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BATTERIES, THE INSIDE STORY

 

INTRO

Ever wondered what lurks inside the battery pack on the back of your phone? Probably not, but it’s worth finding out, so at least you’ll know what to blame the next time the ‘low battery’ warning starts bleeping...

 

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Most cellphone users hardly give their phone batteries a second thought, other than to curse them when they expire -- usually in the middle of an important call. Sad to say for a lot of people re-chargeable battery packs are merely black plastic boxes, notable for being short-lived and expensive to replace. To everyone here at What Cellphone (well, almost everyone...) they’re a never-ending source of fascination. There’s always keen competition in the office to be the first to try out a new pack, and to open it up, to see the works. We thought it was high time you joined in the fun, and shared with us a peek into the exciting, and usually very private world of a mobile phone battery...

 

By the way, please don’t try opening up a battery pack yourself. They’re not meant to come apart. The two halves of the case are glued or ultrasonically welded together -- you’ll almost certainly destroy the battery trying to separate the shell. There’s also a very good chance you’ll damage or short-circuit the cells inside, which may burst, releasing toxic and/or caustic chemicals, and you don’t want to be anywhere near when that happens!  

 

The subject of our attentions is the Varta T432. This is a 6 volt, 600mAh pack for Ericsson GH337 and 338 models, but apart from the shape of the case, and the electrical contacts, the bits and pieces inside are the same for almost all similar styles of battery pack. This one uses nickel metal hydride cells (NiMh), as opposed to the more common nickel cadmium (nicad) type of rechargeable cell. However, apart from being slightly smaller -- for a particular capacity -- the design and layout of nicad and NiMh packs are virtually identical.

 

The first point of interest is the number and type of cells this battery uses, (technically a battery is a collection of cells). The cells in the T432 are rectangular in shape -- known as ‘prismatics’ in the trade -- so they can be packed closely together, compared with bulkier cylindrically shaped cells. There are five cells in this particular pack. Each one is rated at 1.2 volts so that when they’re wired together in series, the battery has an output of exactly 6 volts. Higher capacity batteries either use larger cells, or two or more ‘banks’ of cells, connected together in parallel.

 

Inside a prismatic cell there’s a tightly-packed block, made up of layers of thin flexible sheets of chemically impregnated materials. The positive electrode is made of nickel hydroxide; the positive electrode is a hydrogen storage alloy, they’re kept apart by a ‘separator’ made of a permeable non-woven insulating material. The whole caboodle is soaked in a alkaline electrolyte gel. Cells are housed in an ABS plastic coated nickel plated steel case, which also acts as the negative terminal. The positive terminal on the top doubles up as a re-sealable vent. This allows hydrogen gas to escape, if the cell over-heats or is short-circuited.

 

Normally that should never happen. Phone batteries contain a number of safety devices, or at least they should -- it’s not unknown for manufacturers of cheaper phone batteries to skimp... The first line of defence against an accidental short circuits is the Polyswitch or thermal fuse. This is a fuse, that will blow if the battery is subject to a sudden high current discharge. This is unlikely to occur whilst the battery is connected to a phone, but hands up everyone who has carried a spare phone batteries in a coat pocket, along with a bunch of keys? Okay, we’ve all done it, just don’t do it again. If the keys touch the battery contacts the cells will be short-circuited. On a cheapo pack, without proper protection, they could burst, or vent super-hot hydrogen gas, which could cause a very nasty burn. There’s enough energy in a fully charged phone battery to melt a metal key-ring. Trust us, the last thing you want in your trouser pocket is a bunch of red-hot keys!

 

The polyswitch or thermal fuse is a ‘non-recoverable’ device. If it blows the pack is kaput and there’s nothing you can do about it, except to return it to a dealer, or your local amenity tip for safe disposal. On no account put expired batteries -- of any type -- into normal household refuse. The chemicals they contain -- even ‘greenish’  NiMh packs -- are hazardous;  you don’t want to be responsible for poisoning future generations, do you?

   

A thermostat or TCO (temperature cut off) device protects the cells against overheating, due to a constant high current discharge. This can be caused by a faulty phone etc., or from overcharging. Inside the thermostat there’s a bi-metal strip, that breaks the circuit when the cells reach a pre-set temperature, usually between 55 to 75 degrees centigrade. The thermostat will reset when the temperature comes down.

 

The cells inside a battery pack can become quite hot during fast-charging so an additional temperature-sensing component, called a negative temperature coefficient (NTC) thermistor, is normally incorporated into the circuit. The thermistor’s electrical resistance increases as it heats up, limiting the amount of current the cells can draw from the charger. As the cells cool down the thermistor’s resistance reduces, so the system is effectively self-regulating.

 

The final element is the electrical connections between the cells and the external terminals. Most battery packs use thin strips of metal or wires, soldered or welded to contact points on each cell. The T432 is slightly different in that it has  a one-piece flexible printed circuit. This has a number of advantages, including making the pack more stable by preventing the cells from shifting around inside the case. Moreover, because there’s fewer joins, it is more reliable than a conventional pack with soldered or welded connections.  

 

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Several recent cellphones now come with lithium-ion re-chargeable battery packs. These have an even higher power density than nickel-metal hydride packs, consequently they’re a lot smaller -- for a given capacity --  resulting in even longer standby and talk times. However, unlike nicad and NiMh batteries, which have similar characteristics, and are generally interchangeable, phones and chargers have to be specially designed to use lithium-ion packs. The new batteries have a higher internal resistance -- compared with nicad batteries -- so they can’t be used on devices like power tools, that have a continuous high current demand. Lithium ion packs are also quite expensive, costing between two and three times as much as nicad packs of equivalent capacity. However, manufacturing costs are coming down and the plus points now outweigh the negative ones. In addition to being smaller and lighter, they have a very predictable discharge curve. That means the battery meters on phones can be very accurate. Time will tell, but it looks as though they’ll last as long, if not longer than nicads, moreover they’re completely immune to debilitating memory effects and do not mind repeated top-up charges.   

 

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Ó R. Maybury 1997 1201

 

 


 

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