2013 Joint International Conference on Rural Information & Communication Technology and Electric-Vehicle Technology (rICT & ICeV-T)

November 26-28, 2013, Bandung-Bali, Indonesia

978-1-4799-3365-5/13/$31.00 ©2013 IEEE

Transformerless High Voltage and Controllable Current Battery Charger for e-Car

Purnomo Sidi Priambodo #1, Wahyudi Purnomo *2, Aries Subiantoro #3, Abdul Muis #4 and Feri Yusivar #5 # Department of Electrical Engineering, Universitas Indonesia, Indonesia

[1] p.s.priambodo@ieee.org [3] biantoro@ee.ui.ac.id [4] muis@ee.ui.ac.id [5] yusivar@ee.ui.ac.id

* Politeknik Manufaktur Negeri Bandung, West Java, Indonesia [2] yud_prn@yahoo.com

Abstract— Charger is an important part in e-car. For a massive number of e-cars in the future, battery charger should have characteristics of high efficiency, rapid charging capability, portability, low-cost and compatible with the national standar electricity. In order to cover thus five parameters, the battery charger design should be transformerless, employing buck swith converting method, and having a microprocessor to monitor and control charging voltage and current during charging process. In this paper, we do simulation and experiment with a battery charger prototype, where the input is 220V-50Hz (PLN) and transformerless. To control the charging voltage and current output of buck switch converter, we use a microprocessor circuit. The charging voltage and current of the battery charger are 36-96V DC and 0 s/d 20 A, respectively. Keywords— Battery charger, e-car, transformerless, buck converter, portability

I. INTRODUCTION In electical vehicles or e-cars, battery or electrical energy

storage is the main component to support the car’s operation. The importance of battery is in the same level with electrical motor to move the e-car. In e-car concept, the battery is required to have high energy capacity and density in Wh/kg and Wh/l (note: Wh is Watt hour). Therefore, ideally the e-car should have battery(s) with characteristics of compact (small volume) and light, however has large energy storage capacity. Since developed the first time in 1859 by Gaston Plante [1], the shape and types of rechargeable battery had been evoluted from lead-acid to various shapes with their own advantages and disadvantages [2-4]. In reality, nowadays, there exists various types of batteries in market, non rechargeable and rechargeable types and from small to large capacities. The small capacity battery types, specially for non-rechargeable (disposal) ones, mostly are made by materials based on alkaline [2]. While for rechargeable small capacity battery types, in general are made by materials based on Ni-Cd and Li-ion [3-4]. In general, small capacity and size batteries are used for small devices or gadgets, includes smartphones, camera, MP3 players, toys and etc. On the other hand, instrument with large power, for instances, uninterruptable power supply (UPS), energy storage for renewable energy resources and e-car, use types of lead-acid based batteries, dry free-maintenance batteries and the one based on Li-Ion.

Lead-acid battery, even though has a lower energy capacity and density compared to the newer types, especially

Li-Ion, however, lead-acid battery are still used and popular demanded for applications, which are not constrained by space and weight. The main advantage of lead-acid battery is relatively low price, since the material components are abundance and easy to be produced. The disadvantages are its properties of low energy capacity and density, hence to have the same energy capacity, the lead acid battery will be heavier and larger volume. Moreover, it requires additional maintenace to add distilled water to dilute the electrolyte after a certain charging process. The maintenance-free battery or semi-dry battery[5] is in the form of electrolyte gel, has higher price, however, it has advantages of maintenance-free and compactness.

The most ultimate battery technology is Li-Ion battery, which has many advantages, includes higher energy capacity (Wh/kg) and energy density (Wh/l), much higher compared to its competitors. So far, Li-Ion battery has reached energy capacity higher than 100 Wh/kg [6]. However, Li-Ion has its own disadvantages, includes very high price and very dependent to the rare-earth material, where not all countries have it. Recently, even though the price relatively still very high, however, the direction of battery development for e-car, is going to Li-ion based battery.

The very importance position of battery in e-car, then the battery charger for e-car must be designed such that becomes effective and efficient to support battery charging process. Beside of energy storage capacity, the charging properties of the battery include maximum charging current, maximum operating temperature, operational voltage, and internal battery resistance at charging and discharging processes. Every battery type has its own specifications, hence the charging method can be different from one battery type to the others. The main requirements for e-car battery are: large capacity electric storage, small volume, light and large charge rate (CR). The e-car users will demand to charge and fill the battery up as rapid as possible from empty to full. Rapid charging can be applied and realized on battery with large CR battery property. However, the maximum charging current must be considered and limitted to avoid battery degradation on both, capacity and lifetime.

In general, CR is determined by the battery type and its capacity in Ah (Ampere hour). A small battery capacity of 1.6 Ah charged with CR = 0.8 A, it is required 2 hours to fill the battery up. When it is charged with CR = 3.2 A, it is required 0.5 hours to fill the battery up. It shows the invers

proportional relation between CR and charging time. The rapid charging process is limited by the maximum limit charging current permitted by the battery specifications.

In general, charging process can be divided into 3 regions. The first one is the region from empty to half-full condition. The second region is almost full condition and the third one is full condition. In electrical perspective, the charging process can be represented or modeled as voltage-drop in internal electrical resistance due to chemical process. When the battery in empty to half full condition, the internal charging resistance due to chemical process is low. It shows that when at empty condition, chemical process is easy to capture the electrical charging. However, once the battery has been fully filled-up then the process to capture the electrical charging becomes more difficult, hence the internal charging resistance becomes higher.

In constant voltage charging method, when charging the empty battery where it has a low internal charging resistance, if not careful controlled, it may cause a very large charging current. In common, it will cause high temperature raising due to over charging current (Vc-Vbattempty) / Rint. The total heat dissipated is (Vc-Vbattempty)2/ Rint (where Vc is constant charging voltage, Vbattempty is empty battery voltage and Rint is a low internal charging resistance at empty battery). For such reason, it is recommended to use the constant current charging method, when the battery is empty up to half-full condition. This constant current charging method avoids the excessive temperature during charging an empty battery. Moreover, this method also protecting from condition of unbalanced voltage charging, which very much happen in constant voltage charging on series of empty batteries, which end up to over charging voltage at several batteries in series.

When the battery “almost full” condition, the internal charging resistance becomes high, this is due to chemical process that is getting few for the charging process. If it keeps to use constant charging current, then there is no more charge to be stored, this makes the charging energy to be converted to heat as large as Ic

2. Rint even Ic.Vbattfull (where Ic is constant charging current, Vbattfull is full battery voltage and Rint is internal charging resistance when battery full, where the value is relatively high), which can generate excessive temperature raising on the charged battery. In order to avoid thus problem, for battery charging close to full condition, it is recommended to apply constant voltage charging, where charging current is adjusted as small as possible for charging and compensating internal discharging process in the battery. Charging method on the full battery, sometimes called as a trickle charging method, which provides a relatively small amount of current, sufficient to counteract self-discharge of a battery that is idle for a long time.

From explanation above, it is recomended to use constant charging current for empty battery and constant charging voltage for nearly full battery. We can modified thus recommendation becomes adjustable constant charging current or adjustable constant charging voltage based on the charge content in the battery.

In this paper, we propose a transformerless high voltage battery charger with automatic variable charging current method, where the automatic process is controlled by charge content in the battery.

II. MODELING AND DESIGN The needs of rapid charging for e-car, pushing to design

battery charger by considering several conditions, such as: maximum current charging, voltage balancing of battery cells in series, maximum operating temperature, operational voltage, and internal battery resistance at charging and discharging processes. In this research, we used 4 lead-acid batteries with each 24-V, 64 Ah capacity, connected in series to produce 96-V DC with 65 Ah capacity or about 6-kWh. First of all, based on prior experiment we determined that the maximum charging current for lead-acid battery is about 30% of its capacity. For 65 Ah capacity, then the maximum charging current is about 20 A.

The next consideration beyond technical is low-cost and robust consideration. The PLN (Indonesian State Owned Company for Electricity) input voltage to the charger is 220-V AC 50 Hz. Since the battery voltage is 96-V DC, then the charging process is not necessary to use step-down transformer. The 220-V AC is rectified with high breakdown voltage rectifiers and filtered by using 2nd order L-C low-pass filter, such that results in 311-V DC with small ripple. This transformeless charger makes the charger price cut down lower. Over all, the High Voltage Battery Charger with Automatic Variable Charging Current is shown in Fig.1 below.

Fig. 1 High Voltage Battery Charger with Automatic adjustable Charging Current.

In order to be used as 96-V battery charger, the 311-V DC must be stepped down to the voltage about 15% above 96-V DC. It is proposed to use switching method, by controlling the duty cycle [7-8]. The switching method has efficiency near to 100%. The output of switching, then to be filtered by using 2nd order L-C low-pass filter, to obtain 110 V DC output with small ripple.

In order to obtain relatively stable current charging, if there exists fluctuation on PLN voltage, it is expected to be suppressed by implementing feedback loop on the charger circuit. Vsense, which is a part of PLN DC input, is compared to a constant reference voltage Vref as shown on Fig.1 above. The output of comparison is used to control switching duty-cycle.

We propose an automatic adjustable charging current controlled by the battery charge content. When the battery is empty, the maximum permitted current will charge the battery. At condition when the battery is half full, charging current is decreased. At the full condition, charging current must be as small as possible, only for compensating internal current discharge in battery, until the battery is released from the charger. This method requires an intenssive monitoring and controlling system to assess the battery voltage that corresponds to charge content and to control the charging current realtime. It is recommended to apply microprocessor

system for monitoring and controlling system. The common method to measure the charge content in battery, is by measuring battery voltage (note: the detection voltage range is very narrow). By implementing microprocessor system, command and controlling the charging current can be conducted by fully automatic. This charger system can be explained and represented as a control system, where the control flow approach is shown in Fig. 2.

Fig. 2. Control flow diagram for battery charger.

The simulation of Transformerless High Voltage Battery Charger with Automatic Adjustable Charging Current is represented in electronic circuit shown in Fig. 3.

Fig. 3 High Voltage Battery Charger with Automatic Adjustable Charging Current Circuit.

Fig. 4 shows the electronics circuit realization of the design and has been tested to satisfy the technical requirements.

Fig. 4. Transformerless battery charger circuit, dimension 17 x 11 x 9 cm3 and weight less than 0.5 kg.

III. SIMULATION AND EXPERIMENT RESULTS A. Simulation:

The circuits are designed with Power Sim dan MultiSim ver10 softwares. We simulate the designs with several conditions. The first condition is stable input at 311 V DC without feedback loop to control the output voltage, then we

obtain output at 110 V DC before we feed to the current control circuit, such as shown on Fig. 5 below. Transient or settle time to obtain stable output is about 0.03 seconds with overshoot peak about 10-V.

Fig. 5. Stable DC input at 311 V without feedback control

The second condition is when the PLN input DC fluctuated about 100-V peak-to-peak, ratio Vpp in fluctuated to the Vin average is about 0.322, as shown on Fig. 6 below

Fig. 6. Unstable DC input at 311 V 100Vpp fluctuation, without feedback control.

Transient or settle time to obtain the stable output is about 0.03 seconds and overshoot peak about 10-V, similar to the one non-fluctuated input. However, the output fluctuation is 20 V peak to peak. The ratio of fluctuated Vpp out to Vout average is about 0.182. It means that the switching system reduce the fluctuation ratio to the Vaverage as 1.77 times.

The next simulation is applying a feedback loop to compensate the fluctuation exist at Vin. The simulation result is shown on Fig. 7 below, where the transient or settle time to obtain stable output is about 0.06 seconds and overshoot peak about 50-V.

It is shown that by applying feedback control, causes transient time becoming 2 times longer in comparison to the non feedback loop. Moreover, the overshoot peak is getting higher, reaching 50-V. However, if we evaluate the output fluctuation that is only 7 Vpp, it means the ratio Vpp out fluctuated to the Vout average is less than 0.064, meaning that 0.35 from condition of non feedback loop.

Fig. 7. Unstable DC input at 311 V 100Vpp fluctuation with feedback control.

It is shown that by applying feedback control, causes transient time becoming 2 times longer in comparison to the non feedback loop. Moreover, the overshoot peak is getting higher, reaching 50-V. However, if we evaluate the output fluctuation that is only 7 Vpp, it means the ratio Vpp out fluctuated to the Vout average is less than 0.064, meaning that 0.35 from condition of non feedback loop.

It is concluded that feedback loop suppress fluctuation generated by Vin as 5 times, more than without feedback. However, there are two consequences or cost of applying feedback loop, those are transient or settle time become twice of the non-feedback loop and overshoot current reachs 50-V DC, which is much higher in comparison to the non-feedback loop one. Hence, to avoid the overshoot, it is recommended to apply a protection delay switch in the begining of charging operation. B. Experiment Results:

The relation between duty cycle control and the output DC voltage is shown in the following measurement, as shown on Fig. 8, below. The clock rate to control the switching circuit is designed for 10 kHz and input DC to switching circuit is adjusted to 100V-DC. As shown on Fig.8a, for duty cycle of 21% results in 14.5V-DC and Fig. 8b, duty cycle of 47.3% results in 67.9V-DC. Other measurements are not displayed in this paper. However, the relation between duty cycle control and the DC output voltage is summarized on Fig. 9.

(a)

(b)

Fig. 8. The relation between duty cycle control and the output voltage,

Fig. 9. The relation between duty cycle control and the output voltage,

As shown on Fig. 9, the relation between duty cycle control and the output DC voltage is not exactly as a linear line as expected. However, to control the charging, it does not

need any linear relation. In our design, the charging process is devided into 3 regions. When the battery is empty, it is charged by highest current represented by highest duty cycle about 90%. When the content of battery is reaching about 80% of the maximum, it is turned to continuation charging method, where duty cycle 50%, results in lower charging current to ensure continuation of charging. However, it will keep low heat dissipation. Once the battery reach almost 100%, then the charging process turned to trickle charging step, to ensure the battery not losing the content due to leaking, keeps the battery not over charged.

IV. CONCLUSIONS From the simulation and experiment, it is concluded that it

is more efficient if the charging battery system is transformerless, employing switching and feedback control. Feedback loop in steeping down DC voltage is applied to suppress the possible output fluctuation due to input fluctuation. Therefore, charging current can be kept stable when the input voltage is fluctuated. The current charging adjusment can be conducted by adjusting the output DC voltage charging through switching method in the form of duty cycle adjustment. The relation between duty cycle control and the output DC voltage is not exactly as a linear line as expected, however, it does not matter, since the designed charging system only defines 3 charging regimes, no needs any linearity.

ACKNOWLEDGMENT This research is funded by Mobil Listrik Nasional 2013

Grant from the Directorate General of Higher Education, Ministry of Education, Republic of Indonesia

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