DC/DC Converter, Charging System, PDU for Electric Car Lithium Battery
1. DC/DC Converter
DC / DC converter is a converter that converts a certain DC power supply voltage into any DC voltage. As a very important part of electric vehicle power system, one of its important functions is to provide power for power steering system, air conditioning and other auxiliary equipment; Another kind of function appears in the composite power supply system, which is connected in series with the super capacitor to regulate the power output and stabilize the bus voltage.
When selecting the type of DC / DC for on-board electrical power supply, it does not directly add all electrical power together to calculate the demand, but divides the electrical equipment into long-term power consumption, continuous power consumption, short-term intermittent power consumption and additional power consumption equipment, and gives different weights. The following table lists the power consumed by various equipment for reference.
At present, the power of electric vehicle DC / DC converter produced by the manufacturer includes 1kW, 1.2kW, 1.5KW, 2kW, 2.5kW, 3KW, 5kW, etc.
Power distribution unit (PDU) is the high voltage power distribution unit in the high voltage system solution of new energy vehicles. There are copper bars, circuit breakers, air switches, contactors, soft start, frequency converters, transformers, high-voltage relays, fuses, surge protectors, mutual inductors, ammeters, voltmeters, transfer switches, etc. in the PDU box of pure electric vehicles. In electric vehicles, the high-voltage components connected with the PDU include: power battery, motor controller, frequency converter, inverter, electric air conditioner, electric defrosting, charging seat, etc.
PDU can also integrate BMS master control, charging module, DC module, PTC control module and other functions. PDU has flexible configuration and can be customized and developed according to customer requirements, which can meet the needs of different customers and different models. BDU (battery disconnect unit) battery pack circuit breaker unit is specially designed for the interior of battery pack and is also a kind of high-voltage distribution box.
3. Charging system
According to whether the charging system is installed on the vehicle, the charger is divided into on-board charging system and off board charging system.
On board charger (OBC) is an electric energy conversion device which is fixedly installed on electric vehicle to control and adjust battery charging. The on-board charger has the ability to safely and automatically charge the electric vehicle power battery. It dynamically adjusts the charging current or voltage parameters according to the data provided by the battery management system (BMS), performs corresponding actions and completes the charging process.
The on-board charger consists of two parts: the power supply part and the charger control mainboard. Among them, the main function of the power supply part is to convert 220V AC into DC; The charger control mainboard is mainly used to control, monitor, calculate, correct, protect the power supply and communicate with the external network. It is the “central brain” of the vehicle charger.
According to the time required for charging, the charger is divided into slow charging system and fast charging system, corresponding to DC power supply and AC power supply respectively.
The slow charging system is mainly composed of cables from the outside of the vehicle to the power supply end, charging interface and cables, on-board charging, high-voltage harness, high-voltage distribution equipment, power battery and its controller. The charging pile or household AC power supply is connected with the on-board charger through the vehicle interface and harness to convert the AC 220V power supply into DC to charge the power battery. During the charging process, the on-board charger communicates with the BMS through can communication to ensure the safety of the charging process.
The architecture of the fast charging system is relatively simple, and the vehicle end parts involved are only the charging interface, fast charging harness, power battery and its controller. The power supply equipment of the fast charging system is the charging pile, which includes power module, billing system, communication and control system, card reading and authorization system, etc. The fast charging system directly converts the three-phase 380V industrial power into DC power to charge the power battery. In the charging process, the communication module of the charging system communicates with the BMS to ensure safety.
Different charging methods will affect the battery life in varying degrees. Adopting appropriate charging methods is of great significance to prolong the service life of the battery.
Common charging methods of vehicle chargers include constant voltage charging, DC charging, phased charging, pulse charging, etc.
Constant voltage charging: during the whole charging process, the charging voltage remains unchanged, and the charging current gradually decreases with the increase of charging time. When the charging current is less than a certain value, stop charging. The energy consumption in the whole charging process is small, which can effectively avoid battery overcharge, simple control and easy operation. However, the initial voltage of the battery to be charged is often small, resulting in a large charging current at the initial stage of charging. On the one hand, excessive current will cause battery polarization and affect the charging speed; On the other hand, it causes the battery temperature to rise rapidly, which is easy to burn out the battery in serious cases, resulting in accidents. Therefore, at the beginning of charging, it is necessary to limit the charging current value to keep the battery charging within an acceptable current range.
At the beginning of DC charging, the power battery is charged with a constant current. When it is about to be fully charged, it is used for floating charging with a constant small current to sufficient the remaining power and compensate for the self discharge of the battery. When the charging voltage reaches the rated voltage, stop charging. Constant current charging avoids the problem of excessive constant voltage charging current, and the current is always limited to the acceptable range of the battery pack.
Phased charging can be divided into two or three stages according to the actual application. The first stage is constant current charging, which uses high current to quickly charge the battery, Make the battery voltage reach a certain voltage value (according to the voltage setting of the power battery pack; the second stage is constant voltage charging, which continues to charge the battery with a current smaller than the constant current to reduce the gas production of the battery; the third stage is floating charging, which charges the battery with trickle to ensure that the battery can be fully charged. When the control system detects that the charging current is less than a certain set value, the charging is ended. Phased charging combines constant voltage and constant current charging The utility model has the advantages of reducing the polarization of the battery and avoiding overcharge and large current charging impact. At present, phased charging is mostly used for charging.
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