LMLMLPLP Battery Charging Literature Number SNVA - PDF document

power supply design. The complexity (and cost) of the charging system is primarily is typically a system that can recharge a battery in aboutboth the battery chemistry and cell construction. When the cell is fully charged, contin-ued charging causes gas to form within the cell. All of the gas formed must be able totry, but also on the construction of the internal electrodes. This has been improved in the cell's A-hr rating) indefinitely with no damage to the cell. At this rate, a typical continuous trickle charge currents as high as c/3. Applying c/3 would allow fully charg- NI-MH: Ni-MH cells are not as tolerant of sustained charging: the maximum safe tricklecorresponds to a charge rate of about 1.2c. The vast majority of applications whereC is typically considered optimal for charging. Fast charging at (meaning it makes the cell get cooler), while (it makes the cell heat up). The importance ofR. The internal sense resistor (with a gain ofminate fast-charge: voltage or temperature is typically the primary method, with a timer INSWGNDFB + ++NI-CD/NI-MH0.05W680mF35VVINGND -15 5A FIGURE 1. 2.6A NI-CD/NI-MH CHARGER T DETECTORFigure 3 shows a schematic diagram of a circuit which measures both the ambient tem-perature and the battery temperature and produces a high signal when the cell tempera-ture is 10°C above ambient.The signal coming from the ambient sensor (10 mV/°C) is level shifted up 100 mV by aunity-gain buffer stage (the 100 mV shift corresponds to 10°C).The signal from the battery sensor is compared to the level-shifted ambient signal bythe second amplifier, which is connected as a comparator. When the two signals are equal, the battery temperature is 10°C above ambient, and ahigh signal is provided on the S/D line which can be used to shut off the high currentcharger shown in Figure 1. rate-of-change (slope) of the cell temperature can be usedfor end-of-charge detection with both Ni-Cd and Ni-MH batteries. This type of circuit isP-based systems: temperature read-ings are taken at timed intervals, and stored in memory. The present temperature read- + -+ INOUT GNDINOUT BATTERYPACK VS/DVBAT+VBAT-5.1K470K9.1K1M1M1M1M1M5.1KLM3539K662LPC AMBIENTSENSOR BATTERYSENSORLM35 78L05 IN (8-30V)(FAST-CHARGE TERMINATION SIGNAL TO CURRENT SOURCE)DT DETECTOR CIRCUIT then averaged to get the data value to be stored for comparison. This greatly reduces Decreasing Positive Slope: This termination method looks for the part of the voltagecurve where it is rising more slowly, but still has positive slope. This point on the curve 0 D slope, which is the peak of the curve. The detector system would identify zero slope - D V: The part of the curve which has negative slope (just past the peak of the curve) isflowing into the battery. The switching regulator maintains good efficiency over a widefrom one of four different charging currents by changing the logic levels at two bits. Therates. The recommended trickle-charge rate for a Ni-Cd is typically c/10, but for Ni-MH, control loop is implemented using op-amp U2. The voltage drop across the sensepower dissipated in this resistor. If a higher Ohmic value is used (more resistance), aat 1.23V. The battery charging current that corresponds to this voltage is dependent oncurrent. The load current is higher with Q1 on because R2 and R3 divide down thefeedback line of U1. Higher voltage at the output of U2 means that more chargingThe operation of Q2 is similar to Q1: when Q2 is turned on by putting a logic "1" oninput "B", the load current is increased by a factor of 10:1. This is because when Q2 isbattery is disconnected. These components prevent the voltage at the cathode side of coming from U2. Adding this component improved the accuracy of the measured constant voltage (C-V) charger sources current into the battery in an attempt to force orcharge cycles the battery can complete is reduced (shortened battery life). If the volt- 012 01002401C% CHARGEDELIVEREDCELLVOLTAGECHARGERATE LIMIT 3 also possible (with a correspondingly longer charge time). The design example pre-Ion battery in a portable stereo. The customer specification for the charger was 150 mAover temperature. Also, the LP2951 has built-in current limiting that prevents the output portion of the INOU T S/DGND The output voltage is set by the resistive divider made up of R3, R4, and R5. Since an ing back through the internal diode of Q2 when the DC input is removed. A 16A diode 13 - 16VDCINPUT WITHCURRENTLIMITING INCOMPOUTGND16A3-CELLLI-IONP-FET(0.1W)20k20k1N9141N9143.9k6.9V100kN-FET(2W)1N9144.7nF2N39042K-12.6+-+ +- +mFZenerQ2Q1D1D2R1R2R3R4R5D3D4D5C1C2Q3FIGURE 10. 3-CELL, 3A CHARGER USING THE LM3420 IMPORTANTNOTICE TexasInstrumentsIncorporatedanditssubsidiaries(TI)reservetherighttomakecorrections,modifications,enhancements,improvements, andotherchangestoitsproductsandservicesatanytimeandtodiscontinueanyproductorservicewithoutnotice.Customersshould obtainthelatestrelevantinformationbeforeplacingordersandshouldverifythatsuchinformationiscurrentandcomplete.Allproductsare soldsubjecttoTI¶stermsandconditionsofsalesuppliedatthetimeoforderacknowledgment. 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