Introduction
Lithium Ion (Li-Ion) batteries are becoming more avail-able in the marketplace, allowing system designers to u both nickel metal-hydride (NiMH) and Li-Ion bat-tery types to power their portable equipment. The bat-teries, however, require different charge schemes. NiMH batteries are usually fast-charged at a constant current and terminated by either peak voltage detection, PVD, or the increasing ri in temperature at the end of full charge, ∆T/∆t. Li-Ion batteries are usually charged at a constant voltage with a 1C current limit. Charge is usu-ally terminated by time or when the charging current drops to a very low rate, typically less than C ⁄30, indicat-ing that the battery is full.
In addition to battery charging, the designer has the task of battery monitoring and capacity reporting. NiMH bat-teries are typically monitored for end-of-discharge volt-age, battery temperature, and charge and discharge current. With a fairly flat discharge voltage over about 80% of its capacity, capacity gauging for NiMH is done by determining the Amp-hour capacity removed during dis-charge and replaced during charge. NiMH batteries loo capacity due to lf-discharge, which is determined by the temperature of the battery and is about 1.5 to 2
Li-Ion batteries also require monitoring for capacity and state of charge. Li-Ion batteries using coke electrodes have a sloping discharge as shown in Figure 1. In some cas, the voltage during discharge can be ud as an indicator of state of charge, but the voltage must be cor-rected for charge/discharge rate and ambient tempera-ture. Voltage is acceptable for full or empty indication,but the better approach would be to monitor the capacity removed and the capacity replaced to determine the bat-tery state of charge. This method would be more applica-ble to the ot
her type of Li-Ion battery, which us graphite electrodes. The graphite Li-Ion battery has a much flatter discharge profile making voltage-bad gauging much less accurate than the coke Li-Ion batter-ies. The lf-discharge for both types of Li-Ion batteries is about 1⁄10th of that for NiMH batteries.
Benchmarq Microelectronics is developing charge con-
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trollers, battery protectors, and capacity gauging ICs specifically tailored for the Li-Ion battery. Today, how-ever, Li-Ion batteries can be charged and monitored us-ing existing Benchmarq products. The purpo of this paper is to describe how a subsystem can be developed that will support both NiMH and Li-Ion batteries using existing Benchmarq ICs and easily transition to the new IC developments.
Figure 1. Li-Ion Battery Discharge Curve (Coke Electrodes)
Using NiMH and Li-Ion
Batteries in Portable Applications
Apr. 1995
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Charger
The charger is bad on the bq2004 operating in the switch-mode topology as shown in Figure 2. The charger can be controlled by either a bq2004 or a bq2004E. The charger is operated in a buck configuration where BAT+ is the battery pack positive contact and BAT– is the bat-tery pack negative contact. When the battery is a NiMH pack, the SELC connection is not connected. When the battery is a Li-Ion battery, then the SELC contact is tied to the BAT+ contact within the battery pack. The bat-tery also provides a thermistor contact so that charging can be qualified by the battery temperature and ∆T/∆t can be ud for charge termination.
L1 is made using a composite core, MICROMETALS PN ST50-267, in a toroid geometry (e attached data sheet). The toroid is wound with 70 turns of 22 gauge copper magnet wire. The initial inductance is about 3mH. Be-cau the inductance is a function of the current, the greater the current, the lower the inductance. This prop-erty allows for a greater range of current with smaller changes in switching frequency. The current range is needed for the lithium battery where the charg
e current decreas during charging as the battery EMF ap-proaches the maximum allowable charging voltage. The switching frequency is about 30KHz.
When the SELC contact is floated, the charge lection is made for NiMH. In this mode, the bq2004 is configured for 1C charging with top-off and pul trickle. The charge current is t to 2.25A. In this example, the bat-tery divider is configured for nine cells. The ∆T/∆t nsi-tivity is configured using R8 and R9, and the maximum charge temperature is t by the resistors R5 and R6. The bqCharge disk provides a program to calculate the proper values for the resistors depending on the appli-
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聚集的意思是什么cation requirements and the thermistor choice. The
Figure 2. Li-Ion/NiMH bq2004 Switch-Mode Charging System
Apr. 1995 Using NiMH and Li-Ion Batteries in Portable Applications
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functional operation of the bq2004 and bq2004E is de-scribed in their respective data sheets.
When the SELC contact is at the BAT+ potential, the Li-Ion mode is lected. The battery pack is configured for three-by-three battery configuration, three strings of three cells in ries connected in parallel. The TM1 and TM2 pins are t to provide a six-hour time-out with no top-off or trickle. The battery starts charging at the cur-rent limit t to 1.9A and is voltage-limited to 4.225V. For graphite Li-Ion cells, R19 and R20 are changed to limit charge voltage to 4.125V. During charging, the cur-rent varies as the battery EMF reaches the voltage limit. Full charge is indicated after the time-out of six hours. Capacity Gauging
Capacity gauging is an important ur feature for both NiMH and Li-Ion. Capacity gauging is provided by the bq2014 for NiMH batteries and can be configured using the information in the bq201
4 data sheet. The bq2014 can also be ud for Li-Ion capacity gauging and is dis-cusd in this paper.
Figure 3 shows the bq2014 monitoring an NiMH battery configured similar to that described in the above charger ction. The application can identify the battery pack as NiMH by bit 4 of the PPU register. This bit is 0 for nickel-bad chemistries. The bq2014 provides the proper compensation for charge and discharge rates with temperature compensation and lf-discharge correction. The bq2014 provides software-adjustable end-of-discharge voltage lection. Battery voltage is also available. The capacity of the battery is reported in a 8-bit register pair. The typical application scales this value bad on the n resistor ud to get the Amp-hour capacity. The application can also scale the capac-ity to Watt-hours by using the battery voltage. During discharge, the battery voltage is read from the bq2014 and averaged with the end-of-discharge voltage. This is
the average available voltage for the remaining dis-
宽面条Figure 3. bq2014 NiMH Battery Capacity Monitoring System
Apr. 1995
Using NiMH and Li-Ion Batteries in Portable Applications
3
charge at the current discharge rate. The average volt-age is then multiplied by the remaining capacity to get the remaining Watthours. Although NiMH batteries are usually gauged in Amp-hours due to their relatively flat discharge profile, Watt-hour capacity can also be ud for consistency with Li-Ion batteries.
Li-Ion battery capacity can be obtained using the bq2014 as shown in Figure 4. The primary difference is the con-figuration for the capacity and pulling PROG5 high to d isable lf-discharge compensation. The lf-dis-charge for Li-Ion batteries is about 1⁄10th of that for NiMH and can be neglected in most applications. For tho applications that choo to compensate for lf-dis-charge, the BATID register can be written with the week of the year so the time that the battery might have been expod to lf-discharge can be measured; however, in most applications, this correction is small enough to be neglected. Although the capacity for Li-Ion batteries is usually reported in Watt-hours, the capacity can be com-puted as described above. Figure 5 shows the cycl
e pro-file for a Li-Ion battery that has been discharged to 2.7 volts per cell after various levels of partial recharge. The battery capacity is determined properly, and the ur can be comfortable with using the battery near the end of capacity.
Summary
单位证明格式模板
Benchmarq is developing a family of compatible Li-Ion chargers, protectors, and capacity gauges. Using existing products, manufacturers can go to market today with chargers from Benchmarq that support both NiMH and Li-Ion batteries. Battery capacity can be determined for
both NiMH and Li-Ion batteries using the bq2014.
Figure 4. bq2014 Li-Ion Battery Capacity Monitoring System
Apr. 1995 Using NiMH and Li-Ion Batteries in Portable Applications
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大学总结
Using NiMH and Li-Ion Batteries in Portable Applications
Figure 5. Li-Ion Battery Discharge and Capacity Profile
Apr. 1995
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