Safety circuits for modern batteries (BU10) 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.
Figure 1: A common safety circuit for cellular phone applications. Because of improved thermal stability of manganese-based lithium-ion, manufacturers relax the need for an external safety circuit on packs with two cells in series. 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.)
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.
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. ________________________ 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.
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