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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...
COPY
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.
BOX COPY 1
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.
---end---
Ó R. Maybury 1997 1201
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