Rural lighting
Practical Action
distribution to rural areas raising the cost even more and making the supply unreliable. There is
also an issue of waste disposal once they are finished with.
Rechargeable batteries are more cost-effective than disposable batteries but recharge equipment
then becomes necessary. The two main options available are
nickel-cadmium batteries
lead-acid batteries
Nickel-cadmium batteries are generally less widely available (except as dry-cell substitutes) and
cost more, but they can be more robust and tolerant of abuse than lead-acid batteries. However
they self-discharge quite quickly if not used. Electrical energy from a lead-acid battery can cost
as little as one twentieth to one fifth as much for the same amount of energy delivered from
primary (dry) batteries.
Lead-acid batteries are similar to the batteries used in cars and in many cases car batteries are
used in lighting systems as these are the most widely available type of battery. For most lighting
purposes these are the easiest and cheapest option. Lead-acid batteries are also available as
deep-discharge batteries which have a longer life than car batteries, and if looked after tend to be
better for general electrical storage.
Most lead-acid and nickel-cadmium batteries require regular checking of their electrolyte level
and topping up with distilled or deionised water, (not with acid). Rain water can be used for this
purpose, providing it has not been contaminated in any way. Low maintenance and maintenance-
free lead-acid batteries are also available, at slightly increased cost.
An important point to note with lead-acid batteries is that their life is considerably shortened if
they are over-discharged. Ideally they should only be discharged to about 50% of their full rating;
i.e. a 60Ah (ampere-hour) battery should only be discharged to 30Ah before recharging it.
Typically deep-discharge lead-acid battery costs are in the region of £60/kWh of total rated
capacity or £120/kWh of usable storage, while nickel-cadmium will be about three times this
level. The cost increases with some specialist types and very small batteries.
Batteries are generally provided with nominal voltages in multiples of 2V; common larger capacity
lead-acid batteries will be 12V or 24V nominal voltage.
Cable runs should be kept as short as possible with low voltage supplies, (or heavier cable must
be used) otherwise significant losses will occur in the cables. A 1.2V voltage drop in a 12V
system represents a loss of 10% of the power transmitted down the cable.
The voltage drop is numerically equal to the current in amps multiplied by the resistance of the
cable in ohms. A 10m length of 0.75mm² cable with a specific electrical resistance of 0.042
ohms carrying 1.25A will have a voltage drop of 1.25 x 0.042 x 10 = 0.525V. This represents
about 96% cable efficiency, which is acceptable. However, 100m of the same cable will cause a
voltage drop of 5.25V which will cause the light not to work and in any case represents a quite
unacceptable loss of nearly 50% of the energy supplied.
A 30W fluorescent light (for example) running off a battery with an inverter for six hours per night
will consume 180Wh each 24 hours. Losses in the inverter, cables and the battery will increase
this requirement to about 300Wh/24h. To avoid more than 50% discharge and provide a nominal
24 hours storage capacity would require, with the above example, a battery with a usable capacity
of 600Wh (1200Wh to total discharge), costing in the region of £80.
The energy sources
The main methods by which the electricity for charging may be provided:
taking the battery to the nearest mains supply and putting it on charge
using a small engine-powered generating set
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