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Development of a Universal Adaptive Battery Charger as an Educational Project

Heike Barth2, Christoph Schaeper1, Tim Schmidla2, Hannes Nordmann1, Martin Kiel1, Heinz van der Broeck2

Yusuf Yurdagel1, Christoph Wieczorek1, Frank Hecht1, Dirk Uwe Sauer1

1 Institute for Power Electronics and Electrical Drives (ISEA), RWTH Aachen University Jaegerstr. 17/19, D - 52066 Aachen, Germany

2 Institute for Automation, Cologne University of Applied Sciences Betzdorferstr. 2, D – 50679 Köln, Germany

Abstract - The number of small portable electronic devices (such as notebooks, mobile phones, etc.) has risen continuously over the last years. For charging the batteries of these devices a mains charger is required. Unfortunately each portable device requires an own special battery charger which shows a low efficiency and a poor power factor in many cases.

In order to solve this problem the IEEE has organised theinternational student competition “Future Energy Challenge 2007” aiming at the development of a highly efficient universal battery charger. This charger should be operated at all AC voltages between 90 V and 270 V and it should be able to optimal charge four different battery types (Lead Acid, NiCd, NiMH, Li-Ion) with an unknown number of cells. As a very challenging task the type and number of cells of the battery has to be determined by the terminal behaviour of the battery only.

Within the paper the competition contribution of a combined student team from the RWTH Aachen University and the Cologne University of Applied Sciences is presented. This includes organisation, support and experiences of the student team, design and development of the universal battery charger as well as measurements at the prototype.

I. INTRODUCTION

During the last ten years the number of portable electronic devices has risen continuously. Today, in most households at least one notebook, a digital camera, several mobile phones and MP3-Players can be found. All these devices are fed by batteries which have to be charged regularly by mains appliances. With each mobile device an own battery charger is delivered which can normally be used for this certain device only. In addition, many battery chargers operate inefficiently and cause high stand-by losses. The latter is especially problematic as many users do not disconnect their battery chargers from the mains after use. For the consumers it would be desirable to need only one battery charger which can be used to charge batteries of all portable devices. From the ecological point of view this device should work at a high efficiency.

A. Student Competition In order to find a solution according to this problem the

IEEE has announced the international student competition “Future Energy Challenge in 2007” with the task to develop a universal battery charger. This device should operate with an almost unity power factor on all mains voltages between 90V and 270V AC and frequencies

between 48 Hz and 400 Hz. Furthermore it should provide galvanic isolation and charging currents up to 2 A. The special challenge in developing such a battery charger is to detect different battery types with different cell numbers and to charge the batteries independently of their polarity. In addition, the battery type (Pb, NiCd, NiMH, Li-Ion) and the cell number should be identified from the terminal behavior only.

B. Student Team: Organization and Experience The supervisors of the student team [*] decided to take

part in the competition in autumn 2006 with a combined student team of the RWTH Aachen University and the Cologne University of Applied Sciences. Finally, nine students were found [**] who were willing to spend a big part of their spare time in the development of a universal battery charger beside their study. The supervisors prepared the team members during special lectures on switch mode power supplies and battery technologies for the competition. By visiting electronic trade fairs the students contacted several companies which supported the project by free software, components and devices.

Meetings in Cologne and Aachen took place regularly and the developing work was planned and split up so that the students could do most of their developing work independently of each other. Thus, the group in Cologne designed the main power converter with galvanic isolation and the auxiliary voltage supplies. In Aachen the charging unit with the measuring system was built up. Furthermore the battery-detection algorithms were developed and programmed in the microcontroller.

Figure 1 Final version of the developed battery charger

978-1-4244-1668-4/08/$25.00 ©2008 IEEE

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During the first months only little progress could be achieved, because the students had to familiarize themselves with the different components and circuits as well as with the software. Thus, only smaller circuit parts were built up hand-wired. Many measurements on several battery types were carried out using a professional charge and discharge system. Not all attempts were directly successful: Electronic components were destroyed by overload or errors in the control circuit. The regulation and control functions were disturbed by missing shielding as well as by too long connecting wires. Only after several redesigns the circuit parts could be realized on two printed circuit boards during the final stage. These layouts were designed by the students in Cologne (power supply: fig. 3) and in Aachen (charging system: fig. 4) independently.

The progress of the project work was documented in two reports for the IEEE and was sent to reviewers in the USA. In spring 2007 two team members were invited to take part in a workshop of the Future Energy Challenge in Anaheim, California. There they presented the progress of the team and discussed the subject with other students from Australia, Bangladesh, the USA, Brazil and China. During the last months before the final competition in Dallas most progress was done. In July both boards were connected together for the first time and worked properly The whole universal charger is presented in figure 1.

II. BATTERY CHARGER

A. Overall Topology and Control Figure 2 shows the circuit schematic of the charger with

the most important components. The input part consists of a rectifier with PFC function, which works in the discontinuous mode. It draws a nearly sinusoidal current from the mains after HF filtering (not shown in fig. 2) Additionally, it regulates the DC link voltage U1 to approximately 370V independently of the mains voltage.

As the second power stage a flyback converter is applied, which generates a galvanic isolated and stabilized output voltage Uo = 32V. Both converters are controlled by the NXP Green Chip TEA1750. This IC minimizes the switching losses by “valley switching” and by reduction of the switching frequency at low load. During stand-by mode the PFC function of the rectifier is disabled and the flyback converter works in “burst mode”. Thus the stand-by losses can be diminished to less than 0.5 W.

Figure 3 PCB layout of the PFC & flyback converter

Figure 4 PCB layout of the transistor H-bridge and the measurement and C unit.

Another loss reduction is achieved by the application of a synchronous rectifier (T3) on the secondary side. Figure 3 presents the corresponding PCB with all mounted components. The two SMD controller ICs are placed on the bottom side.

The third power stage (Fig. 4) includes an H-bridge consisting of four transistors. According to the polarity of the battery the lower transistor of one half bridge is in on-state and the current is regulated by the other half bridge via the PWM unit of the microcontroller ADuC7026 as well as by a current control circuit.

Figure 2. Overall circuit topology of the universal charger

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In order to detect the battery type a defined discharge current of 2 A has to be taken for at least one minute. This current is fed into the lower DC link Uo. The related power can be used to supply the μC as well as the control and measuring unit. If the discharge power exceeds the need of the auxiliary circuits, the voltage Uo further rises. At a value of Uo = 36 V the discharge power is taken over by the bipolar transistor T4 mounted on a heat sink. In worst case losses up to PV = 32 V · 2 A = 64 W have to be dissipated in heat.

B. Performance of the PFC and the Flyback Converter In order to combine the control and drive signals for the

PFC and the flyback power stages by one single chip the Green Chip controller TEA1750 was applied. This new advanced controller was introduced by NXP on the fair of the APEC conference in Anaheim, the student team visited after the IFEC07 workshop. In addition, another controller IC, the TEA1761 has been inserted on the secondary side of the flyback converter. The secondary control IC allows synchronous rectification and provides output voltage feedback control respectively.

Special functions of both controller ICs allow high efficiency of the converter over a wide power range. This is obtained by quasi-resonant or valley switching operation at high and medium power and by reducing the switching frequency for a low power load. At low power levels and low mains voltage the power factor controller operates in burst mode control in order to maintain a high efficiency in the PFC part.

The boost converter is controlled by a constant on-time method of transistor T1. The output voltage at capacitor C1 is stabilised to U1 = 370 V by adapting the on-time of transistor T1 to the given load. The converter operates in the discontinuous switching mode. This means, the transistor voltage Uds starts oscillating when the secondary current becomes zero. In conventional DC-DC converters a fixed switching frequency and duty cycle is set for a certain load point and the transistor is turned-on at certain time instants, where the oscillating transistor voltage may be high. This causes high turn-on losses. In the new controller IC TEA1750 the transistor is only turned on at the minimum values of the oscillating voltage (valley switching) which reduces the switching losses and EMI contribution substantially.

The triangular current of the boost converter is smoothed by a small differential mode LC filter. In addition, a common mode filter is inserted between the mains and the rectifier bridge.

Fig. 5 shows the voltage and the current of the mains together with the discontinuous HF current in choke L1 for a mains period. It confirms the performance of the LC filter.

The operation modes of the PFC depend on the value of the mains voltage and of the load.

At low loads but higher mains voltages the PFC function is deactivated. The corresponding curves measured at a mains voltage of 220 V are shown in Fig. 6. As can be seen the DC link voltage U1 at C1 is equal to the peak mains voltage.

The normal PFC operation at different mains voltages is illustrated in Figure 7 and 8. In both measurements the converted power was about 50 W. As the power stays the same, the mains current changes with the input voltage. Nevertheless the mains current is in phase with the voltage and shows an almost sinusoidal wave which leads to a high power factor.

Furthermore the different operation modes of the flyback converter will be described. The converter basically stabilises and isolates the flyback output voltage to Uo = 32V.

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Similar to the PFC, the flyback converter is either operated in the burst mode (low power), in a frequency control mode with fixed ton-time or in the critical conduction mode.

In all cases valley switching is applied to reduce the switching losses. This is illustrated in the measurement of Fig. 9 which shows the voltage at transistor T2 and the secondary current of the flyback converter. The period of the flyback operation changes on each cycle to maintain valley switching.

Figure 9. Operation of the flyback converter: transistor voltage(UT), and secondary winding current (I2)

The converter performance has been measured for a mains input voltage of 100 V and 230 V RMS. In both cases the output power was varied from zero to 64 W. Under all conditions the output voltage was stabilised to Uo = 32 V (Iout = 0 ... 2 A). The efficiency is above 85% for a large power range.

C. Voltage Measurement Circuit In order to detect the battery type and to charge it

properly the battery voltage is measured repetitively by the microcontroller. The voltages of the battery lie in a range between 1V and 32V and have to be measured with a resolution of at least 5mV. This cannot be achieved by using the 12 Bit A/D channel resolution. In order to increase the resolution of the A/D conversion, a programmable reference value provided by the D/A output of the microcontroller is subtracted from the measured voltage of the battery by a special operational amplifier circuit. The basic function of this unit is illustrated in Figure 10.

The microcontroller sets an appropriate subtraction voltage to fit the measuring window to the battery voltage. The resulting signal (red curve) is now acquired by the microcontroller. By means of this easy to implement and cost effective circuit an improved resolution of two additional bits is achieved. As a result an absolute resolution of about 2mV can be obtained.

D. Battery Detection The four battery types NiCD, NiMH, lead-acid and Li-

Ion need only two different charging regimes: Constant Current (CC) for NiXX and Constant Voltage (CV) for Li-Ion and lead-acid batteries. The goal is to detect the correct battery type and the number of cells during the charging process. For Li-Ion and lead-acid batteries it is necessary to set a maximum charging voltage.

The identification algorithm has been developed using high advanced battery test equipment at ISEA. Later it has been adapted to the universal charger hardware and implemented into the microcontroller.

After a system reset the four transistors of the H-bridge are switched off to high impedance in order to measure voltage at both connection poles. Once a voltage value higher than 1V is detected, the controller determines the polarity of the connected battery. Depending on the polarity one inverter leg is connected to ground and the other one is used to control the current with PWM.

The first step for detecting the connected battery is to measure the open circuit voltage (OCV). This is done when there is almost no further change in the voltage value which may take a few minutes. Based on this information, the possible number and type of the cells connected in series can be achieved.

battery NiXX Lead-Acid Li-Ion V min 1.1 V 1.95 V 3.2 V V max 1.35 V 2.15 V 4.2 V

TABLE 1 OPEN CIRCUIT VOLTAGE RANGE PER CELL

Table 1 shows minimum and maximum expected cell voltages for three battery technologies. Multiplied with the number of cells the voltage ranges for all possible cell combinations are calculated. These open circuit voltage ranges are shown in Figure 11. For example an OCV of 12.4 Volt is within the voltage ranges of 3 Li-Ion cells, 6 Lead-Acid cells and 10 or 11 NiXX cells. The relative “position” within this range gives a rough estimation of the state of charge (SOC) of these particular cell combinations. So the possible cell combinations are assigned in three categories: Full, empty and medium state of charge.

The charging process always starts with the constant current mode. To avoid an overcharge of Li-Ion or Pb cells a safety voltage limit has to be set. In case of three Li-Ion cells the maximum charge voltage can be calculated to VVU 6.122.43max . This voltage limit must not be exceeded as long as a Li-Ion battery can not be excluded.

Figure 10. Voltage window fitting

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The second detection algorithm is based on the difference in the open circuit voltage OCV after charging and discharging. In order to measure the so called hysteresis voltage the OCV has to be measured twice at the same state of charge. This sequence is shown in Figure 12 and has to be performed by the charger. The sequence starts with a discharge pulse to avoid a possible overcharge of a full battery. Firstly, the battery is discharged for a certain time (black curve). Afterwards, the charger waits, until the voltage has reached a steady state again (point A). Then, the battery is discharged again for 30 seconds with 2A and afterwards recharged for the same time with the same current. This leads to point B. The state of charge between point A and point B is the same, but as shown in Figure 12, the open circuit voltage is different due to the hysteresis phenomena. This effect is distinct for Nickel based batteries [1, 2].

If an empty battery is connected which can be detected by a large discharge voltage drop, the hysteresis measurement is carried out at a higher SOC. A set of own experiment results shows, that the hysteresis voltage is a function of the cell technology and capacity. Batteries with high capacities show lower hysteresis voltages.

If a Ni based battery can not be detected properly by the hysteresis measurement, the battery is further charged with constant current considering the safety voltage determined for a possible Li-Ion battery pack. If the battery voltage decreases during charging (-dV/dt criteria) a full Ni battery is detected and the charging process is stopped. If the safety voltage is reached during constant current charging, constant voltage charging is applied until the current is almost 0A.

During searching for suitable detection algorithms, other possible methods have been tested. One of them is shown in Figure 13. During constant current charging with 1A, the charger applies short discharge and charge current pulses of 2A for ten seconds.

Figure 11 Open circuit voltage range for various numbers of series connected cells of the batteries to be detected

Figure 12. Hysteresis measurement sequence Figure 13. Voltage response of current pulses

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This is carried out in time intervals of ten minutes. As shown in Figure 13, the corresponding voltage response of a lead-acid battery is much higher at high SOC [3]. The voltage response of the other battery types is much lower under the same conditions. Therefore the current pulses method is suitable to detect lead-acid batteries. During this detection method with constant current charging it is also possible to detect nickel-based batteries when the voltage decreases.

Figure 14. Detection algorithm

Figure 14 shows a flowchart of all applied detection algorithms. After the startup the charger begins with the OCV measurement. The possible cell combinations as well as safety voltage limits to avoid overcharging of a Lithium-Ion battery are set. This voltage limit remains during the complete charging process, until a Li-Ion battery can be excluded. Thus, in the worst case the connected battery may not be charged completely.

III. EXPERIMENTAL RESULTS

The input and output performance of the developed universal charger will be shown by two measurements. Figure 15 presents the output current response if the charging current changes from zero to 2A. The measurement has been taken by charging a battery pack of 10 NiMH cells which results in a battery voltage of about UB = 14.5 V. The switching frequency of the PWM is fs = 50KHz as can be seen by the inverter output voltage Uwr(t) in figure 15. The rise time of the output battery current (0 to 2A) is less than 150 s.

The next measurement shows the behavior of the mains connected battery charger in a transient from charge mode ( AI B 2 ) to discharge mode ( AIB 2 )In this case a 12 V lead-acid battery was supplied. In figure 16 the battery current IB(t), the inverter input voltage (DC link) Uo(t), the mains voltage Um(t) and the mains current Im(t) is shown ( See figure 2 as a reference) The time scale is set to 10ms/div and 4 periods of the 110V-50Hz mains are recorded as can be seen by the mains voltage Um(t) (blue curve). During charging mode the DC link voltage is stabilized to Uo = 32V and the converter draws a sinusoidal current Im(t) from the mains. In the discharging mode all auxiliary power is taken from the battery so that the mains current becomes zero. In this operation point most of the discharging power has to be dissipated in heat in the transistor current source (T4) which becomes active as soon as the DC link voltage Uo exceeds 36V ( see brown curve in figure 16).

Figure 16. Measured current step: Mains voltage (Um), mainscurrent (Im), battery current (IB), link voltage (Uo)

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Red curve: battery current IB(t).

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IV. FINAL COMPETITION

The whole battery charger was successfully presented to the jury by two students during the final competition in the laboratory of Texas Instruments in Dallas, USA. The work of the Cologne-Aachen team was recognized with an award of 2,500 US $.

V. EDUCATIONAL BENEFIT FOR THE STUDENTS

The students involved in the project could use their individual work on the battery charger for certain exams or for a diploma thesis. Others supported the project as collegial assistants. Although the amount of work extended the demand of a regular thesis, the experience gained within the project was worth it for all participants.

The students had already gathered information and knowledge of power electronics or battery systems during their former study. Though, special lectures were established at the RWTH Aachen as well as at the FH Köln to prepare and support the students sufficiently for their participation in the Future Energy Challenge 2007. The topics were chosen according to the different tasks the students were working on at the two institutes. In addition, students gained a lot of experience in soft skills: During the competition the participating students had to give concise presentations during meetings where complex issues had to be explained in a comprehensible way. By visiting electronic trade fairs the students learned how to get in contact with companies which thereupon supported the project by free software, components and devices. Most students were attracted by this project since it offered a unique chance to experience practical work and teamwork; two aspects which most students often miss in the regular study course in electrical engineering.

Participation in this competition enabled the students to gain valuable experience for their later professional career. The project work and the university collaboration in a larger team as well as the international contacts will be very beneficial for them in view of their future work in industry. Apart from solving technical problems they had been involved in project management, scientific documentation and in the way of presenting technical results.

VI. CONCLUSION

This paper presents a mains connected universal battery charger which has been developed by a joint Aachen-Cologne student team within the IEEE international competition “Future Energy Challenge 2007”. The universal charger operates under all AC voltages between 90V and 250V and shows a high power factor. The output of the charger is galvanically isolated and it can be used to charge batteries with a settable controlled DC current up to 2A. Three different battery types (lead- acid, NiXX and Li-Ion) can be charged up to a maximum

voltage of 32V. The type, the number of cells and the polarity of the connected battery is automatically detected from the terminal behavior within the charging process. This also requires discharging cycles. The probability of detecting the type and the number of cells of the connected battery correctly was found to be about 80%. This percentage may be increased by an improved software. Depending on the battery voltage an overall efficiency between 70% ( UB = 12V) and 80% (UB = 24V) could be measured at the prototype which is not yet optimized concerning minimum losses.

ACKNOWLEDGMENT

The team would like to thank all companies and institutions which have promoted the project by providing free components and devices as well as by generous financial support: Philips Research Aachen, NXP, Ferroxcube, NORWE, MacService, LEM, Tecxus, Analog Devices and IEEE- PELS, IEEE German Section, ISEA-RWTH Aachen, Fak IME – Cologne University of Applied Sciences

[*] Supervisors: Prof. H. van der Broeck (IA FH Köln & ISEA RWTH Aachen), Prof. D. U. Sauer (ISEA RWTH Aachen), Prof. R. de Doncker (ISEA RWTH Aachen), Dipl.-Ing. M. Kiel (ISEA RWTH Aachen), Dipl.-Ing. D. Pingel (FH Köln)

[**] Participating students: Heike Barth, Tim Schmidla, Andreas Anschütz, Raffael Kuberczyk (FH Köln), Hannes Nordmann, Christoph Schaeper, Christoph Wieczorek, Frank Hecht (RWTH Aachen), Yusuf Yurdagel (FH Aachen).

REFERENCES

[1] Chester G. Motloch, Gary L. Hunt, Jeffrey R. Belt, Clair K. Ashton, George H. Cole,Ted J. Miller, Calvin Coates, Harshad S. Tataria, Glenn E. Lucas, Tien Q. Duong, James A. Barnes, and Raymond A. Sutula, Implications of NiMH Hysteresis on HEV Battery Testing and Performance, 19th International Vehicle Symposium, 2002

[2] Mark Verbrugge, Edward Tate, Adaptive state of charge algorithm for nickel metal hydride batteries including hysteresis next term phenomena, J. Power Sources,Volume 126 (2004), 236-249

[3] -eling of the Charge Acceptance of Lead Acid Batteries, J. Power Sources, 168 (2007) 31-39

M. Thele, J. Schiffer, E. Karden, E. Surewaard, D. U. Sauer, Mod