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POWER CRAZY...

 

INTRO

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...

 

COPY

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 on.

 

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 charger.

 

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.

 

---end---

R.Maybury 1993 2206

 


 

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