Design and Implementation of Lithium-ion/Lithium-Polymer Battery Charger with Impedance Compensation S.-Y. Tseng, T.-C. Shih GreenPower Evolution Applied Research Lab (G-PEARL) Department of Electrical Engineering Chang Gung University Kwei-Shan, Tao-Yuan, Taiwan, R.O.C E-mail: sytseng@mail.cgu.edu.tw Tel: 886-3-2118800; Fax: 886-3-2118026 Abstract -- In this paper, an impedance-compensated battery charger is presented to increase the effective capacity of the lithium-ion/lithium-polymer battery. The proposed charger adopts a pulsating current source associated with a pulsating voltage detector to dynamically estimate compensated impedance of battery. With this approach, the discharge-time is extended 12% and the overall battery capacitor is increased 10%. The experimental results based on a lithium-ion battery charger with 11.4V/2.4Ah battery capacity verify these significant improvements. Index Terms--T impedance compendation; lithium-ion battery; lithium-polymer battery; charger I. INTRODUCTION Nowadays, the explosive increase in the number of portable electronic devices has results in massive demand for the secondary batteries. Three chemistries are widely used for secondary batteries: N i C d, N i MH and lithium-ion (Li-ion) batteries. However, N i C d and N i MH batteries do not satisfy people s requirements due to the low-energy capacity, bulky size, and environmental concerns. In contract, advantages of no memory effect, high operation voltage, and high energy density promote the Li-ion batteries in becoming the acceptable power source for portable electronic systems. [1] Many battery charger strategies have been proposed, such as constant trickle (CTC), constant current (CC), constant voltage (CV), and constant-current constant-voltage (CC-CV) battery charge strategies. Because the life cycles of Li-ion batteries are easily affected by undercharging and overcharging, the CC-CV charging method is conventionally used to charge Li-ion batteries. [2] As shown in Fig. 1, the I-V curves of Li-ion battery charge profile, battery is first charged at a constant current until the battery voltage reaches the predefined upper voltage V MAX (4.1 or 4.2V), then charged by a constant voltage until the current reaches a predetermined current I MIN (0.1C). [3] With the CC-CV charging method, the battery is charged by a degrading current after switching from CC stage to CV stage, preventing the battery from overcharging. Fig. 2 shows the typical Li-ion battery pack, which includes the Li-ion battery cell, protection controller, and external impedance. Due to the presence of highly reactive electrodes and non-aqueous electrolytes in a full charged Li-ion battery, a full charged Li-ion battery can vent with flame at *S.-Y. Fan, *G.-K. Chang * Linear Motor Research Laboratory Department of Electrical Engineering Wufeng Institute of Technology Ming-Hsiung, Chia-Yi, Taiwan E-mail: syfan@mail.wfc.edu.tw TEL: 886-5-2267125 high temperatures. [4] This is the reason why protection loop in Fig. 2 is required by the UL Standard 1642 and the upper voltage V MAX is predefined as 4.1 or 4.2V. [5] Fig. 3 shows the conceptual schematic of Li-ion battery pack based on typical Li-ion battery pack shown in Fig. 2. As illustrated in Fig. 3, the external impedance includes contacts, fuses, PCB trace wire, switches, and cell resistance. The external impedance can be in the range between 110 to 250 mω according to the used Li-ion batteries. The variations of voltage drop across the external impedance vary the transition time from the CC stage to the CV stage, which may damage the battery or reduce the energy capacity of the battery. Hence, the transition time for switching from CC stage to CV stage is serious issue for the charger due to the external impedance. [6,7,8] Fig. 1. Ideal waveform of the transition between CC and CV stage. Fig. 3. Fig. 2. Typical Li-ion battery pack. Conceptual schematic of Li-ion battery pack. 866
Several efforts have been devoted to study the external impedance In [6], a resistance-compensated phase locked battery charger is proposed to compensate the voltage drop across the pack impedance by using the inherent frequency-tracking, phase-tracking and phase-locked characteristics of phase-locked loop which needs complexity controller and calculation to realize. [9] uses the dynamic voltage-compensation technique to solve the voltage drop problem with a specified control IC. To simplify the voltage compensation circuit, an impedance compensation circuit realized by buck converter with pulsating current source is introduced into the conventional Li-ion battery charger in this paper. Here, buck converter operated in DCM mode can generate pulsating charge current with fixed frequency for battery charging. Then, compensated voltage obtained from the voltage difference caused by the pulsating charge current is feedback to the battery charger for charging current regulation. II. TEST ENVIRONMENT DESCRIPTION The batteries used in this study are Samsung Li-ion batteries. Nominal capacity of these batteries is 0.8Ah. Three series-connected batteries are used in the test. A test-bench shown in Fig. 4 was specifically designed to study charging/discharging characteristics of the Li-ion battery. Based on the requirement of standard for Li-ion, the charge rate is chosen to be 1C (2.4A), discharge rate is set to be 0.55C (1.35A). Environment temperature is one of the important factor in the battery charging and discharging test. Hence, as it can be seen from Fig. 4, a temperature-controlled thermal chamber is used to keep the environment temperature of 23 C during the test. 7 is proposed in this paper to solve this problem. Here, compensation voltage V BC caused by the external impedance can be obtained by charging the battery with pulsating charge current and detected by using the voltage detector. This voltage is used to regulate the reference voltage V ref for the impedance Fig. 5. Block diagram of a Li-ion battery charger without impedance Fig. 6. Waveform of the transition between CC and CV stages due to voltage drop. Fig. 4. Test-bench block diagram. III. SYSTEM DESCRIPTION Fig. 5 shows the block diagram of a Li-ion battery charger without impedance This comparator shown in Fig. 5 compares the voltage at the output of the battery pack and a reference voltage. Due to the voltage drop caused by the external impedance of battery, as shown in Fig. 6, the predefined upper voltage V MAX may be lower to V REG, which causes the battery charging process entering the CV stage early and in turns reducing the battery capacity. To prevent the battery from the effects of external impedance, an impedance compensation circuit constructed by pulsating current source, voltage detector and adder and shown in Fig. Fig. 7. Block diagram of a Li-ion battery charger with impedance Fig. 8. Circuit diagram of a Buck converter. I. Identify applicable sponsor/s here. If no sponsors, delete this text box. (sponsors) 867
IV. IMPEDANCE COMPENSATION CIRCUIT The pulsating current source shown in Fig. 7 is realized by the Buck converter shown in Fig. 8. To obtaining the pulsating charge current, the Buck converter is operated in the discontinuous current mode (DCM). A. Principal of Impedance Compensation Fig. 9 shows the steady-state waveforms of buck converter under DCM, where is switching voltage V G, is inductor current I L, (c) is battery voltage V CHG, and (d) is compensation voltage V BC. Here the inductor current I L is the pulsating charge current used to charge the battery. On the other hand, since the high energy density characteristics of a Li-ion battery, the battery voltage V CHG can be the pulsating voltage shown in Fig. 9(c). Then, having the battery voltage V CHG pass through a capacitor, the compensation voltage V BC can be obtained and is shown in Fig. 9(d). (c) (d) Fig. 9. Steady-state waveforms of buck converter under DCM : switching voltage, inductor current, (c) battery voltage, and (d) compensation voltage. B. Circuit Design To generate pulsating charge current for charging the battery, the peak value of output current needs to be keep constant for the PWM controller. That is, boundary conditions for the DCM operation mode and the specifications for the used component play essential roles in the circuit design. For Buck converter operated in the DCM, the relationship between output voltage V o and input voltage V i is given as V V o i D1 = (1) D + D where V o is output voltage, V i is input voltage, D 1 and D 2 are the duty ratios when the switch in Fig. 8 turns on and turns off respectively. For buck converter operated in DCM, the duty ratio D 1 should be restricted as Vo D 1 < (2) Vi Referring to Fig. 9, the peak value of the inductor current can be determined and given as 2IL Δ I = (3) D1 + D2 where ΔI is the peak value of the inductor current and I L is the inductor current. For buck converter operated in DCM, the boundary condition for the value of inductor can be expressed as ( Vi Vo ) D1T s L < (4) ΔI V. EXPERIMENTAL RESULTS To verify the performance of the proposed impedance compensation circuit, a pulsating current source realized by a buck converter under DCM operation mode was implemented. With the function of peal current control mode, PWM control IC UC3845 has been used to achieve the requirement of pulsating charge current. The specifications of the proposed circuit are as following: Input voltage V i : 19V, Output voltage V o : 9V ~ 13.5V, Switching frequency f s : 40KHz, Peak value of pulsating charge current I : 2A, Inductor L : 25uH, and Battery : Samsung Li-ion 11.1V/2.4Ah. Experimental waveforms of switching voltage V G and pulsating charge current I L are shown in Fig. 10. It reveals that a pulsating charge current with peak value of 1.9V has been generated and used to charge the battery. Fig. 11 shows the experimental waveforms of charge voltage V CHG and the inductor current I L. It can been seen from the Fig. 11 that the charge voltage V CHG appears to be in the form of pulsating due to the pulsating charging current. After filtering out the DC level of the charge voltage V CHG, the compensation voltage V BC shown in Fig, 12 can be obtained, and then was feedback to the battery charger for compensate the voltage drop caused by the external impedance. 1 2 868
(V G: 25 V/div, I L: 1 A/div, T S: 10 µs/div) Fig. 10. Experimental waveforms of switching voltage and the inductor current for buck converter under DCM. A Li-ion battery charger with a model of BenchMarq BQ24105 is used in the experimental test. The charger uses voltage detected type controller and its reference voltage for CC-CV transition is 2.1V. Also, the charge current during the CC stage is kept at 0.4A and the cut-off current during the CV stage is set at 0.24V. Hence, the total charge current provides to the Li-ion battery is 2.4A. With the test-bench shown in Fig. 4, test results of the charge voltage, charge current, and battery temperature under 1.5A charge current are shown in Fig. 13, where is without impedance compensation and is with impedance Comparing the curves shown in Fig. 13, it reveals that charger with impedance compensation has regulated the upper voltage V MAX for CV stage from 12.6V to 12.9V only at the cost of increasing the maximum battery temperature from 28 C to 30 C. From Fig. 13, it also can be found that the transition time has been prolonged for more than 11 minutes, hence increasing the battery capacitor for 275mAh, that is, 11.5% effective capacitor of the battery used. (V CHG: 5 V/div, I L: 1 A/div, T S: 10 µs/div) Fig. 11. Experimental waveforms of charge voltage and the inductor current for buck converter under DCM. ( V: 500 mv/div, I L: 1 A/div, T S: 10 µs/div) Fig. 12. Experimental waveforms of compensation voltage and the inductor current for buck converter under DCM. Fig. 13. Measured curves of Li-ion battery charge profile under charge current of 1.5A in 11.1V/2.4Ah battery pack without impedance compensation, and with impedance Fig. 14 shows the test results of the charge voltage, charge current, and battery temperature under 1.35A discharge current. Comparing the discharging time, the one for charger with impedance compensation is 10 minutes more than that for charger without impedance Estimating with the battery capacitor, it is 225mAh or 9.4% effective capacitor of the battery used. Table I shows the comparison of the experimental results for charger with and without impedance Obviously, it can be seen from Table 1, charger with the pro- 869
posed impedance compensation technique can provide the following advantages: Larger charger voltage for CV stage Prolong the transition time from CC to CV stage Longer discharge time Better effective capacitor Fig. 14. Measured curves of Li-ion battery discharge profile under discharge current of 1.35A in 11.1V/2.4Ah battery pack without impedance compensation, and with impedance test-bench was specifically designed to study charging/discharging characteristics of the Li-ion battery. Experimental results show that the discharge-time is extended 12% and the overall battery capacitor is increased 10% by using the proposed charger. REFERENCES [1] Yi-Hwa Liu, Jen-Chung Li, and Jen-Hao Teng, An FPGA-based lithium-ion battery charger system, IEEE TENCON Conference, vol. 4, pp. 435-438, Nov. 2004. [2] V. L. Teofilo, L. V. Merritt, and R. P. Hollandsworth, Advanced lithium ion battery charger, IEEE Aerospace and Electronic Systems Magazine, vol. 12, Nov. 1997, pp. 30 36. [3] Yi-Hwa Liu, Jen-Hao Teng, and Yu-Chung Lin, Search for an optimal rapid charging pattern for lithium-ion batteries using ant colony system algorithm, IEEE Transactions on Industrial Electronics, vol. 52, Oct. 2005, pp. 1328 1336. [4] J. Lopez, M. Gonzalez, J. C. Viera, and C. Blanco, Fast-charge in lithium-ion batteries for portable applications, INTELEC 26th Annual International Telecommunications Energy Conference, vol.2, 19-23 Sept. 2004, pp. 19 24. [5] UL Standard for Lithium Batteries, UL 1642, Third Edition, Dated April 26, this standard contains revisions through and including June 24, 1999. [6] L. R. Chen, J. Y. Han, J. L. Jaw,. C. P. Chou, and C. S. Liu, A Resistance-Compensated Phase-Locked Battery Charger, IEEE Conference on Industrial Electronics and Applications, 24-26 May 2006, pp. 1 6. [7] D. Salerno, and R. Korsunsky, Practical considerations in the design of lithium-ion battery protection systems, Applied Power Electronics Conference and Exposition, vol. 2, 15-19 Feb. 1998, pp. 700 707. [8] C. H. Lin, C. Y. Hsieh, and K. H. Chen, A Li-Ion Battery Charger with Smooth Control Circuit (SCC) and Built-in Resistance Compensator (BRC) for Achieving Stable and Fast Charging, IEEE Transactions on Circuits and Systems. (Accepted for future publication) [9] R. Saint-Pierre, A dynamic voltage-compensation technique for reducing charge time in lithium-ion batteries, The Fifteenth Annual Battery Conference on Applications and Advances, 11-14 Jan. 2000, pp. 179 184. TABLE I COMPARISONS OF THE EXPERIMENTAL RESULTS FOR CHARGER WITH AND WITHOUT IMPEDANCE COMPENSATION Design of battery Without impedance With impedance compensation compensation Charge time 127 Min 128 Min Impedance detection No 160mΩ Upper voltage 12.6 V 12.9 V Discharge time 83 Min 93 Min Discharge capacitor 2175 mah 2400 mah Effective capacitor 90.6% 100% VI. CONCLUSIONS The paper proposed a Li-ion battery charger that adopts a pulsating current source associated with a pulsating voltage detector to dynamically estimate compensation impedance of battery. A prototype of the proposed charger was implemented to verify the impedance compensation function of the proposed charger. Here, the pulsating current source was realized by a buck converter under DCM operation mode. A 870