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The Fast Charger Configuration
Both the charging voltage peaks are noticed when initial decline is caused by the decreasing battery resistance during the CC charge interval, with the second decline due to decreasing current. In conclusion of the fast charge it can be ascertained that:
• The commercially available batteries can be fast charged with a temperature rise of less than 25°C • Fast charging does not exhibit detrimental effects on battery cycle life
In the first part of the fast charging process, to about 40% return, resistive heating is the major cause of the initial battery pack temperature rise. In case the battery is designed such that it meets the fiveminute-50% charge return requirement, the best strategy is to design the battery pack with low resistance batteries.
The temperature distribution of batteries in the pack appears nonuniform, with possibly temporary localized hot spots. In the case of a VRLA battery, the temperature on the casing surface follows the internal temperatures more closely during charging than for immobilized electrolyte batteries.
A battery with a larger heat capacity will have a lower temperature rise when it is subjected to fast charging. Assuming that all other battery pack conditions are the same. Therefore, VRLA batteries have a lower temperature rise, since sulphuric acid solutions have a large heat capacity. VRLA batteries will also have less heat production from water vapor decomposition. In addition, the battery heat transfer to the battery exterior is better.
The lowest charging current should be chosen such that it takes full advantage of the time allowed to meet the charging requirement. This lowers the resulting battery temperature and saves energy. In addition, it may also return more charge to the battery in the given time.
THE FAST CHARGER CONFIGURATION
The fast charger for traction batteries provides charging of batteries in 5 to 30 minutes. In order to apply this fast charge in a period of 10 to 30 minutes, the charger must be able to provide voltages up to 450V and currents up to 500A. Such a charger characteristic “envelope” is depicted by a maximum voltage-maximum current profile as shown in Figure 5–3. This envelope implies a peak output power of 225kW. Thus
Figure 5–3 Constant voltage charging profile.
Charge Voltage (V)
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charging a compact electric car in about 6 to 10 minutes, a midsize electric vehicle (EV) in about 25 to 30 minutes provided that the battery quality allows charge acceptance at such rates.
The modified battery charge profile of maximum voltage-maximum power-maximum current limits the power to 120kW, while the maximum current and the maximum voltage characteristics remain the same as the maximum voltage-maximum current envelope, as shown in Figure 5–4.
Using this new charge envelope, the compact size EV will require 10 to 12 minutes to charge. The maximum voltage-maximum powermaximum current profile characteristic has the advantages of (a) lower peak power (which dictates that the size of the battery grid); and (b) spread of charging times, among EVs can be narrowed down in spite of different battery voltages and battery capacities. The charger is designed essentially to deliver the same amount of energy in the same time.
The fast charger requires intimate knowledge of the battery on charge. The battery charger requires knowledge about battery pack, and the faster the charger, the more is the information needed. The charger can prevent unwanted abuse of the battery while achieving optimal charge in the shortest possible time. Such information includes the battery chemistry, number of cells in the battery module, and voltage and temperature characteristics of the battery. The charger control can also
Charge Voltage (V)
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become more “battery specific” on an individual battery basis and monitor the battery under charge.
The fast charger defines the universal charging station for EVs. The salient features are based on the battery charger, comprised of the power section and the controller section.
The power section of the battery charger is a DC-controlled current power source with sufficient voltage compliance to be able to charge a wide range of EVs, not being battery specific. The controller section of the charger is battery specific and is placed on board the EV together with the battery pack.
The control signal connection (a twisted pair or coaxial cable) is required for the battery pack charge control. Additional wires are required to send a control signal for the automatic mode operation, and transmit the battery pack temperature and battery charge acceptance data to the battery monitor (BMON) module. The power cable along with the control pair constitutes the charging cable interface. A standard high-voltage connector interfaces the charging station to the EV battery pack charge receptacle.
A large range of EVs can be charged as long they are equipped with a standard charge receptacle. In such a configuration, the charge is performed in the manual mode. The user provides the voltage and the
current level, within the battery-charging envelope. In addition, the user may also input the charge time and the total Ahr to be provided to the battery pack. This type of control places limits on the output current and the output voltage of the battery pack, allowing the battery pack to be charged using the popular constant current–constant voltage charge profile. In case the values for the charge current and the charge voltage are not available, the user should consult the manufacturer specifications.
The high-level components of the EV fast battery charger include the charging station interface consisting of an electrical connection to the power supply grid capable of supplying several hundred amperes. A rectifier module converts AC input power to DC input power. A switching inverter module regulates the flow of DC charge power to the battery. The charge delivery cable carries the power to the EV battery pack. The termination of the charge delivery cable is a receptacle jack, which carries hundreds of amperes of DC current. This receptacle may require substantial insertion force to achieve a reliable connection to the battery pack. It is thus necessary to provide a locking lever that will assist with the insertion and the removal of the battery charge cable. Such a zero insertion force design will be required for public acceptance. Thus making the insertion and the removal of the plug as simple as the gasoline nozzle used to fuel the internal combustion engines. A connectorlocking lever will also ensure that the control signal is activated upon proper contact before high current is delivered through the battery charge cable.
The station controller regulates the charge current either in the (a) automatic mode, as a slave to the charge controller on board the vehicle; or (b) in manual mode, according to the manual current, voltage and time settings. The switching inverter and the station controller are designed in such a way that they are capable of fast turn-on and turn-off in several milliseconds. A digital readout provides indication for charge voltage (V), charge current (A), delivered energy (kWhr), and the monetary charge in the currency units ($). The EV is equipped with a jack, a battery pack, and a charge controller. The charge controller monitors the battery pack (or its individual modules) by sense control lines. The charge controller communicates with the battery station controller via the signal lines.
Fast Charging Prerequisites
Fast charging prerequisites include:
• All personnel performing fast charging of the battery pack must observe proper safety precautions at all times.
