A Solar Power Assisted Battery Balancing System For ...

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TTE.2018.2817123, IEEE Transactions on Transportation Electrification

A Solar Power Assisted Battery Balancing System For Electric Vehicles Chen Duan, Member, IEEE, Caisheng Wang, Senior Member, IEEE, Zongzheng Li, Jianfei Chen, Member, IEEE, Shidao Wang, Adrian Snyder and Chenguang Jiang

Abstract — This paper proposes a solar power assisted electric vehicle battery balancing system. There are three operation modes of the system: Solar-Balancing, Storage-Balancing, and ChargeBalancing. The Solar Balancing mode charges the battery module with the lowest SOC using the solar power during vehicle driving; the Charge-Balancing mode is operated when the vehicle is parked and being charged by the conventional charger. Under this mode, the balancing circuit discharges the battery module with the highest SOC by transferring the energy to an additional storage cell while the solar panel also charges the storage cell independently at the same time if solar power is available. When the solar power is low, the StorageBalancing mode will be selected to charge the battery module with the lowest SOC using energy stored in the storage cell. This system eliminates the energy loss that would otherwise happen in conventional active and passive balancing schemes by equalizing the battery using solar/stored energy in the storage cell. A 48V battery pack with four 12V battery modules system is simulated and tested. A prototype system is developed to prove the concept. The simulation and experimental results verify that the proposed system not only achieves the same balancing performance as conventional balancing circuits, but also effectively increases the overall usable battery energy by 2.1%~3.3% every 13.2km. Index Terms — Batteries balancing, electrical vehicle, solar power, energy storage, state-of-charge (SOC)

Manuscript received October 3, 2017; revised December 27, 2017 and February 21, 2018; accepted February 27, 2018. This work was partially supported by NSFC under award # U1609216. Chen Duan is with Wayne State University, Detroit, MI 48306 (email: [email protected]). Caisheng Wang is with Wayne State University, Detroit, MI 48306 (email: [email protected]). Zongzheng Li, Jianfei Chen, Shidao Wang, Adrian Snyder an d Chenguang Jiang are with the Wayne State University, Detroit, MI 48306 (email: [email protected]; [email protected]; shida [email protected]; [email protected] and chenguang.jia [email protected]).

I. INTRODUCTION

B

ATTERY systems have been widely used in industry, transportation, energy storage applications for more than a century. Battery energy storage has been identified as an enabling technology for transportation electrification and smart grid applications and battery systems can further catalyze the synergy between electric vehicles (EVs) and the electric grid [1]. In high power applications such as EVs and plug-in hybrid electric vehicles (PHEVs), the battery packs are usually formed by battery modules/cells connected in series to increase the voltage, and connected in parallel to increase the capacitance. However, due to manufacturing caused variations and varying operation conditions the imbalances reduce the usable energy [1-5]. The imbalances of a battery pack could lead to negative outcomes such as early termination of charging and discharging process [6-8]. Or, it can be even worse that the battery cells overcharged or over-discharged could be permanently damaged [2]. To deal with the imbalance issue of battery packs, various battery balancing topologies and control algorithms have been researched and developed [2-11]. Passive balancing is still one of the most widely used methods in battery management systems (BMS) because of the advantage of low cost [4]. The operating principle of passive balancing is simple: When a single cell/module reaches the charge voltage limit, it will be discharged by a power resistor to allow other cells to be fully charged [3, 4]. However, passive balancing is only applied during the charge process [2] instead of for both charge and discharge. In addition to this limitation, the overall efficiency of the battery system with passive balancing is relatively low due to the balancing energy is dissipated as heat. In contrast, active balancing circuits equalize the battery by transferring energy from cells with higher state-of-charge (SOC) to cells with lower SOC and can be operated during both charge and discharge processes. Three types of state-of-the-art active balancing circuits are summarized in [2]: Capacitive Balancing, Inductive Balancing and Mixed Active Balancing. For capacitive

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based active balancing one or more capacitors are switched in parallel to a cell [9, 10], the energy transfer is the result of voltage difference between cells. The advantage of capacitive balancing is no complex control algorithm is needed. However, the balancing process is very slow. In addition, the capacitance resistance brings power loss and the balancing process is slow. The inductive balancing uses transformers with air-gapped magnetic cores or inductors to transfer energy between cells. Compared with capacitive balancing, the inductive balancing is able to charge a cell with equal or higher voltage with another cell. But at the same time, the iron loss and copper loss of the inductive components brings power loss of the battery pack. The mixed active balancing uses DC-DC converters, e.g. Cŭk converter [11, 12] to transfer energy. For mixed active balancing circuits, the power loss cannot be eliminated due to the resistance of capacitive components, iron and copper loss of inductive components and switching loss of MOSFETs. The main disadvantage of conventional active battery balancing system is the power loss during the balancing operation. The power loss wastes the useable energy of the whole battery pack. For EVs, the result is the drop of driving range. To deal with the power loss issue, some other battery balancing circuits have been studied and developed. For example, in [13], a hierarchical cascaded multi-level inverter was proposed to achieve uniform SOC operation; In [30], Hu et.al developed a unified cost-optimal approach for charging, power management and battery degradation mitigation in PHEVs. An ideal solution to the energy loss and efficiency issues of conventional battery balancing schemes is to use the energy from an external source to charge the low SOC cell/modules. For example, the electric energy generated by an internal combustion engine (ICE) [18], or the solar energy from photovoltaic (PV) panels, can be used for the purpose. On the one hand, due to the limited area available for PV installation, it is not feasible to use just solar energy to power the whole vehicle at the current stage. However, on the other hand, the solar energy can be used for battery balancing even if the solar power is limited. In this paper, a solar power assisted battery balancing system is proposed. It has 3 operation modes: (1) when the vehicle is parked and being charged, the solar energy as well as the actively discharged energy from high voltage battery modules will be stored in an independent storage cell. (2) When the vehicle is driving in a sunny day, the solar power is used to charge low voltage or low SOC battery modules, or the whole battery pack when all the modules are balanced. If the solar power is hard to harvest, for example, in cloudy, rainy weather or at night, the battery modules with low voltage will be charged by the storage cell. Because the energy used for the active battery balancing comes from

energy source independent from the battery pack, the extra energy loss of the battery pack during balancing can be eliminated. By taking advantage of the solar energy harvesting, the energy used for battery balancing is also “free.” II. SYSTEM CONFIGURATION Fig. 1 shows the system architecture of the proposed battery balancing system. Take a battery pack with 4 modules as an example. In this system, the solar panel, DC/DC converter, storage cell and the high voltage battery pack share a common DC bus. The maximum output voltage of the DC/DC converter should be higher than the battery pack terminal voltage. It can also automatically recognize the input voltage and output terminal voltage, then charge the battery module connected to the output. A switch box is used to link the battery module with lowest/highest voltage that needs to be charged/discharged to the DC bus. The topology of the switch box is shown in Fig. 2. The storage cell is used to store the solar energy and actively discharge energy during the time when the vehicle is parked for charging. Thus the battery modules can be balanced and fully charged even the solar power is unavailable during charging, for example when the vehicle is parked indoor. When the vehicle is driving at night or under raining/cloudy weather without solar power available, the battery pack can still be balancing by using the energy from the storage cell. The total capacity of the cell depends on the vehicle design requirements. It can be small (10%) when used as a range-extender. For balancing purpose, because the energy stored in the cell is only used for equalizing the SOC variations among the battery modules, the storage cell can be super-capacitors or small battery cells with low voltage and low cost. During the parking and charging period, the battery pack is charged by the conventional plug-in charger, and the battery module with the highest voltage will be actively discharged by the regulator to the storage cell. Thus the balancing discharge energy is not wasted. It is worth noticing that if the rated voltage of the storage cell is equal or higher than the battery cell voltage, the regulator should be replaced by a Buck-Boost DC/DC converter to guarantee the energy can be transferred from the battery module to the storage cell. When the solar power is low, for example, during rainy, cloudy time or at night, the solar panel is disconnected from the DC bus, and the storage cell acts as the balancing power source. The different operation modes are selected by 4 dual-switches on the DC bus, i.e., DS1-DS4.



Fig. 1. System Architecture of the Proposed System.

Fig. 2. shows the circuit topology of the switch box. There are 8 digital controlled switches S1-S8. If the total battery module number is n, the number of switches will be 2n. For practical EVs, the module numbers are limited. For example, Tesla Model S and Toyota Prius have 16 and 28 modules in series, respectively [26, 27]. These switches can be packaged into a small circuit board with MOSFETs, installed out of the battery pack or integrated with the solar panel/DC-DC converter. Thus no modification or redevelopment is needed for battery modules, which makes the proposed system easy to be added on. Initially, all the switches are normally open. The control unit measures the voltage or estimates the SOC of each battery module and closes the corresponding switches to link the battery module needed to be charged/discharged to the DC bus. Only 2 switches will be closed at the same time. For example, to charge /discharge module 1, S1 and S3 are closed. Switches linked to the same DC bus terminal and same battery module terminal will never be closed at the same time to avoid short circuit of DC bus or battery modules.

Fig. 2. Switch Box Circuit Topology.

III. OPERATING MODES AND CONTROL ALGORITHM In addition to the Solar-Balancing mode that charges the battery modules at low voltage (or low SOC) by solar power, due to the limitation and unpredictability of the solar power, the proposed system also has a StorageBalancing mode to balance the battery modules during discharging using the stored energy and a ChargeBalancing mode to save the active discharge energy and store together with the solar energy to the storage cell during the vehicle’s parking period. This is a unique feature of the proposed system, with which the solar energy is utilized as much as possible. The operating mode of the proposed system is selected based on the vehicle and weather conditions. The different operating modes are shown in Fig. 3 where the orange arrows indicate the energy flow.

TABLE I

SOLAR-BALANCING MODE SWITCH STATUS

S1 CLOSE OPEN OPEN OPEN CLOSE

S2 OPEN CLOSE OPEN OPEN OPEN

S3 CLOSE OPEN OPEN OPEN OPEN

S4 OPEN OPEN CLOSE OPEN OPEN


S6 OPEN OPEN OPEN CLOSE OPEN


S8 OPEN OPEN OPEN CLOSE CLOSE

Charged Module Module 1 Module 2 Module 3 Module 4 Balanced



Mode 1: Solar-Balancing Mode Fig.3 (a) shows the Solar-Balancing mode of the system. When the weather is sunny and the vehicle is under driving, DS1 and DS4 will be closed. Under this condition, the battery pack is discharged for energizing the vehicle powertrain. The battery module with the lowest SOC/voltage will be linked to the output of the DC/DC converter and charged by the solar panel. Once all the battery modules are balanced to the same SOC/voltage, the whole battery pack will be connected to the DC bus and charged. Thus the solar energy can still be harvested. Table I shows the switch box status of the Solar-Balancing mode. The maximum power harvested from the solar panel and charged to the battery module is given by Pm = (ns × Vm) × (np × Im) × ηc

(1)

Where Pm is the maximum power charged to the battery module by the solar panel, ns and np are the number of series and number of parallel panels in array. Vm and Im are the module voltage and current for each panel at MPPT. ηc is the efficiency of the DC/DC converter. Mode 2: Storage-Balancing Mode When there is little or no solar power to harvest (such as during cloudy days or at night), DS1 and DS3 are opened and DS2 and DS4 are closed to run the StorageBalancing mode, shown in Fig. 3(b). Under this mode, the energy saved in the storage cell will be transferred to the battery module at the lowest SOC/voltage through the DC/DC converter. Since the energy saved in the storage cell is limited, once the battery modules are balanced, the energy flow from the storage cell will be cut. Table II shows the switch box status of the StorageBalancing mode. The balancing charging power of this mode is controlled by the output voltage of the DC/DC converter and given as Pc =

𝑉𝑜 −𝑉𝑜𝑐 𝑅𝑖𝑛

× 𝑉𝑡

(2)

Where Pc is the charging power to the battery at the lowest voltage or SOC. Vo is the output voltage of the DC/DC converter, Voc, Rin and Vt are the charged battery module open-circuit voltage, internal resistance and terminal voltage, respectively. Mode 3: Charge-Balancing Mode The two modes demonstrated above are used when the vehicle is driving and the battery pack is being discharged. When the vehicle is parked and charged, the Charge-Balancing mode will be selected by closing DS1 and DS3, shown in Fig. 3(c). Under this mode, the battery pack is being charged by a plug-in charger. The system monitors the battery module voltages and links the module with the highest voltage to the DC bus. The battery module will be discharged by the regulator. Table III shows the switch box status of the ChargeBalancing mode. The discharging power of the battery module with the highest voltage under this mode is controlled by the output voltage of the regulator and given as

Pd =

𝑉 ′𝑜 − 𝑉 ′𝑜𝑐 x 𝑉𝑡′ 𝑅 ′𝑖𝑛

𝜂r

(3)

Where Pd is the discharging power of the battery with the highest voltage. 𝑉𝑜′ is the output voltage of the ′ voltage regolator, 𝑉𝑜𝑐′ , 𝑅𝑖𝑛 and 𝑉𝑡′ are the storage cell open-circuit voltage, internal resistance and terminal voltage respectively. 𝜂r is the efficiency of the regulator. The discharging energy as well as the harvested solar energy can be saved in the storage cell. The maximum charging power from the solar panel is also given by (1). By operating this mode, the system guarantees the energy used for battery balancing are all “free”. Unlike in conventional active balancing systems, the energy for charging low voltage cells comes from high voltage cells. While in conventional passive balancing systems, the high voltage battery cells are discharged by power resistors which waste this part of energy that can be saved.

TABLE II.

STORAGE-BALANCING MODE SWITCH STATUS









Charged Module Module 1 Module 2 Module 3 Module 4 Balanced



(a)

(c) Fig. 3. (a) Solar-Balancing Mode of the Proposed System. (b) Storage Balancing Mode of the Proposed System. (c) ChargeBalancing Mode of the Proposed System.

is the DC-bus voltage. After being charged/discharged for a certain period t, all switches will be opened for a sampling time T and T=1/f, where f is the sampling frequency of the voltage measurement. A new decision on battery module to be charged / discharged will be made based on the module voltage measured on sampling period T. Another reason of doing this is for short-circuit protection. The period T also acts as a dead-band between switches status changing. Thus the switches connected to the same terminal of DC bus or battery modules will not be closed at the same time. It is important that the dead-band T