The battery is the single most expensive item in the investment costs of a firefly system. Also in the running costs, replacement costs for worn-out batteries form a major part. So for the economics of the firefly system, it is essential to choose the most appropriate type of battery and to take proper care of it. Several types of batteries are worth considering.

• Ordinary car batteries: These are cheap, widely available in many different sizes, but they will not last long. They are designed mainly to start a car, so to provide the very large starting current but only for a few seconds, so that still they are discharged only a tiny bit. In a firefly system, the battery must deliver a low current during a long time and will not be recharged immediately afterwards.
  Once they are discharged to 50 % state of charge, they have to be recharged again so only 50 % of the total battery capacity can be used.
• Heavy duty car batteries: These are designed to last longer under heavy use in a car, truck or bus and consequently they are more expensive. The plates are thicker and the separators between plates are made of glass-wool. Also in a firefly system, they should last considerably longer.
• Solar batteries: These are designed for use in solar energy systems. Like with the heavy duty batteries, the plates are thicker and they have glass-wool separators. On top of this, the active material is mechanically stronger and this makes that they can be discharged until only 20 % state of charge, so 80 % of the total battery capacity is available. Even when used so heavily, they still last many more cycles than a car battery. Probably `lighting' and `semi-traction' batteries have virtually the same characteristics.
  However, they also have disadvantages:
  • They are quite expensive.
  • They are not widely available yet.
  • The size that is commonly used in solar energy systems is a bit too large for a firefly system. It is heavy (nearly 30 kg) and such a high capacity is not really needed. A solar battery of half this size for half the price would be much more attractive but these are even more difficult to get.
• Worn-out car batteries: It could be that batteries that are too weak to start a car, can still be used in the firefly system. There are 5 major mechanisms that make a battery wear out or could destroy it in one go (see annex C.3: What makes batteries wear out). Batteries that are not usable for cars because of one of these mechanisms (oxidation of grid of positive plate), should be still useable for lighting lamps. Such batteries have retained a large part of their original capacity to store electricity but due to the weakened grid, they can not deliver the high current that is needed to start a car. Luckily this is the most common mechanism that makes batteries wear out in a car so a good percentage of all worn-out car batteries should be usable.
  Of course the low price of such batteries makes this option very attractive, but there are also problems:
  • Used car batteries should be tested to see whether they are useable (it could be that a battery has become unusable because of one of the other mechanisms). A test procedure must still be worked out and operators have to be trained in performing such tests.
  • There should be a way of setting a reasonable price for a battery that came out as usable from the tests.
  • Even with the above things settled, there is still a larger risk that a used car battery will wear out too fast and the owner will be disappointed.
  Therefor is seems best to avoid disappointments and use new batteries as long as the firefly technology is still new in the area. In a later phase however, it is worthwhile to start experimenting with this. It offers opportunities to reduce investment costs substantially, which makes it possible for poorer people to join in. Even when the life span of used car batteries would be so short that solar batteries seem as cheap in the long run, it could still be advantageous to choose for old car batteries because:
  • The low investment costs make it affordable to poor people even if there are no sources of cheap credit.
  • Used car batteries will be widely available so that no development project is needed to get parts from faraway places.

Then batteries differ with respect of their casing (plastic or bakelite), the type of connections, whether they have plugs for topping up with destilled water or are `maintenance-free'. These things are less relevant so the cheapest design is the best.

Prices of batteries and availablility will vary all over the world and therefor it is impossible to say which type is the most advantageous. In choosing a type of battery, mind the following criteria:

• Costs.
• Availability (also in the long run, when there is no development project to organise things).
• Life span: This is a difficult thing to estimate. In the Cambulo project it was estimated that, with good care, car batteries would last 1.5 year and solar batteries 3 years.
• Effective capacity: This is the part of the total capacity that can actually be used, so 50 % of total capacity for car batteries and 80 % of total capacity for solar batteries. A high effective capacity is advantageous because it means that a battery needs to be recharged less often (saves time and money) and/or more lamps can be connected to it.
• Weight: Of course a low weight is desirable because batteries need to be carried to the charger regularly. The weight is more or less proportional to total battery capacity so only solar batteries can have a high effective capacity combined with a moderate weight.

Especially the weight should not be forgotten. Generally people in mountainous areas are used to carry heavy loads for considerable distances. But this should be no reason for letting people carry excessively heavy batteries in an attempt to cut costs. In western countries, regularly handling weights as little as 25 kg proved to be damaging for health. If batteries are heavy, it is even more likely that carrying them will be seen as men's work. So if men move out to work elsewhere or are unwilling to carry, there is no one to bring them to be recharged. Preferably also women and older children should be able to carry batteries.

Many other issues relate to proper battery care. These are dealt with in other paragraphs and chapters. See also annex D: More about batteries.

The following types of lamps can be used in a firefly system:

• Ordinary 12 V car bulbs: These are by far the cheapest and also the sockets are quite cheap. The type of 15 W (sometimes referred to as `21 CP') is the most applicable. This is the type that is fitted in the indicator lights. Bigger lamps (like headlights) consume too much electricity and smaller types (rear lights) give too little light for use in a normal room. Of course headlight lamps could be used on occasions when a lot of light is wanted, and smaller lamps in cases when many lamps have to be powered by one battery.
  A disadvantage of using car bulbs is that they burn a bit dull. Car bulbs are made for using them at about 14 V (the voltage when the motor is running and the battery is being recharged) instead of 12 V. In the firefly system, the battery is being discharged and the voltage is 12 V or even less. This makes that the efficiency of car bulbs (= amount of light produced per amount of electricity consumed) is quite low.
• Halogen lamps for interior lighting (12 V types). A halogen lamp in itself is more efficient than an ordinary filament lamp and on top of this, such halogen lamps are designed for 12 V and not 14 V. This makes that a 20 W halogen lamp produces a lot more light than a 15 W car bulb while it consumes hardly more electricity. There are two versions:
  • A normal bulb.
  • A bulb combined with a reflector.
  The type with reflector produces a beam that for instance provides ample light at a working table.
  Fitting halogen lamps poses some problems:
  • The bulb itself and its leads get quite hot. That is why special ceramic connectors for halogen lamps are used, but these are about as expensive as the bulb itself. However, with some care, 20 W halogen can also be connected on 2 pieces of a connector block. The type with reflector does not become very hot. With the bare bulb, some extra measures are adviseable to prevent that the connector and the cable isolation will get too hot (fire risc!):
    • Do not obstruct natural ventilation around the bulb
    • Preferably have the heat-sensitive connector and cable besides the bulb and not in the hot air stream above it.
    • Use thick, massive cable for the last bit to the lamp. The copper inside it will conduct heat away from the bulb.
  • The bulb itself should not be touched with bare fingers (or it should be wiped off with alcohol) because the substances that make fingerprints, will noticeably reduce the lifespan of the bulb. With the type with reflector, the bulb itself usually can not be touched because there is a glass plate that covers the reflector and with these, there is no problem
  In Holland, halogen lamps are not too expensive any more: About ƒ 5 for a bulb type and ƒ 10 for a reflector type. The smallest size used to be 20 W but nowadays also bulb types of 10 W and even 5 W can be found.
• Ordinary fluorescent lamps with a special 12 V ballast. Compared to car bulbs and halogen lamps, these are very efficient. In the Philippines, a complete 20 W fluorescent lamp set was available for about ƒ 13.50 but the life span of the lamp itself proved to be disappointingly low. Apparently the simple ballast circuit makes it wear out too fast. Types with a better ballast should not have this problem, but are much more expensive.
  Such electronic ballasts work with a high frequency and at least the cheap ballasts can produce noise on a nearby radio. To prevent this, an `Elco' capacitor of ca. 63µF, 16 V should be connected over the input wires of the lamp. Make shure that the lamp and the Elco capacitor are connected with the correct polarity.
• Ordinary fluorescent lamps with 220 / 110 V ballast plus an inverter to convert the 12 V DC from the battery into 220 / 110 V AC. These lamps themselves are cheap, very efficient and have a long life span. For powering just one lamp, the costs of the inverter are too high to make this an economic solution. But when there are many lamps that are normally on at the same time, this might be an attractive option. This could be the case if there is one large building (e.g a school building that is also used at night, a church or a community centre). It could also be the case if there are a number of houses to be powered from one battery. Cable losses in the 220 / 110 V cable will be minimal so even houses a few hundred meters away from the battery could be connected using very thin cable, as long as this cable is suitable for 220 / 110 V. See under `inverter plus ordinary 220 / 110 V appliances' in par. 5.3 for more technical details.
• Special `PL' types of fluorescent lamps with 12 V ballast. These are extremely efficient and that is why they are used in solar energy systems. As long as the special 12 V ballast can not be made locally and cheap, they seem too expensive yet for a firefly system. Contrary to a solar system, firefly users are probably poor, do not need that much light and producing the electricity (= charging the battery) is cheap.

For choosing the best type of lamp, the following issues should be considered.

• How much light is needed. Is it for cooking a meal or for reading.
• Costs. The money saved by buying cheap car bulbs could easily be offset again when a larger capacity battery is needed to power these lamps, the battery has to be recharged more often (charging fees) and wears out faster. Also the time spent on bringing the battery to be recharged should be treated as costs.
• Preferences for a certain kind of light.

The electricity consumption per day of a user depends on:

• The number of lamps fitted.
• The rated power of these lamps (in Watts).
• The average number of hours per day that each lamp is on.

Quite likely, richer users will want lamps that provide more light because they can afford it. Probably they also have bigger houses with several rooms and will buy more lamps so that there is a light in each room. If they would use the same kind of lamp (probably car bulbs) as poorer users, they would have a much higher electricity consumption so then the charger is not used equally by all users of one group. This could cause disunity within this user group once it comes to dividing the costs of the charger. A way to minimise this problem is, to advise users who want more light to buy more efficient lamps rather than lamps with a higher rated power. This will make the electricity consumption of all users more equal.

For cables, the cheapest type is probably the best. This is usually `twin cable', the simplest kind of cable for indoor use as cord for 220 / 110 V appliances. It consists of two insulated wires melted together and without a coat around both of them. The wires are stranded so it can stand being bent many times. It is not a high quality cable that will last many years under harsh conditions but it is cheap and widely available.

The thickness of cables should be chosen so that the voltage drop over it will remain below 5 % of the total voltage (= 12 V), see table 5.1 for some practical guidelines. In annex 0: Formulas and reference data, an explanation is given on how to calculate voltage drops in cables.

Fig. 5.1: Ruud Portegijs helps Leon Bentican wiring up his house.

Cables can be fitted on wood with electrical staples or nails that are partially hit into the wood and then bent over to hold the cable. When a cable spans a distance outdoors (for connecting a nearby house to the same battery), it is best to tie a cable so that it does not hang on a staple or a nail.

For connecting cables, connector blocks can be used but just twisting the wires together and insulating the joint with electrical tape is cheaper and it is likely that this will be done anyway when users want to expand their system using bits and pieces of cable.

As switches, ordinary 220 / 110 V switches will do, probably are the cheapest and are easily fitted on walls.

Using the cheapest materials for wiring up a house inevitably will lead to more technical failures and a lower life span. Besides this, in principle there are safety issues at stake. With the firefly home system, those safety issues do not depend strongly on the quality of materials. The voltage itself is harmless to touch and the only risk is a cable catching fire due to a short circuit. The protection against this is formed by the battery fuse and consequently this fuse is essential (see par. 5.5). Using a sturdier cable would of course reduce the risc that a short circuit occurs, but in an environment with choppers, spades, axes and children around, even a cable with a steel coat would not be totally safe.

Although lighting will be the main use for practically all users, there are some other uses that are inexpensive and worthwhile:

• Connecting 6 or 9 V appliances to the firefly battery instead of using dry cell batteries. In this way, the money for dry cells for radio's, cassette players and similar appliances that are used within the home, can be saved.
• Recharging rechargeable NiCd batteries or small, maintenance-free lead-acid batteries. Such batteries could then replace dry cell batteries in torches and other outdoor appliances. The need for torches is much higher in an environment without street lights, outdoor lights and cars or bicycles with their own lamps and a good torch is more or less a safety device when one has to go out along a slippery path in the dark. Consequently, the consumption of dry cell batteries for torches is quite high, once people can afford these.
  . NiCd batteries of the size for torches are quite expensive but nowadays small maintenance-free lead-acid batteries from emergency lamps are available reasonably cheap. These do not fit in a torch itself, but could be tied onto it. The lead-acid battery can be connected to a wooden stick with connections on both ends that acts as a `dummy-battery' and fits inside the torch.
• Appliances other than lamps that can be powered directly from 12 V, for instance:
  • Appliances designed for use in cars: Small 12 V electrical fan, car radio/cassette player etc.
  • Appliances designed for use in a solar energy system: Solar refrigerator for vaccine cooling.
  • Other appliances that can use 12 V: Two way radio systems, portable computers with an adapter for 12 V.
• Using an inverter, a device that turns 12 V DC (decent current) into 220 / 110 V AC (alternating current). Most normal 220 / 110 V appliances can be powered by such an inverter, as long as the power they need is within the capacity of both the inverter and the battery.
  The most likely appliance this will be used for, is for powering a video recorder + television set and showing video's for an entrance fee. From the economic point of view, this could be quite attractive (in fact it probably is the only productive end-use that fits well within the firefly system). What kind of movies will be shown and whether this is a desirable development, is another question. In Ifugao prov. in the Philippines, children started stealing money for going to the movies once such video cinema's came in.
  Another possible use is to power a number of ordinary 220 / 110 V fluorescent lamps in one large building, or in a number of houses, see par. 5.2.

The technical aspects of these options will be discussed below.

To power 6 and 9 V appliances from the main battery, the voltage must be reduced by a stabilised voltage supply. This is a cheap electronic component that produces a constant output voltage from a varying, higher input voltage. To prevent oscillation problems (so: radio interference), capacitors are connected to the input and output of the device. See fig. 5.2a for the electronic circuit.

The device, capacitors and connection cables can be fitted into 3 units of a small connector block so no soldering is needed for that. At the end of the output cable there should be a cylindrical plug that fits in the appliance (if the appliance has no socket for this, fit a socket to the connections for the batteries inside). It is adviseable to fit a socket + plug at the input side as well so that the stabilised voltage supply can be disconnected from the battery in case it is not needed. These sockets and plugs have to be soldered on. Since current is so low, cables can be as thin as is practicable, say 0.4 mm² (nr. 20 in American classification). Normally, the power consumed by the appliance is so low that the stabilised voltage supply device needs no cooling. Only when it gets very hot (too hot to touch) it should be mounted on a piece of aluminum sheet that acts as a cooler.

For fire protection, either the battery fuse should blow in case of a short circuit, or a separate fuse (of ca. 1 A) should be fitted in the input side, see annex 0: Formulas and reference data. If there is no separate fuse, the total length of thin cable (input and output) should be no more than 4 m (with 16 A battery fuse and thin cable of 0.4 mm²). Then it is best to have the device itself near the main cable.

For 4.5 and 3 V, a slightly different circuit is needed because a `7804.5' and 7803' stabilised voltage supply does not exist, see fig. 5.2b. In this case, 4 units of a connection block are needed to mount all components, 3 for the connections of the LM 317 and a separate one for the `-' wire.

NiCd batteries should be recharged with a constant current. This can be done simply by fitting a series resistor that limits the current to the desired value, see fig. 5.2c. After the stated charging time, they should be fully charged. To economise on the electricity consumption from the main battery, it is best to recharge as many batteries in series as possible. To make this into a handy device, a battery holder can be made with a different slots for each size and number of batteries that are to be recharged in one go. Then for each slot, the right resistor can be fitted.

A problem with the circuit given in fig. 5.2c is that for each current and each number of batteries recharged in one go, another resistor value is needed. An alternative for this is to use a `Constant current device', see fig. 5.2d. Then with the same device, 1 up to 6 batteries that need the same charging current, can be recharged. The LED will burn less bright if more batteries are recharged, but this does not influence the charging current itself.

Fig. 5.2a: Stabilised voltage supply for 6 V output. For 9 V output, use device `7809' instead of `7806'.

Fig. 5.2b: Stabilised voltage supply for 4.5 V or 3 V.

Fig. 5.2c: Charging NiCd batteries with a series resistor to achieve the right current.

Fig. 5.2d: Constant current device for recharging 1 up to 6 NiCd batteries in one go.

Fig. 5.2e: Circuit for recharging small, 6 V, maintenance-free lead-acid batteries from the main battery. The LM 317 must be mounted on a piece of aluminum for cooling. The part of the circuit with transistor and LED's can be omitted, it only serves to show whether the charger is functioning and whether the battery is charged already.

Like the main battery, maintenance-free lead-acid batteries need to be recharged with the right voltage and current (see par. 4.9.5.1). Instructions for this are written on the battery itself, see with `cyclic use'. On a YUASA NP 4-6 battery (6 V, 4 Ah capacity) it said: `voltage regulation: 7.2 - 7.5 V, initial current: 1 A max.'.

A fine-tuned version of the stabilised voltage supply of fig. 5.2b, could provide the desired voltage while a series resistor in the input wire could limit the current to 1 A maximally. See fig. 5.2e for the electronic circuit. To make things really perfect, a circuit with two LED's can be added that will indicate whether the device is charging (red LED) or that the battery is practically charged (green LED, current has dropped below 0.2 A).

The voltage is adjusted by the 2.5 kOhm trimmer (range: 1.25 - 7.9 V). The current limitation is set to ca. 1 A by the series resistor in the input wire, in this case 4.7 Ohm. For another current limitation value, choose another resistor: First calculate the voltage over the resistor: 12 - 6 (maintenance-free battery) - 1 (LM 317) = 5 V. Then calculate what resistance gives the desired current with Ohm's law, see annex 0: Formulas and reference data.

Of course appliances that can be powered directly from 12 V, can be connected directly to the battery. However, one has to check how much electricity they consume. For instance a small 12 V electric fan of only 12 W but used 12 hours a day, would still drain a small car battery in only 2 days and users might be disappointed by this. But if a notebook computer would consume as much power, being able to work 24 hours on one small car battery might be seen as quite good.

Solar refrigerators consume so much electricity that carrying batteries to the charger is not feasible. Instead, the solar refrigerator should be placed very close to the charger (or the charger to the solar refrigerator) so that the batteries can be connected without moving them. Then in principle, no batteries would be needed and the refrigerator could be connected directly to the charger. But then the charger would run idle once the temperature inside the fridge has reached the desired value and the thermostat switches off the pump. Also the vaccines would be lost in about a day once the charger breaks down. Therefor it seems best to have a buffer of 1 or 2 large solar batteries. The costs of these are low anyway compared to the costs of the solar fridge itself. During the night, the charger can be used to recharge these batteries while at daytime, these batteries can be disconnected and the batteries of other users recharged. Of course the electricity consumption of the fridge + losses in its batteries + electricity need for recharging batteries of other users, should not surpass the maximum electricity production of the charger running day and night.

To charge batteries properly, the cable between the switchboard and the battery should have a specified, low resistance (see par. 4.9.6). which means that a 2.5 mm² cable should be only 2 m long. So either the cable between the charger and switchboard must be made longer so that the switchboard can be placed close to these batteries, or the batteries have to be placed close to the switchboard and charger. In both cases, still the voltage drop over the long cable should be checked (see annex 0).

Generally, solar fridges are too expensive and have too small a capacity (in liters of useful storage room) to be feasible for e.g. storing food or softdrinks, or producing ice for human consumption. Only for the richest people (development workers themselves) or tourist lodged, this might be an option.

Using an inverter with normal 220 / 110 V appliances might seem an attractive option since the normal 220 / 110 V appliances are relatively cheap. But there are some drawbacks:

• Many 220 / 110 V appliances just consume too much power. Examples of these are: Refrigerators, ironing devices, electric heaters, powerful electric motors and large fans running many hours. For these, several large batteries might be needed for one house, still they might need recharging after only a few days and also the inverter needs to have a very high capacity.
  Even with all 220 / 110 V appliances switched off, the inverter will still consume a little bit of power and therefor it might not be feasible to have the inverter switched on 24 hours a day.
• Often special 12 V appliances are more efficient than the usual 220 /110 V appliances. Fluorescent lamps are an exeption to this.
• The inverter produces a rather crude form of Alternating Current and some electrical devices might not work properly on such an input voltage (electrical clocks, some computer equipment).
• Safety. The 12 V from the firefly system itself is safe from the point of touching live wires. People might get used to this and connect the 220 / 110 V cables in the same cheap but unsafe way.

This option might be attractive for powering a number of cheap, efficient 220 / 110 V fluorescent lamps, see par. 5.2.

The current in a 220 V cable will be only 5.5 % of that of a 12 V cable transmitting the same amount of power (11 % in case of a 110 V cable). Therefor the thinnest cable that is suitable for 220 / 110 V can be used for the 220 / 110 V cables and still cable losses within a building will be negligible. Even houses a few hundred meters away could be connected up using such thin cable without significant cable losses.

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