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Each parallel string of cells of a Li-ion pack needs
independent voltage monitoring. The more cells that are connected
in series, the more complex the protection circuit becomes.
Four cells in series is the practical limit for commercial
applications.
The internal protection circuit of a mobile phone while in the ON position has a resistance of 50 to 100 mOhm. The circuit normally consists of two switches connected in series. One is responsible for high cut-off, the other for low cut-off. The combined resistance of these two devices virtually doubles the internal resistance of a battery pack, especially if only one cell is used. Battery packs powering mobile phones, for example, must be capable of delivering high current bursts. The internal protection does, in a certain way, interfere with the current delivery.
Some small Li-ion packs with spinel
chemistry containing one or two cells may not include an electronic
protection circuit. Instead, they use a single component fuse
device. These cells are deemed safe because of small size
and low capacity. In addition, spinel is more tolerant than
other systems if abused. The absence of a protection circuit
saves money, but a new problem arises. Here is what can happen:
Mobile phone users have access to chargers
that may not be approved by the battery manufacturer. Available
at low cost for car and travel, these chargers may rely on
the battery’s protection circuit to terminate at full charge.
Without the protection circuit, the battery cell voltage rises
too high and overcharges the battery. Apparently still safe,
irreversible battery damage often occurs. Heat buildup and
bulging is common under these circumstances. Such situations
must be avoided at all times. The manufacturers are often
at a loss when it comes to replacing these batteries under
warranty.
Li-ion batteries with cobalt electrodes,
for example, require full safety protection. A major concern
arises if static electricity or a faulty charger has destroyed
the battery’s protection circuit. Such damage often causes
the solid-state switches to fuse in a permanent ON position
without the user’s knowledge. A battery with a faulty protection
circuit may function normally but does not provide the required
safety. If charged beyond safe voltage limits with a poorly
designed accessory charger, the battery may heat up, then
bulge and in some cases vent with flame. Shorting such a battery
can also be hazardous.
Manufacturers of Li-ion batteries refrain
from mentioning explosion. ‘Venting with flame’ is the accepted
terminology. Although slower in reaction than an explosion,
venting with flame can be very violent and inflicts injury
to those in close proximity. It can also damage the equipment
to which the battery is connected.
Most manufacturers do not sell the Li-ion
cells by themselves but make them available in a battery pack,
complete with protection circuit. This precaution is understandable
when considering the danger of explosion and fire if the battery
is charged and discharged beyond its safe limits. Most battery
assembling houses must certify the pack assembly and protection
circuit intended to be used with the manufacturer before these
items are approved for sale.
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