Safety
circuits for modern batteries
A modern battery is a delicate storage device that requires protection
to safeguard against damage. The most basic protection is a fuse that
opens on excess current. Some fuses disengage permanently and render the
battery useless once the filament is broken; other safety devices are
resettable. The Polyswitch™ is such a resettable fuse. Connected
into the battery's current path, this device creates a high resistance
on excess current. The Polyswitch™ reverts to the low ON position
when the condition normalizes, allowing operation to resume.
Batteries used in hazardous areas must be intrinsically safe. Hazardous
areas include oil refineries, mines, grain elevators and fuel handling
at airports. These areas are typically serviced with two-way radios and
computing devices. Intrinsically safe batteries prevent excessive heat
buildup and the danger of an electric spark on equipment failure. Because
of tight approval standards, intrinsically safe batteries carry twice
to three-times the price tag of regular packs.
Another battery that contains high-level protection is lithium-ion. This
is done to assure safety under all circumstances while in the hands of
the public. Typically, a Field Effect Transistor (FET) opens if the charge
voltage of any cell reaches 4.30V. A separate fuse opens if the cell temperature
approaches 90°C (194°F). In addition, a disconnect switch in each
cell permanently interrupts the charge current if a safe pressure threshold
of about 10 Bar (150 psi) is exceeded. To prevent the battery from over-discharging,
the control circuit cuts off the current path at about 2.50V/cell. Prolonged
storage at voltages of 1.5V/cell and lower damages the lithium-ion, causing
safety problems if attempted to recharge.
Each parallel string of cells in a lithium-ion pack needs independent
voltage monitoring. In addition, each cell in series must be monitored
for voltage. 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 must be designed to add as little resistance
as possible to the current path. The circuit of a cell phone battery often
consists of two FET switches connected in series. One FET is responsible
for high, the other for low voltage cut-off. The combined resistance of
the FETs in the ON position is 50-100milli Ohms (mW). This virtually doubles
the internal resistance of a battery pack.
A major concern arises if static electricity or a faulty charger destroys
the battery's protection circuit. This may result in permanently fusing
the solid-state switches in an ON position without the user's knowledge.
A battery with a faulty protection circuit may function normally but will
not provide protection. If charged over a voltage limit (4.20V/cell should
not be exceeded) with a defective charger, venting with flame could occur.
Such a situation must be avoided at all cost. Shorting such a battery
could also be hazardous.
Low-cost cell phone batteries have infiltrated the world market since
the beginning of 2003,. These counterfeit batteries often do not have
an approved protection circuit and can vent with flame if the charger
malfunctions. Cell phone manufacturers strongly advise customers to replace
the battery with an approved brand. Failing to do so may void the warranty.
It is also highly recommended to only use approved chargers. (See photos
of an exploded cell phone with a clone battery that was on charge.)
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Cell phone with clone battery that vented withflame while on charge at the back of a car. |
When advising on the choice of batteries and chargers, cell phone manufacturers act out of genuine concern for safety rather than using scare tactics to persuade customers to buy their own accessories. They do not object third parties as long as the products are well built and safe. The buyer can often not distinguish between an original and a counterfeit battery because the label may appear bona fide.
Small lithium-ion packs with spinel (manganese) chemistry containing one or two cells may only include a fuse as protection. Spinel is more tolerant to abuse than cobalt and the cells are deemed safe if below a certain size.
Although less expensive, the absence of a protection circuit introduces a new problem. Cell phone users have access to low-cost chargers that may rely on the battery's protection circuit to terminate charge. Without the protection circuit, the cell voltage rises too high and damages the battery. Excess heat, even bulging can result. Discontinue using the battery and charger if a lithium-ion battery gets hot.
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Bulging of a lithium-ion cell |
To maintain safe operation, manufacturers do not sell the lithium-ion cells by themselves but make them available in a battery pack, complete with protection circuit. The circuit is often subject to exact scrutiny before the manufacturers release cells to the pack assemblers. Although there are a few reported incidents of venting with flame, the lithium-ion battery is safe.
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Created: April 2003, Last edited: September 2005
About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC.
Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.