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NE57610BDH Datasheet(PDF) 9 Page - NXP Semiconductors |
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NE57610BDH Datasheet(HTML) 9 Page - NXP Semiconductors |
9 / 16 page Philips Semiconductors Product data NE57610 Li-ion battery charger control with adjustable thresholds 2002 Nov 05 9 TECHNICAL DISCUSSION Lithium-ion cells: general information Lithium-ion and polymer cells have higher voltage than nickel cadmium (NiCd) or nickel metal hydride (NiMH) rechargeable cells. The average operating voltage of a lithium-ion or polymer cell is 3.6 V compared to the 1.2 V of NiCd and NiMH cells. The internal resistances of the various types of lithium cells are 50 m Ω to 300 m Ω, compared to the 5 mΩ to 50 mΩ of the nickel chemistries. This makes Lithium-ion and polymer cells better for lower battery current applications, less than 1 ampere, such as cellular and wireless telephones, palmtop and laptop computers, etc. Lithium-ion and polymer cells are safe as long as the cell is maintained within a particular set of operating boundaries. The cells have a porous carbon, or graphite anode where individual lithium ions can lodge themselves within the pores. This keeps the lithium ions separated, and any hazardous condition is avoided, if the cell is kept within the safe operating boundaries. A lithium cell protection circuit is placed within the battery pack. It monitors the level of voltage across each cell for overcharge and overdischarge conditions, and the discharge current in the event of an overcurrent or short-circuit condition. If the lithium cell is overcharged, pure metallic lithium plates out onto the surface of the anode. Also volatile gas is generated within the cell. This creates a hazard. Conversely, if the cell were allowed to over-discharge (Vcell less than typically 2.3 V), the chemistry of the cell changes and the copper metal used in its construction enters the electrolyte solution. This severely shortens the cycle life of the cell, but presents no future safety hazard. When the cell experiences high charge or discharge currents, then the internal series resistance of the cell creates heating and generation of the volatile gas which could again present a hazard. Charging lithium cells An integral part of any Li-ion battery system is a battery charger specifically designed for the lithium cell being used, with its particular over and undercharge limits, capacity, etc. The battery charger should be viewed as a part of the entire lithium battery system so that safe cell operation can be ensured. Lithium cells must be charged with a dedicated charging controller such as the NE57610. The charging ICs, in general, can be described as performing: a current-limited, constant-voltage charge process. When the cell is very discharged, the charger IC outputs a constant current into the battery, which limits the internal heating of the cells. The maximum charge rate is typically the capacity rating of the cell. That is, the maximum charge current is the mAHr rating of the cell(s), that is, a 1000 mAHr cell will be charge with a maximum of 1000 mA. When the cell voltage approaches its full-charged voltage rating (VOV), the current entering the cell begins to decrease, and the charger IC provides a constant voltage-mode of charge. The charge current begins to exponentially decrease over a long period of time (approximately 1.5 – 2.0 hours). When the charge current falls below a preset amount, the charge current is discontinued. If charging begins with the cell voltage below the overdischarged voltage rating of the cell (VUV), it is very important to slowly raise the cell voltage up to this overdischarged voltage level. This is done with a reconditioning charge. A small amount of current is allowed into the cell, and the cell voltage is allowed, for a pre-set period of time, to rise to the overdischarged voltage (VUV). If the cell voltage recovers, a normal charging sequence can begin as described above. If the cell does not reach the overdischarged voltage level, then the cell is considered too damaged to charge and the charge is discontinued. It is important to allow enough time to charge the cell to take advantage of the higher energy density of the lithium cells. When the charger switches from constant current charge to constant voltage charge (Point B, Figure 16) the cell only contains about 80 percent of its full-rated capacity. When the cell is 100 mV less than its full rated charge voltage, the capacity contained within the cell is about 95 percent. Allowing the cell to slowly complete its charge takes advantage of the larger capacity of the lithium cells. The complete charging curve can be seen in Figure 16. SL01554 1.0 0.5 1.0 2.0 3.0 4.0 1.0 2.0 Point B Vov TIME (HOURS) TIME (HOURS) CONSTANT VOLTAGE CONSTANT CURRENT Figure 16. Lithium-ion charging curves. |
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