Pocketable cellular telephones may be the technology of the nineties but
most of them depend on batteries
developed whilst Queen Victoria was still on the throne...
The next time you pick up your portable phone, offer up a little prayer
of thanks to an obscure Swedish scientist called Jungner. Almost exactly one
hundred years ago he established the chemistry and working principles of the
re-chargeable nickel-cadmium cell. His work led to the development of the
battery that powers your portable phone, and countless other electrical
gadgets, but in spite of their many
shortcomings, and competition from rival battery technologies, the 'nicad' is
likely to be with us for some considerable time to come.
Nicads didn't happen overnight, and the first practical 'sealed'
batteries didn't appear until after the second World War; even then they were
far too expensive for anything other than military and industrial applications.
By the mid 1960s manufacturing techniques had improved, costs had begun to fall
and the first consumer products appeared, in the shape of re-chargeable torches
and power-packs for radio-controlled model aircraft. Throughout the seventies
and eighties nicads went from strength to strength, turning up in all sorts of
unlikely places, from portable vacuum cleaners to cordless telephones. Today
nicads are everywhere and we take them completely for granted, which may
explain the frustration we experience when they let us down, so perhaps it's
time to take a peek inside that black plastic box, to get a better
understanding of how it works.
INSIDE THE BLACK BOX....
A nicad battery pack is made up of a number of individual cells, wired
together in series, each cell has a nominal output of 1.2 volts. You're not
supposed to open up these packs, burn them or damage them in any way, for
reason's we'll discuss in a moment, but if you're interested you can tell how
many cells there are in a particular battery by taking note of the voltage,
it's usually stamped somewhere on the box, along with it's capacity rating,
(expressed in amperes, or milliampere-hours). A six-volt pack, for example,
will contain five cells, (i.e. 5 x 1.2), a 7.2 volt pack has six cells, and so
The reason you shouldn't mess with nicads is simple, each cell contains a
remarkably unpleasant cocktail of highly toxic chemicals, including the
nastiest of them all, Cadmium metal. Sealed inside tough steel canisters it's
all perfectly safe, but if they get hot, either through a short-circuit, or
being thrown on the fire, they can burst, and you don't want to be anywhere
near when that happens! The safest thing to do with dead nicads is to take them
back from whence they came, to your mobile phone dealer, who will be able to
dispose of them safely for you.
POWER TO THE PEOPLE
Nicads are tough, hard-working and generally very reliable but they do
have a habit of letting you down at the most inconvenient moment, and few
hand-portable users can have failed to notice how their battery doesn't last as
long as it used to. Most of the time it's not the fault of the nicad pack, but
part of the problem is their very steep discharge curve, (compared with other
types of batteries), which means one minute they appear fully charged, and a
moment later they're as flat as a pancake. For this reason the charge
indicators on most phones can give only a relative indication of the power
remaining and should always be treated with a pinch of salt.
The mains chargers that come portable phones can be another source of
trouble. Nicads like to be charged slowly and gently, preferably over a period
of 14 to 16 hours at what is known as the C/10, or overnight rate. Clearly
that's not going to be very popular in the want-it-now society we've created
for ourselves, where even an hour or two is too long to wait, hence most phones
are supplied with fast chargers as standard.
Rapidly charging a nicad battery brings with it a number of problems, not
least the risk of overcharging, which can gradually reduce the cell's capacity
to hold a charge. Unfortunately most fast-chargers use the specific set of
conditions that arise when a battery is being overcharged, (sudden drop in
voltage, rise in temperature etc.), to determine when to switch off. That means
a battery which has a potential to be re-charged 1000 times, say, might only
last for 500 cycles. The moral is simple, if you've got a slow or
trickle-charging facility on your charger, use it!
NO THANKS FOR THE MEMORY
Nicads like to go through complete charge-discharge cycles, but this
rarely happens with mobile phone usage, even when the power level indicator
says flat there will be a residual charge left. Worse still are repeated top-up
charges, partially charged batteries receive as a matter of course. This leads
to a condition known as cell-imbalance and the so-called nicad 'memory', where
the battery can appear to loose as much as fifty percent of its capacity in
just a few months. Fortunately both effects can be reversed by taking the
battery through a series of controlled charge-discharge cycles, using
specialised battery chargers or conditioners, now available from most mobile
phone accessory dealers.
BOX-COPY 1 -- NICKEL HYDRIDE BATTERIES
The first serious challenge to nickel-cadmium's long and illustrious
career is coming from the nickel-hydride battery, now being supplied with
Panasonic's I-series phones. Nickel-hydride, or NiH batteries, have two
distinct advantages: they contain much less toxic material, and they are
largely immune to the memory effect, retaining their full capacity throughout
their useful lives. Originally it had been hoped that NiH cells would have
significantly increased capacity, compared with nickel-cadmium, but ongoing
improvements in nicads have reduced the gap. On the debit side NiH batteries
require specialised chargers, and original Panasonic replacement batteries
currently cost between two and three times as much as similarly-specified
nicads, though prices should start to fall if and when other manufacturers
adopt the technology.
Sony have come up with their own alternative to nickel cadmium, called
Lithium-ion. These batteries are presently only being used in camcorders,
though it is conceivable they could find their way into telephones, given
Sony's involvement in this market. Like NiH Lithium Ion batteries do not suffer
from memory problems and there is a significant improvement in the power to
weight ratio, making them approximately 30% smaller and lighter than comparable
nicads. The trade-off's once again, are price, and dependency upon a specialised
BOX COPY 2 -- REFRESHING THE PARTS
Discharging a nicad battery can be a tricky business, if the voltage
across each cell falls much below one volt then it can go into a state known as
reverse-polarity, a normally fatal condition that leads to irreversible damage.
For this reason it's is wise not to attempt a DIY discharger out of an old
light bulb, and a couple of wires -- you would be surprised how many do!
Purpose-built dischargers, or conditioners as they're sometimes known do
the job properly, slowly draining the charge on the battery through a resistive
load, safely dispersing the energy as heat. Voltage sensing circuits
automatically stop the discharge, well before any damage could occur. Many
conditioners have LED charge level indicators, to show the amount of charge
remaining, though the accuracy of these displays is questionable. Some
conditioners also double up as chargers, with sockets for car-power leads or
mains adaptors. Conditioners are available to fit most popular makes and styles
of phone battery, prices start at below £50 for simple one-pack designs.
R.Maybury 1993 2206