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NE57600CD Datasheet(PDF) 6 Page - NXP Semiconductors |
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NE57600CD Datasheet(HTML) 6 Page - NXP Semiconductors |
6 / 12 page Philips Semiconductors Product data NE57600 One-cell Lithium-ion battery protection with over/undercharge and overcurrent protection 2001 Oct 03 6 TECHNICAL DISCUSSION Lithium Cell Safety Lithium-ion and lithium-polymer cells have a higher energy density than that of nickel-cadmium or nickel metal hydride cells and have a much lighter weight. This makes the lithium cells attractive for use in portable products. However, lithium cells require a protection circuit within the battery pack because certain operating conditions can be hazardous to the battery or the operator, if allowed to continue. Lithium cells have a porous carbon or graphite anode where lithium ions can lodge themselves in the pores. The lithium ions are separated, which avoids the hazards of metallic lithium. If the lithium cell is allowed to become overcharged, metallic lithium plates out onto the surface of the anode and volatile gas is generated within the cell. This creates a rapid-disassembly hazard (the battery ruptures). If the cell is allowed to over-discharge (Vcell less than approximately 2.3 V), then the copper metal from the cathode goes into the electrolyte solution. This shortens the cycle life of the cell, but presents no safety hazard. If the cell experiences excessive charge or discharge currents, as happens if the wrong charger is used, or if the terminals short circuit, the internal series resistance of the cell creates heating and generates the volatile gas which could rupture the battery. The protection circuit continuously monitors the cell voltage for an overcharged condition or an overdischarged condition. It also continuously monitors the output for an overcurrent condition. If any of these conditions are encountered, the protection circuit opens a series MOSFET switch to terminate the abnormal condition. The lithium cell protection circuit is placed within the battery pack very close to the cell. Charging control versus battery protection The battery pack industry does not recommend using the pack’s internal protection circuit to end the charging process. The external battery charger should have a charge termination circuit in it, such as that provided by the SA57611. This provides two levels of overcharge protection, with the primary protection of the external charge control circuit and the backup protection from the battery pack’s protection circuit. The charge termination circuit will be set to stop charging at a level around 50 mV less than the overvoltage threshold voltage of the battery pack’s own protection circuit. Lithium Cell Operating Characteristics The internal resistance of lithium cells is in the 100 m Ω range, compared to the 5–20 m Ω of the nickel-based batteries. This makes the Lithium-ion and polymer cells better for lower battery current applications (less than 1 ampere) as found in cellular and wireless telephones, palmtop and laptop computers, etc. The average operating voltage of a lithium-ion or polymer cell is 3.6 V as compared to the 1.2 V of NiCd and NiMH cells. The typical discharge curve for Lithium cell is shown in Figure 6. SL01553 100 50 2.0 3.0 4.0 NORMALIZED CELL CAPACITY (%) VUV VOV Figure 6. Lithium discharge curve. |
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