A study on optimizing the characteristics of lithium-ion battery power source and operating cost for hybridmotorcycle

Use your smartphone to scan this QR code and download this article ABSTRACT This study presents a research related to a Plug-in Hybrid Electric Motorcycle (HEM) which renovated from a Honda Lead 110cc with rear wheel is driven by original internal combustion engine and continuously variable transmission, while front wheel is directly-driven by a 48V – 1,000WBLDC Hub-Motor. The research focuses on optimal calculating, designing, prototyping and testing an electric power supply using Lithium-ion (Li-ion) battery pack to replace the lead–acid battery. The simulation and experimental results are able to evaluate the dynamic characteristics and operating cost of HEM. The study has been designed and prototyed a 48V – 33Ah Li-ion battery pack with a Battery Management System (BMS) circuit embeded on the vehicle. The battery pack is 10.84 (kg) weight and 8.11 (l) volume, reduced 30 (kg) and 12.89 liters compared to the lead-acid battery where battery life is greater than 2,000 cycles. In only electricmotormode, the longest distance of the HEM is 78.77 (km) for a half load, only driver, and 65.83 (km) for full load, one driver, and one passenger. Maximum speed is 52.67 (km/h) for a half load and 48.42 (km/h) for full load. In hybrid mode until SOC reduce to 50%, HEM can travel 64.366 (km) for a half load and 54.477 (km) for full load. The fuel consumption in the each case is 2.162 and 2.425 (l/100km), 0.5 liter lower than the original one and 0.3 liter lower than lead-acid battery. The cost of investment in HEM is 56 million VND and the operating cost is 1,106 VND/km, while the original vehicles are 40 million VND and 1,352 (VND/km). For every 1km using hybrid vehicles, 246.88 VNDwill be saved, after about 3.1 years, it will recoup the spending on investment. At the end of the motorcycle life cycle is about 200,000 km, and 43 million VND will be saved.


INTRODUCTION
Currently, people on the world is facing increasingly exhausted fossil fuel sources and global climate change, the urgent requirement is to reduce greenhouse gas emissions and use energy resources effectively [1][2][3] . In the previous studies, the authors presented the current situation of the number and growth trend of motorcycles in Ho Chi Minh City [4][5][6] as well as the impact of motorcycles using internal combustion engines on environmental pollution and its fuel economy [7][8][9][10] . From there, the authors proposed a plan to renovate the 110cc Honda Lead into a gasoline-electric hybrid motorcycle. The main issues have been solved such as: Selecting a general layout schematic; choose electric motors, generators, batteries; propose principles and operating mechanisms of the power management system 11 ; Modeling and simulating the operation of vehicles after renovation with Matlab/Simulink software 12 ; Researching overview of Lithium-ion battery and its applicability on hybrid motorbikes, proposing a design solution for Lithium-ion battery pack for renovated hybrid electric motorcycles 13 . In this study, the authors continue to solve the following outstanding issues: Manufacturing and testing of Li-ion battery pack; Model the Li-ion battery pack and simulate vehicle operating modes when installing the Li-ion battery pack; Assessing the dynamic characteristics of the vehicle and optimizing the operating characteristics of the battery pack; Calculate the cost of renovation and operating of the vehicle. Raise issues that need further study to conduct wide-ranging application.

PROBLEM IDENTIFICATION AND METHODOLOGY
The prototype of Lithium-ion battery pack must be conducted to test some specifications susch as funtion of battery management system, SOC, etc.

Pair the Li-ion cells into a battery pack
In previous study 13 , authors calculated and proposed a plan to design a Li-ion battery pack that is paired from 180 Li-ion cells type SANYO UR18650F-SCUD-3 14 to get a 48V-33Ah battery pack. As seen in Figure 1, 180 cells are paired into 12 serial blocks, each block consists of 15cells in parallel.

Figure 1: Paired cells into a battery pack
This option gives the advantage of good cell selfbalance 15 , simple pairing, just need to use only one BMS 48V-33Ah circuit for the battery pack. The downside is the battery pack connected to one unit, so it is difficult to disperse to take advantage of the vehicle free space. After pairing (excluding the box), the battery pack weighs 8.64kg and has dimensions (L x W x H) = 240mm x 200mm x 136mm. The volume is 6,528 (l). This volume accounted for 17.6% of the vehicle's U-Box, meeting the size requirements

Prototype BMS, Battery Management System
The requirements and features have to be achieved of battery pack mentioned in the previous study 13 . In Figure 2, the authors designed and manufactured a BMS 48V-33Ah circuit for the battery pack consists of following blocks: -Cells-balancing and multiplexer voltage signal block; -Regulator and power supply block -Central control unit; -Communication block -Protection block such as: overcurrent, overvoltage and overheat, short circuit… Cells balancing block is using 2 cell-balancing control ICs LTC3300 from Linear Technology. Each IC receives signal and controls 6 serial nodes. 12 cell voltage sensor to get signal and 12 executive structure to balance cell. Central control unit is using a programmable MCU to control charge-discharge cycles, read-write battery logs and communicate with external interface… In this circuit, authors have chosen MCU PIC18F47J53.
Regulator and power supply block, 3.3V power is supplied by a regulating IC named LT1761ES5-3.3. Communication block: BMS circuit communicate with external interface by USSB protocol through a transceiver LTM2884CY. Charge-discharge controller and protection block: To monitor charge-discharge current and voltage, authors used a pair of LTC 6804 and LTC 6802. Power amplifier block: to control cell balancing, consist of 12 MOSFET RQJ3003PGDQALT, rate voltage is 30V, rate current is 3A. Added an RC type Low-pass filter to filter the cell voltage signal. In addition, to ensure operational and safety features, BMS circuit is included other auxiliary components such as: current sensors, self-healing fuse, power filters, and temperature sensor in Figure 3. Connection of the BMS circuit to battery pack and solder power connector, communication jack and battery capacity level indicators. Then pack the battery pack with electrical insulation papers. Choose a plastic toolbox that fits a laptop size, fits the battery pack, and reprocess the case. Because the battery pack and BMS circuitry need heat dissipation, the cover is made of aluminum, thickness 3mm, screwed in holes, hand-held holes and milling the power connector plug holes. Then insert the battery pack into the box. Completed battery pack has a weight of 10.84kg and dimensions of 308 x 183 x 144 mm (L x W x H), volume of 8.11 liters accounted for 21.9% of the U-Box that illustrated in Figure 4.

Testing battery pack
The power source of DC60V -40A can be used to adjust the power supply, an adjustable Tungsten resistor set to create the load. Testing to figure out charge-discharge characteristics, cell balancing, lifetime cycle, temperature rising test with high current. Because the experimental conditions could not keep the room temperature fixed at 25 0 C, authors chose a closed room and adjust the air conditioner at 25 0 C for testing.  Then discharge in C 1 mode (I p = 33A) and stop discharge when SOC = 20%, repeat in 500 cycles, battery pack capacity falls down to 23Ah (69.7% original capacity). This result is not much different when compared with the cell charge-discharge test mentioned in the previous study. So we can conclude, if the discharge with mode C 5 or C 10 , and stop discharge at SOC = 50%, the longevity of the battery pack can reach over 2000 cycles while the capacity still over 60% original capacity.

Li-ion battery pack modeling
In the study 12 , authors modeled HEM assemblies and systems including: -Modeling the gasoline engine and power train systems; -Modeling the driver; -Modeling electric motors

SI83 -Modeling the vehicle dynamics
In this study, authors model Li-ion battery pack to predict the state of charge level (SOC), capacity, voltage and current when testing.
For designing battery pack and doing simulating of vehicle performance, the SOC is calculated from the following current integration: Where Q i is battery pack capacity (Ah) The relationship between terminal voltage and current can be expressed following equation: Where U r is dropped voltage caused by internal resistance of battery pack Where R is internal resistance of battery

Modeling controller
Rule-based algorithms are used to control the power management system of the HEM. There are two important rules in this controller: -Internal combustion engine has to operate in optimal performance area; -The battery pack has to be optimized SOC, SOC is kept in: Controller has three input signals: Demand power of driver; state of charge of battery pack; actual speed of HEM. Two output elements are demand power of gasoline engine and demand power of electric motor.

Simulating HEM operation
To evaluate dynamic characteristics and fuel consumption of HEM when mounted Li-ion battery pack. Models of parts in system are concreted to blocks in Matlab/Simulink as shown in Fig 6. Then simulated HEM under some motorcycle basic testing cycles such as: Japan 10-15 Mode, WVUCITY, FTP75, ECE-R15.

SIMULATION RESULTS AND DISCUSSION
In order to compare the results of HEM using leadacid battery with previous study [10][11][12] . In this paper, authors focused on the results of HEM simulation under the Japan 10-15 Mode cycle from t = 0 with the step increase time of 0.1 to the end of the cycle (660 seconds) and then repeat until SOC = 50%. The typical simulation results are shown as in Figure 7.
In simulation cycle, the HEM's response speed is very close to the required speed of the cycle, at the points that speed changes suddenly, the vehicle's speed is still late or overshoot, but the level of deviation is less than 0.5%, better than the deviation when the HEM mounted lead -acid battery is~2%. The reason may be the weight of the power supply unit was reduced and reduces the vehicle's weight 30kg, leading to a decrease in HEM's inertia.
As seen in Figure 8, when the required power (P k_demand ) stays in the optimal efficiency area of internal combustion engine, the internal combustion engine will drive rear wheel, the electric motor will be turned off. In case P k_demand smaller than the minimum limited point of high efficiency zone, the electric motor will drive front wheel, the gasoline engine will be turned off. In case P k_demand higher than the maximum limited point of efficiency zone, both engines work together, electric motor supplies power to bring internal combustion engine work in the optimal area. In simulation cycle, total consumed electrical energy accounts for 32% of total vehicle demand energy. The remaining energy (68%) is provided by gasoline engine. Electrical energy is equivalent to 48.15% of the energy from gasoline engines as shown in Figures 9 and 10 and Figure 11. Fuel consumption of HEM is 2.162 (l/100km) foe a half load, and 2.425 (l/100km) for full load. Estimated range until SOC reduced to 50% is 64.36 km for a half load and 54.47 km for full load given in Table 1.

Renovating cost
The investment on buying and renovating include: -Cost of buying new scooter: $1,709 -Battery pack: $414 -Other supplies and costs: $275.2 Details shown in Table 2.

Operating cost
In order to calculate the operating costs of base scooter and renovated HEM, authors choose the calculation method and conversion all of depreciation and basic operating costs per 1 kilometer.

Evaluation of renovating and operating cost
According to the calculation results above, we set up a table to calculate the cost, figure out the break-point when renovating and operating vehicles given in Table 3.
The initial investment of the renovated HEM is nearly VND 16 million higher than the original vehicle. However, in the operating life, every 1 km of opera-tion, 246.88 VND will be saved by the HEM. Breakpoint when investing in renovating and operating HEM with Li-ion battery pack is about 26,000 km. According to the research results of Mr. Tung 16 too, the average distance traveled by motorbike of people in Ho Chi Minh City is 23 km per day, equivalent to 8,395 km/year. Thus, if investing in renovation of Honda Lead to a HEM, after about 3.1 years, it will recoup the spending on investment. After 3.1 years, for every additional 1 km of operation, 246.88 VND will be saved. So by the end of life of the original vehicle (200,000km), equivalent to 23.8 years, 43 million VND will be saved by the HEM, equivalent to the money to buy a new motorcycle as in Figure 12.
In Ho Chi Minh City with about 8.6 million motorcycles, if investing to renovate traditional gasoline motorbikes into HEM, it will cost 137,600 billion VND,