Edited by Rachel Duncan 4th Edition ISBN 0-907649-91-2 London ...

Communications 71
The selection of the battery will be dependent on the continuous or peak current
required, period of use, temperature, weight allowances and sometimes cost limitations.
Modern technical breakthroughs in battery design are changing the battery capacity
to weight ratio, whereas a lead acid battery was one solution to medium and heavy current
needs in the past, today nickel-cadmium, lithium-based, air-polarised and aluminium-based
batteries are now practicable alternatives.
The choice of battery will also be dependent on whether the battery is of primary
type, i.e. throw away after use – not being able to be recharged, or secondary, where there
is a need to recharge the battery using solar panels, a wind turbine, generators or a
combination of all three.
Dry cells (or the higher capacity at lower temperatures Alkaline types) as used in
flashlights, radio receivers or similar devices can be used down to 0°C. Below this
temperature the chemical reaction reduces, lowering the current that can be supplied.
Nickel-cadmium batteries can produce large currents with a stable voltage until the
end of battery life, and are an excellent solution down to temperatures of –20°C, but need
to be charged at a higher temperature, preferably above 0°C otherwise the capacity will be
reduced. Partial discharge and recharging can cause a lowering of capacity (the ‘memory
effect’).
At lower temperatures all batteries suffer lower storage capacity, however selfdischarge
is reduced. It is better to raise batteries off the ground and mount as high as
possible, as an uncharged or partially charged battery may suffer damage due to the
internal electrolyte in gel or liquid form freezing, expanding or cracking the surrounding
case material.
7.5.2 Battery selection
Lithium-based primary cells each producing a stable 3.3 volts can be put in parallel or
series to produce solutions to very long low current and high pulse current needs, but
generally are not so good at long term continuous high current demands.
Lithium cells are expensive but can give outputs down to below –40°C. 14-volt
banks of cells each with a diode have been assembled providing a safety route if a cell
fails. This weighs only 1kg but provides a capacity of over 35amp/hr. The battery cell
should not be exposed to mechanical and thermal stress and can self-ignite if abused or if
internal safety fuses are bypassed or pressure vents are blocked. Make sure the maximum
current you draw does not go below the maximum allowable depression voltage of the
battery, otherwise damage can occur.
There are still transportation difficulties with some types of batteries and strict
adherence to International regulations with reference to the Transport of Dangerous goods
should be observed. Special packing is often required, and some batteries are banned from
passenger flights.
Plugs, sockets and cabling should be suitable for low temperatures and have
adequate dimensions and cross sectional area for the current requirements or length of
cables. Beware of tapping off large cells at different voltage points, especially if the metals
cases of equipment are at different potentials. Unequal discharge in this configuration can
be problematic and can cause early battery failure and low capacity.