if also an highly overall efficiency could reach output will be 1.5 times higher than given 0.45, meaning that electrical power in current would be too high for just one fig. 4.3 and the table above. Then fuses might blow and charging battery. Of course the solution is simple: Fit a larger blocking timber.

Fig. 4.3: Characteristics of standard charger.

To keep costs low, the charger is designed for fitting a second-hand car alternator. Many different alternator types exists and it is no use to recommend one type: It might not be available, while other types that are just as good, are. Alternator types could differ with respect to:

1. Their manufacturer, meaning that an alternator type from one manufacturer could be exchanged for a type from another manufacturer without problems.
2. Physical dimensions, of the attachment points, pulley etc.
3. Electrical characteristics like rated capacity (= maximum charging current), starting speed, characteristics that influence its efficiency.
4. The electrical circuit inside.

With respect to their suitability for use in a firefly charger, the following things are relevant:

1. Clearly the manufacturer is not important as long as the characteristics are O.K.
2. The design of the frame and seal can be adapted to a wide variety of alternator types.
3. The electrical characteristics are relevant, as they determine how well the charger will perform, but they are not critical. Even with an alternator that is less suitable, the charger will probably function satisfactorily.
   An exeption to this is the starting speed: Nowadays modern, very compact alternator types exist that probably have a too high starting speed for using in a firefly charger.
   Of course the alternator should be a 12 V type since the whole system works at 12 V.
4. In the main text of this book, only one electrical circuit will be discussed: The one drawn in fig. 4.25. For an alternator to fit into this circuit, it should have the following characteristics:
   • No internal voltage regulator, so an external voltage regulator should be bought separately, to be fitted onto the switchboard.
   • One of the brushes is connected to earth and the other one to an outside connection called `F', `DF' or `EXC'.

   For checking whether alternators have such a circuit and for modifying the circuit of alternators with a different electrical circuit inside, see annex A.

Generally speaking, older alternator types of Japanese brands (Hitachi, Nippon-Denson and Mitsubishi) have the right electrical circuit. Also older types of some europese brands (Bosch, Ducellier, Paris-rhone and Fiat) have an electrical circuit that can be used without problems. However, not all alternator types of these brands will have this circuit and to find out, one could look at the codes that identify the external connections:

• Hitachi: `E', `A', `F', `N'
• Nippon-Denso and Mitsubishi: `E'` `B', `F', `N'
• Bosch: `D-', `B+', `DF', `D+'
• Ducellier and Paris-rhone: `-', `+', `EXC'
• Fiat: `30', `67', `85'

Of course one could also open up the alternator and see how things are connected inside, or make some electrical measurements on the outside connections that reveal how things are connected inside, see annex A.

When buying an alternator, mind the following points: Choose one of the brands named above and with the right codes for connections. If no such alternator is available:

• Ask for a type that needs an external voltage regulator.
• If you have a tester, check whether one brush is connected to earth.
• Choose a cheap type: The most expensive types probably are modern types that have a more complicated electric circuit which could be confusing, or special features that make them less suitable. Also a good second-hand one could be just as good as a new one.
• A normal shape and size of the V-belt pulley and fan. This is important for two reasons:
  • For building the seal. If the pulley does not resemble one of the types given in fig. 4.12, one has to find out a technical solution by oneself.
  • Nowadays modern, compact, high capacity alternators are being used that probably have a too high starting speed (the minimum speed at which it can just generate a small charging current). Those types can be recognized by a small diameter pulley for a multi-vee-belt, with a number of small grooves besides one another. Also they usually have the fan built inside the alternator itself.
• A rated capacity of 35 to 65 A (this is the maximum charging current the alternator can produce). Built into a firefly charger, the alternator will never even come near its rated capacity because it needs a very high speed (about 6000 RPM) for this. Besides, the turbine is not strong enough to drive the alternator with so much power. Still a rated capacity of say 35 A is needed to make sure that it can generate enough power at much lower speeds. Alternators with a somewhat higher rated capacity probably have a better efficiency.
• A type that is commonly available (e.g.: The type that is fitted in a type of car that is used widely in the area). This guarantees that spare parts will be available and that the same type can be bought again when one would like to build more chargers later.
• The condition the alternator is in. Either one has to trust the seller in that he won't sell rubbish, or one could make some fast checks oneself, see box 4.1.

The physical dimensions of the alternator are not so important because the frame can be adapted in such a way that it will fit properly. Only the largest alternator types might give a problem if they do not fit inside the 200 x 200 mm cover.

If the supplier has a test bank for alternators, ask whether he could demonstrate the alternator you are about to buy. Relevant in this are:

• The starting speed. This is the lowest speed at which the alternator produces just enough current to supply its own field current. The starting speed should be around 1000 RPM, preferably even lower.
• The charging current at some 1430 RPM. This is the optimum speed for a head of 7 m and then the charger should just produce 11.8 A (see par. 4.1). Preferably, on the test bank the alternator should produce a lot more at this speed already.
• The charging current at some 3000 to 6000 RPM. At such a high speed, the alternator should reach its rated capacity of 35 to 65 A and if it does, this means that there is nothing wrong electrically. Often there is no type plate that reveals the rated capacity, so then one has to rely on the data from the supplier.

If the alternator performs worse than expected, either the type is unsuitable or the alternator is defective inside, probably conductor rings and brushes are in bad shape, stator coils are burnt or diodes have blown out.

Make notes on speeds and charging currents, and also about whether it was connected to a mechanical or an electronical voltage regulator during testing. Try to figure out whether alternator speed is measured accurately. Either a measuring device must be connected directly to the alternator shaft or the operator should measure the transmission ratio of the V-belt transmission between the driving shaft and the alternator shaft, and adjust the speed indicator accordingly.

It is best to buy the (external) voltage regulator together with the alternator. Important characteristics are:

• A 12 V type.
• Compatible with an alternator with one brush connected to `E'.
• Preferably with just 3 outside connections. Many types have extra connections that have to do with the charge indicator lamp on the dashboard. These extra connections are unnecessary and confusing.
• Preferably an electronic type with an adjustable voltage setting. Electronic types are more accurate and, when bought new, cheaper. However, electronic types with an adjustable setting are rare. If only non-adjustable regulators are available, it is better to buy a mechanical one since it is easier to change the setting of such a regulator.

When looking at them from the pulley side, almost all alternators are designed to run to the right (in the direction of the hands of a clock). This must be taken into account in building the turbine parts so that in the charger, the alternator will run in its proper direction. All drawings are correct for alternators running to the right.

Often the proper direction of rotation can be seen from the fan: When running in the right direction, the blades will push the air outwards and the fan will work efficiently. Sometimes there is also an arrow on the fan indicating the proper direction of rotation. Sometimes the brushes are not directed straight towards the hart line of the shaft and then these make that it is meant for one direction only, see box 4.2.

index page

next