5 The home system previous
5.1 The battery index page

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.

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:

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.


5.2 Lamps, switches and cables

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

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

The electricity consumption per day of a user depends 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.

Table 5.1: Selection of cable type.
Cable between battery connection and indicator:
  • Always 2.5 mm (or no. 14 in American Wire Classification)
  • maximum length = 2 m.
Cable cross section: Cables between indicator and lamps:
Maximum cable length (in m) for total power is:
10 W 20 W 30 W 40 W 60 W 80 W 100 W
0.75 mm 15 7.7 5.1 - - - -
1.31 mm (no 16) 27 14 9.0 6.7 4.5 - -
1.5 mm 31 15 10 7.7 5.1 - -
2.08 mm (no 14) 43 21 14 11 7.1 5.3 4.2
2.5 mm 51 26 17 13 8.5 6.4 5.1
3.31 mm (no 12) 68 34 23 17 11 8.5 6.8


5.3 Minor uses for electricity

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

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 mm (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 mm). 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. 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 mm 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:

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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