Since the necessity of the wireless and mobiles electronic devices, the estimation of state

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1 State of Charge Introduction Since the necessity of the wireless and mobiles electronic devices, the estimation of state of charge is being one of the most relevant researches on engineering field. One example of this kind of electric application is the hybrid car of Toyota: Toyota Prius. This documentation contains the design of a system prototype to estimate the state of charge of the battery stack on the backside of the hybrid car by Toyota. The following paragraph includes the explanation and the development of the application designed to estimate the state of charge of one 400 V battery stack with maximum rate charge/discharge of 10A. Toyota Hybrid system II Toyota has been developing new technologies for the purpose of achieving energy security and diversifying energy sources. Some of this technologies are electric vehicle (EV), that use motor as source; hybrid vehicles, that combine engine and electric motor fusing the advantages of each one; and fuel cell hybrid vehicles (FCHV), that use a chemical reaction between hydrogen and oxygen in the air generating electricity used for the motor by the fuel cell (FC). In March 1997, Toyota announced the completion of a new power train called Toyota Hybrid System (THS) which combines gasoline engine and electric motor reaching almost twice the fuel efficiency of conventional gasoline vehicles. 3

2 This new system was included in the passenger vehicle Toyota Prius, which world marketing began in 2000 acquiring the reputation of a highly innovative vehicle. Toyota was improving this technology and in 2001 installed THS-C, which combines THS with continuously variable transmission in Estima Hybrid minivan and in THS-M, a mild hybrid system, was installed in the Crown, luxury sedan. Toyota has developed the concept of Hybrid Synergy Drive and, based on it, THS II was created as a new generation of THS which increases 1.5 times the motor output and achieves high levels of efficiency between environmental performance and supplied power. How does it work? Two power sources: o High-efficiency gasoline engine o Permanent magnet AC synchronous motor Figure 1 The power control unit contains a high-voltage power circuit for increasing the voltage of the power supply system for the motor and the generator to a high-voltage of 500V. This system also includes a AC/DC inverter between generator and electric motor (AC) and hybrid 4

3 battery (DC). The power split device transmits the motive power from engine, motor and generator by combining them in order to get a high efficiency. The power control unit is the responsible of controlling these components. In deceleration, the battery is recharged because the braking system recovers kinetic energy as electrical energy. When it is necessary, the engine drives to the generator to recharge the battery. High-voltage system. The voltage of the motor and the generator has been increased to 500V thanks to the high-voltage circuit. This allows that the electrical power can be supplied with a smaller current thus a higher efficiency. Power control unit, battery and regenerative braking system As the necessary current is smaller, the size of the inverter has could be reduced. As well the control circuits have been integrated and the power control unit has almost the same size. The internal resistance of the battery has been decreased by improving the electrode material and by using a new connection structure between (battery) cells. The input/output density is 35% better now, achieving the highest output one per unit of weight of the world. Aside of the DC/AC inverter, there is a DC/DC high-voltage power circuit that boosts the voltage and as consequence the electrical power increases with the same current. During engine braking and footbraking, the electrical motor operates as a generator converting the kinetic energy into electrical energy, used to charge the battery. The regenerative braking system is the responsible of this process. 5

4 Figure 2 The regenerative break system is used preferentially in front of the hydraulic brake of the ECB when the footbrake is being used. It makes possible recovering energy at low speeds. Hybrid transmission The hybrid transmission is based on a power split device that divides the power into two. One of them is an electrical route and the other one is a mechanical route. The first shaft is connected to the motor and the wheels while the other one to the generator. 6

5 Figure 3 The speed can be changed by an electronically controlled continuously variable transmission while varying the rpm of the engine as well as the rpm of the generator and the motor. System control The system control manages the energy of the entire vehicle including the energy used by the motor as well as the energy used by the auxiliary services. The system control monitors the requirements and the status information of the whole hybrid system (engine, motor, generator and battery). The driver instructions, such as throttle opening or shift lever position, are also sent to the system control as well as braking information. The main function of the system control is to report energy consumption statuses information of the hybrid system in real-time in order to get a high efficiency as well as a safely and comfortably suitable driving. 7

6 Figure 4 The driver instructions, in order to be used in the integrated control, are converted into electrical signals which are transmitted by wires, which is named as by-wire control. When one smart key sends information that a driver is inside the car, the system power supply in turned on. Then, hybrid computer is monitored to work properly before the ignition button is pressed. The start-up control sequence is, in that moment, started with the checking of the generator, the engine, the motor, the battery and some sensors such as the high-voltage system. If the whole system is right, the components are turned on and the vehicle ready to run. If the ignition button is pressed again, the high-voltage system is turned off and the hybrid computer shuts down. During the vehicle is moving, it is also being checked in order to maintain the safety in any unlikely event. 8

7 Engine power control is the basic control mechanism of the THSII for managing the energy control of the entire vehicle and minimizing it. Energy management control determines whether to stop the engine or run it depends on the depression of the accelerator by the driver. Under normal conditions, the vehicle begins to operate by the electrical motor. To operate with the engine, firstly, the generator starts it and the system calculates the power required to run the entire vehicle with the highest efficiency in order to produce it and send the rpm instructions to the engine. For the engine power control, it is taken in account the direct driving power, the motor power, the power needed for the auxiliary devices and the recharging battery power. When the vehicle s driving power changes abruptly, a protection control similar to the conventional traction control is used to prevent abrupt voltage changes in the power split device. In THS II, it has got the world s first motor traction control with the goal of restoring traction in case of slippage and informing of this situation. Graph 1 9

8 Another unique service of THSII is the called uphill assist control. It basically consists in preventing the vehicle from sliding downward in a steep slope in the moment that brake is released. It is carried out by increasing the motor s torque when sensing a vehicle s descent for the angle of the slope. Output enhancement An improvement of 1.5 times of the motor s output (from 33 kw to 50 kw) has been achieved due to raising the motor performance and the control voltage up to 500 V. United to this enhancement, the motor can supply electrical power also at medium speed owing to the fact that the maximum revolution has been increased to rpm. At-start acceleration and overtaking acceleration has been improved close to the behaviour of a 2.0 litre gasoline engine getting a better hybrid driving experience with a high response and smooth acceleration. Graph 2 10