Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

Charging mode

Energy supply system of electric vehicle consists of power supply system, charging system and power battery. Also, include a charging monitoring, battery management and smoke alarms monitoring. Charger is an important part of the charging system. Charge ways for vehicles by charging station are generally divided into three ways: normal charging, fast charging, battery replacement. Common charging mostly is AC charging, for the capacity of AC charger is not over 5kW, the input is rated voltage 220V, 50Hz single-phase AC power, for the capacity of AC charger is greater than 5kW, the inputis  the nominal line voltage 380V, 50Hz three-phase AC. Directly put the AC plug in electric vehicle charging port, the charging time takes about 4-8 hours. Quick charging mostly is DC charge, DC charger input is line voltage rated 380V, 50Hz three-phase AC , output voltage is generally not more than 700V, the output current is generally not more than 700A. Output voltage of AC input Isolated AC / DC charger is 50% to 100% of the rated voltage, and the output current is rated current, power factor should be greater than 0.85, the efficiency should be not less than 90%.

Charger should be able to ensure that in the process of charging power , battery cell voltage, temperature, and current does not exceed the allowable value. Charger should have functions of preventing output short-circuit proof and anti-reverse. Charger at least can charge for following three types of motive power batteries: lithium-ion batteries, lead acid batteries, nickel-metal hydride batteries.

Charging mode of motive power Battery havs "constant current - constant voltage" two-stage charging mode. The charging of  beginning, generally use optimal charge ratio (lithium-ion battery is 0.3C) to do constant current charging. (C is the battery capacity, such as C = "800mAh", 1C charge rate is charge current 800mA) at this stage, due to low battery electromotive force, even if the battery voltage is not high, the battery charging current will be large , charging current must be limited. Therefore, charging in this stage is called "constant" charging, the charging current is maintained to be the limited current. With the continuation of charging, the battery electromotive force rising, charging pressure has been increasing. When the battery voltage rises to the maximum allowed charging voltage , maintain constant voltage charging. At this stage, the electromotive force is still rising, while maintaining the same charging voltage, so the battery charging current hyperbolic trend was declining, has been reduced to zero. But in the actual process of charging, when the charge current is reduced to 0.015C, indicating full charge can stop charging. Charging in this phase is called "constant" charging, charging voltage in this stage: U = E IR is constant value. This is the basic requirement of lithium-ion battery charging mode. In addition, the charging system must also automatically adjusts the charging parameters, automatic control and automatic protection. Especially in the constant voltage charging phase, if the single battery charge voltage exceeds the allowable charge voltage, the charger should be able to automatically reduce the charging voltage and current, so that the battery charging voltage does not exceed the allowable charging voltage to prevent the battery pressure charging. Charging process and charging voltage and current changes shown in Figure 1.  

Figure1 charging curve(n is the number of tandem single batteries in the Battery pack)

According to the battery charging characteristics and the electric vehicle battery charging requirements, commonly used charging equipment is charger which can be divided into two types, DC charger and pulse current charger. DC charger is the output DC power source isolated voltage stabilization, which is through rectified and filtered by electric network source, supply to battery pack to charge. Currently the most used DC charger is the high-frequency switching DC power charger. It has small size, light weight, reliable work, high efficiency, high power factor, good adaptable power grid , small or large power, easily intelligent advantages and so on. Pulse battery charger can reduce polarization phenomena when battery is chargeing, thereby enhancing the efficiency of the battery charge and reduce the charging time, fast charging, but the pulse charger technology needs further study.

Electric vehicle charging time is long , it’s a problem of promoting electric vehicles usage that charging is hard. A large lithium-powered electric bus, for example, the battery capacity of 700Ah. Maximum charge current 210A (equivalent to 700AH battery capacity of the 0.3C charge rate), the maximum charging voltage of 700V (the equivalent of 165 maximum charge voltage of about 4.2V lithium battery voltage single series), then maximum power output must be 245kW. According to the best requirements for electric vehicle charging time, it’s at least 3 hours. Therefore, the electric vehicle charging method is not as fuel vehicle refueling at gas stations as charging. If the 20-minute quick fully charging, need to use at least 3C charge rate to charge, which it’s possible for lithium iron phosphate lithium-ion battery .

In summary, the electric vehicle charging mainly focused on the ordinary charge, supplemented with fast additional charging method. For the electric bus, the charging station is located in bus termini. In the evening after work, charging in low charge time of 5-6 hours. Day running vehicle, driving range is not enough, can be charged in a break supplement class time. The number and capacity of the charger is decided according to the scope of the fleet, the charging station is managed by the fleet team. For example, 12 large-scale lithium-powered electric buses require 12 charger. When fast charging, six chargers is needed to parallel charging, the maximum output power is 1470kW, the maximum charge current is 2100A (the equivalent of 700AH batteries 3C charge rate). Or 8 chargers charge for the eight electric vehicles, the output maximum charge voltage for each is 700V, the maximum charge current is 500A (the amount equivalent to 700AH battery charge rate of 0.7C). 1C ~ 3C fast-charge mode have been to discussed to put into use, but it should be ensured to use under the premise.of the safety and life of the battery Accordance with the above configuration of the maximum power charger, effective total power of the power transformer is about over 3000kW.

Currently auto manufacturers have developed on the major production of oil and electric hybrid vehicles and pure electric vehicles. BYD E6 electric vehicles, for example, battery type is cobalt-iron phosphate lithium battery, Configuration of battery capacity is 200Ah, 3C charging current is 600A, the nominal voltage is 316.8V (3.3V charging voltage is equivalent to about 96 iron phosphate monomer series voltage lithium cobalt). Charger output power is 192kW. It’s filled with 80% to fast charging time of 15 minutes. Energy consumption of one hundred kilometers is about 21.5 kWh, equivalent to 1 / 3 to 1 / 4 of the consumer price for fuel cars

Structure of the system

Input of high-power electric vehicle charger is the nominal line voltage 380V, 50Hz three-phase alternating current ,output voltage 700V, rated current 600A. System uses the 19 "standard rack, compact structure, reasonable layout, nice look. Dimensions: height × width × depth of 2200mm × 600mm × 600mm. The use of 60 modules in parallel, each module 10A/700V, module size : H × W × D is 133mm × 425mm × 270mm, 15-layer 4, four cabinet placed, put in four cabinets and transport separately, use a compact arrangement. rack front door, back doors are double doors for easy maintenance. Power coil in and bus-bar both input in the bottom. Power input breaker and monitoring unit touch screen are installed in the middle of the front of the host. charger control structure shown in Figure 2.

Electric vehicle charger uses high-power high-frequency switching power supply block diagram shown in Figure 3, the three-phase bridge uncontrolled rectifier circuit filter rectifying for the three-phase AC input, power factor correction pre-regulator 800V is by the high frequency DC / DC half-bridge power converter, the filter output DC 700V is for powered battery charging. After analysis and calculation, transformer use E65 dual magnetic core, primary coil 12 turns, then can be obtained secondary winding turns N2, N2 = (12/780) × (700/0.95) = 11.33, by the maximize output voltage 700V, the minimum input voltage 780V, the maximum duty cycle of 0.95 , consider the factors of the leakage inductance, the secondary rectifier drop, etc., take the N2 for 12 turns.

As the electric car charger is with non-linear loads, it produces harmonics which is a kind of pollution for power grid. It has to take effective measures, such as power factor correction or reactive power compensation techniques to limit electric car charger into the total amount harmonics of power grid. To improve the power factor and reduce input harmonics, can use the active power factor correction circuit, shown in Figure 4. It uses three-phase three switch three-level BOOST circuit, operating in continuous mode, using two MOSFET switches combined into a two-way switch. The figure shows, the switch S1, S2, S3 is a two-way switch. As the circuit symmetry, the potential midpoint VM of capacitor is approximate to potential midpoint of power grid, so can control separately the corresponding current by two-way switch S1, S2, S3 .

The corresponding current amplitude increase when switch on ,the corresponding bridge arm diode current flow when switching off (current is positive, the upper arm diode current flow; current is negative, the lower arm diode current flow). The current on Boost inductance decrease under the effect of output voltage , so that achieve to control current . The controlling circuit uses three control chips UC3854A, phase voltage offers sync signal and pre-correction signal to UC3854A by three-phase isolation transformer ,the current feedback use the Hall current transformer, respectively control the three switches, to be three current feedback inners rings and a closed-loop multi-system of voltage feedback outer ring. The circuit has the advantage of simple structure, each with only a power switch. With a small three-level feature harmonic current , a small stress of switch tube voltage and current. No middle, no third harmonic, high power factor at full load. Switch stress is small, off pressure drop, low switching losses, low common mode EMI.

DC / DC power converter use half-bridge circuit topology, a few low power devices and simple control and high reliability. Shown in Figure 5, use the technology of MOSFET and IGBT in parallel , full use of the fast MOSFET switching speed and low voltage to turn-on IGBT. In the circuit to take measures to make the MOSFET turn-off time delay a little more time than that of the IGBT, greatly reducing the IGBT current tail, reducing the switch on-state losses, improve efficiency and reliability, making the half-bridge circuit of the output power can reach 7kW. The output side use the rectifier with half wave rectifier, center-tapped full-wave rectification and full-bridge rectifier. As the output voltage is high, full-bridge rectifier is have a high utilization on the transformer, suitable for such occasions.

System uses the PWM forced flow equalized method , working block diagram shown in Figure 6. This is a improved method of combination system voltage control and forced flow, and its working principle is error voltage generated from comparison between the system bus voltage Us and the system reference voltage Ur ,and use this error voltage control PWM modulator, to get PWM signal to control the current of each module. The current required signal for each module are the same ,PWM signal regulate module reference voltage through the comparison between optocoupler and the module's output current, thus changing the output voltage to regulate the output current and achieve current equalizing. In this way, each module is equivalent to the voltage-controlled current source. This current-mode ahs a high accuracy, good dynamic response, many control modules, can easily compose to redundant systems. Forced current sharing depends on a module ,it can not current sharing if the module fails, so should design exit function if module fails. In forced flow , the system modules can be up to 100, even if the module voltage have a big difference, without any adjustments after setting up parameters, all current accuracy is better than 1%, load fast response, no oscillation, to meet application needs.