Electric vehicle charger part of the principle, standards, terminologyUnderstanding the Principles, Standards, and Terminology of Electric Vehicle Chargers

 

[1] Basic charging principles

From a technical perspective, new energy electric vehicle charging methods are currently divided into the following categories:

 

Alternating current (AC) charging

Direct current (DC) charging

Direct battery replacement

Wireless charging

The fourth type of wireless battery charging technology is not yet perfect and cannot be popularized.

 

So now when we charge our cars, we basically use the first two types of DC charging or AC charging.

 

If AC charging is used, the current passes from the socket through the AC charging interface and reaches the car's onboard charger (OBC) through the charging cable. The OBC converts the AC current into DC and charges the battery through the battery management system (BMS).

 

If DC charging is used, the OBC is bypassed and the current is sent directly to the battery through the BMS. Therefore, DC charging does not use OBC but uses its own AC/DC conversion components, so it is not subject to the charging power limit of OBC. Of course, it also has higher requirements for BMS.

 

AC charging vs DC charging

The battery pack of an electric vehicle can actually be simply understood as thousands of small batteries connected in series and parallel. The actual charging process is to divide the input large current into thousands of "tricks" and then charge thousands of battery cells separately. This process depends on the exchange of voltage and current related parameter information between the BMS and the charger, and the control of the charging of each terminal battery.

 

In addition, the cooperation of various systems such as battery auxiliary group, cooling, AC/DC conversion, etc. is also required.

 

[2] Basic knowledge of on-board charger (OBC)

OBC is the abbreviation of on-board charger (On Board Charger), which refers to a charger fixedly installed on new energy electric vehicles. It converts the AC power input from the AC charging pile into high-voltage DC power. According to the data provided by the battery management system (BMS), it dynamically adjusts the current and voltage parameters input from the charging pile and performs corresponding operations to complete the process of charging the high-voltage battery of the car.

 

The principle of the simple circuit diagram is as follows:

 

 

The core is the AC-DC converter composed of PFC (Power Factor Correction) power factor corrector + isolated DC-DC. PFC realizes the conversion of AC voltage of the power grid into DC voltage, and ensures that the input AC current is in phase with the input AC voltage; DC/DC realizes the conversion of PFC-level output DC voltage into the required charging voltage, realizes constant current/constant voltage charging function, and ensures electrical insulation between the AC high-voltage side and the DC high-voltage side. In addition, there are AC input filtering, DC output filtering and control circuit components.

 

Since OBC is responsible for the conversion of AC/DC and low-voltage and high-voltage circuits, it determines the power supply quality for charging the battery, so it is safe, stable and EMC (electromagnetic compatibility standards).

 

In addition to the OBC that can be charged in one direction, there is also a bidirectional OBC that can charge and invert and discharge. It can be divided into V2L type and V2G type.

 

V2L (Vehicle to Load): Take power from the vehicle power battery-OBC inverter-AC charging port-dedicated V2L AC socket board-220V electrical equipment. That is, the DC power of the power battery can be inverted into 220V AC power for household use. It is suitable for outdoor camping and other scenarios such as taking power from the car.

 

V2G (Vehicle-to-grid) is the process of taking power from the vehicle's power battery and returning it to the grid.

 

[3] Battery Management System (BMS)

The battery management system (BMS) can monitor the status of the vehicle's battery in real time, manage the charging and discharging process, prevent the battery from being overcharged or over-discharged, enhance the battery's efficiency and extend its service life.

 

It is a comprehensive system that includes sensors, CPUs and execution units. Various sensors are used to obtain battery status information and pass this information to the CPU, which then processes and sends the operation command to the execution unit for processing, adjusts the battery status, places it in a suitable working environment and a safe environment, and meets the vehicle's power requirements. It also provides overcharge protection, over-discharge protection, overcurrent protection, and temperature protection for the battery.

 

Therefore, BMS is a key factor in ensuring the performance and safety of electric vehicles.

 

The battery management system performs the following four functions:

 

1. Monitor battery status

Including the following parameters

 

voltage, temperature, current.

 

Battery charging status, used to display the battery's charging level.

 

The state of charge (SOC) of a battery refers to the available state of the remaining charge in the battery, or simply how much power is left in the battery.

 

SOC is the most important parameter in a BMS, because everything else is based on SOC

 

Battery health (SOH). Or the degree of battery degradation, it is the ratio of the actual capacity of the current battery to the rated capacity.

 

Battery power state (SOP)

 

Battery safety state - determined by paying attention to all parameters and determining whether the use of the battery poses any danger.

 

Others such as the flow of coolant and its speed.

 

2. Management control

BMS calculates various battery status values ​​based on the various parameters mentioned above to determine the battery's charge and discharge current limits.

 

Battery pack balancing, estimated charging strategy control, battery safety and protection.

 

3. Cooling system management

4. CAN (Controller Area Network) bus communication

Mainly, BMS communicates with the on-board charger OBC to monitor and control the charging of the battery pack, and adjust the current and voltage of the OBC input to ensure that the battery is not overcharged or over-discharged.

 

Battery pack balancing technology, fast charging technology and battery SOC estimation are three key technologies of the battery management system.

 

[4] Calculation of voltage and power matching of charging piles

The meter voltage is divided into two types: 220V and 380V:

 

220V voltage is also called single-phase electricity. The standard configuration is 1 live wire, 1 ground wire, and 1 neutral wire. The relative voltage between the live wire and the neutral wire is 220V.

 

380V is called three-phase electricity. The standard configuration is 3 live wires, 1 ground wire, and 1 neutral wire. Because the relative voltage between any two live wires is 380V for the sinusoidal wave of alternating current, the voltage is called 380V. The voltage of any live wire relative to the neutral wire is actually 220V.

 

So how is the power of the charging pile calculated?

 

For a single-phase 220V charging pile, the maximum charging current is 32A, so the corresponding power is 220*32=7040w=7KW.

 

For a three-phase 380V charging pile, the corresponding calculation formula for 32A current is 220*3*32=21KW or 380*√3*32=21KW.

 

If a 380V charging pile only uses a pair of wires with 220V voltage + 32A, it is 7KW, just like single-phase electricity.

 

If 16A current is used, then 220*3*16=11KW.

 

This is why a 380V 21KW charging pile can be backward compatible with all types of power.

 

But if it is a 11kw charging pile, it can generally only be charged with 380V 11kw and 220V 3.5kw, not 7kw. But there are exceptions, see the next section.

 

Three-phase electricity has an advantage over single-phase electricity. Because there is a phase difference in three-phase electricity, the same charging current has resistance in the cable, and the voltage drop in the circuit is smaller, which means less loss in the circuit.

 

So try to apply for a 380V meter, which can install three-phase electricity and bring expansion advantages.

 

[5] Compatibility of 11KW charging piles

 

The 380V 11KW charging pile products on the market generally indicate that they are not compatible with 7KW and are only compatible with 3.5KW.

 

According to my understanding, when designing a three-phase 11kw charging pile, the power supply line of the charging pile is generally only 2.5 square meters and only supports 16A current.

 

According to the formula we mentioned in the previous section, 220*3*16=11KW.

 

The incoming line does not support 32A current, and 7KW requires a 32A high current. It is impossible to make only one line support high current. If both are made larger, the specifications will be the same as 22KW.

 

Therefore, considering cost and product positioning, it is generally not compatible with 7KW. In addition, it seems that some patented technologies in structural design are also involved.

 

[5] Charging power fluctuation and loss

If the charging pile has real-time parameter display, we often find that the actual charging power is different from the nominal power or fluctuates.

This is actually because the actual charging efficiency is affected by many practical factors.

The first is the conversion efficiency of the on-board charger OBC.

This is where the AC point is actually converted into high-voltage DC to charge the battery. Factors such as internal resistance and circuit conversion will cause losses. According to general experience, there is a loss of about 5%, which is the largest loss link.

 

Voltage fluctuation

The voltage input from the meter actually fluctuates within a range. For example, the 220V AC power we use may actually fluctuate within the range of 200V-250V. This will of course affect the current change and the charging power.

 

Line loss

Wire loss, voltage drop and heat generation,

A 7KW charging pile should be matched with a 6-square copper wire, but if the distance exceeds 100 meters, it should be considered to upgrade to a 10-square copper wire.

 

Temperature influence

 

When charging at low temperatures, the temperature control system will adjust the battery charging capacity, and heating power consumption will increase.

 

Therefore, there is a suggestion to charge the car immediately after using it. At this time, the battery temperature is relatively high and the charging performance is better.

 

In northern areas with lower temperatures, it is recommended that users charge in a heated room.

 

There are also some other loss factors with a smaller impact, such as the working loss of the charging pile, the loss of the on-board charger, the loss of the battery heat dissipation and cooling system, and the loss of the BMS system.

 

[5] Basic description and requirements of cables

 

Wire specifications 4 square/6 square The "square" in it does not refer to the square meter of the house area, but refers to the cross-sectional area of ​​the wire, the unit is square millimeter, but it is commonly referred to as "square".

 

For example, the ordinary socket in our home is connected to a 2.5 square wire, which generally does not exceed 10A.

 

The socket of a high-power appliance such as an air conditioner is connected to a 4 square wire, which can support a current of 16A.

 

And the wires entering the house are required to be 6 square or 10 square wires.

 

The safe current carrying capacity of the charging pile cable is very important for long-term safe use. The greater the power, the higher the safety load-bearing cable specifications (that is, the thicker), and the more expensive the unit price.

 

At the same time, if the distance is longer, the cable requirements are also higher, otherwise the current transmission attenuation will increase and the charging efficiency will not be achieved.

 

For 32A single-phase current, at least 4 square wires are required, and now it is generally recommended to use 6 square wires.

 

For example, a typical requirement:

 

For 380V 22KW charging piles, it is recommended to use 5-core 6-square wires within 60 meters, and 5-core 10-square wires from 60 meters to 120 meters

 

For 220V 7KW charging piles, it is recommended to use 3-core 6-square wires within 30 meters, and 3-core 10-square wires from 30 meters to 100 meters

 

For longer distances, of course, the specifications may need to be improved.

 

Wire specification parameter reference

 

How to check the quality of wires? Generally, it can be checked from the following points:

 

1- Look at the length. There is a length mark on the packaging of the wire. Some unscrupulous merchants will change the length mark, so be careful when buying.

 

2- Look at the wire diameter. That is, the diameter length of the copper wire, which can be directly seen on the cross section of the copper wire.

 

3- Look at the quality of the copper wire. The best copper wire is red copper, and yellowing is not good.

 

4- Look at the 3C mark. The wire must have a 3C certification mark, otherwise it is an unqualified product.

 

Material type of wire:

 

The following two types of charging pile wire materials are generally better:

 

RVV: PVC insulated PVC sheathed flexible cable

RVV power cord is a commonly used wire and cable in weak current systems. Two or more RV wires are added with a layer of sheath. The internal wire core is not only 2 cores, but also 3 cores, 4 cores, 6 cores and other specifications of power cord.

 

YJV: Cross-linked polyethylene insulated PVC sheathed power cable

The copper core conductor of the YJV power cable has a non-corrosive outer layer like the RVV copper core conductor, and there is also a sheath on the outermost layer. It is an environmentally friendly cable. The copper core wires in the YJV power cable are generally parallel and not twisted.

 

YJV cable has cross-linked insulation material and has the characteristics of high temperature resistance; while RVV cable has good flexibility and bendability.

 

[6] Waterproof and dustproof IPXX standard

IP is the abbreviation of Ingress Protection. The IP protection level is an important evaluation standard for the safety protection of electrical equipment. It provides a method to classify products based on the dustproof, waterproof and collision-proof degree of electrical equipment and packaging. This system is established by the International Electrotechnical Commission IEC and has been recognized by most European countries.

 

The IP rating format is IPXX. The first digit after IP is the dustproof level, and the second digit is the waterproof level. The larger the number, the higher the protection level.

 

IP54 is the protection standard required by China for outdoor charging piles. This standard can ensure that the normal operation of the charging pile is not affected by dust (but dust may still enter the interior). At the same time, it can also provide safety protection under low-pressure water spray for 3 minutes, and also protect against vertically falling rain, ensuring the safe use of the charging pile in a normal rainfall environment.

 

A higher standard is IP 65, let's compare them.

 

Dustproof level 5 means: it cannot completely prevent dust intrusion, but the amount of intrusion dust will not affect the normal operation of the product

 

Dustproof level 6 means: completely prevent foreign objects and dust intrusion.

 

Waterproof level 4 means: prevent splashing water intrusion, prevent splashing water from all directions.

 

Waterproof level 5 means: no harm when rinsed with water.

 

At present, most charging pile products can achieve IP55, and many products will make the gun head IP65-IP67.

 

For charging piles installed outside of indoors, including car chargers, the higher the IP level, the better.

 

Because IP54 housings are only splashproof, they are at risk of problems or even failures when exposed to bad weather. Foreign surveys have shown that IP54 charging piles outdoors can cause premature aging of the power electronics system due to the intrusion of dust and moisture, sometimes shortening the service life to three years. The normal service life of an electric vehicle charging pile is about 10 years, so it is reduced by 70%!

So if your charging pile is installed outdoors, my advice is: the gun head must be selected above IP 65, and if conditions permit, try to install a protective box for the charging pile body.

 

Appendix, IP grade specific standard description -

 

IP dustproof level:

 

1: Prevent large solid intrusion

 

2: Prevent medium-sized solid intrusion

 

3: Prevent small solid intrusion

 

4: Prevent solid objects larger than 1mm from entering

 

5: Prevent harmful dust accumulation

 

6: Completely prevent dust from entering

 

IP waterproof level

 

0: No protection

 

1: Water drops into the shell have no effect

 

2: When the shell is tilted to 15 degrees, water drops into the shell have no effect

 

3: Water or rain from a 60-degree corner to the shell has no effect

 

4: Liquid splashed on the shell from any direction has no harmful effect

 

5: No harm after washing with water

 

6: Can be used in the cabin environment

 

7: Can withstand immersion in water for a short time (1m)

 

8: Can withstand immersion in water for a long time under a certain pressure

 

[7] Advantages and disadvantages of shell materials

As the external protection component of the charging pile, the charging pile shell (including the charging gun shell) needs to meet the mechanical strength requirements of possible collisions during transportation and use in addition to the most basic flame retardant + weather resistance.

 

Weather resistance refers to the ability of a material to withstand the test of climate when used outdoors, such as comprehensive damage caused by light, heat, wind, rain, bacteria, etc., which is called weather resistance. That is, the ability to protect against outdoor wind, sun, rain and rain.

 

Common charging pile shell materials are PC/ABS and PC/ASA.

 

PC/ABS is a thermoplastic plastic made of polycarbonate (PC) and ABS alloy, and is the most classic material for mobile phone plastic shells.

 

ASA is a ternary polymer and belongs to impact-modified resin. ASA is more impact-resistant and fire-resistant than ABS.

 

PC/ASA and PC/ABS have similar toughness, but PC/ASA is better than PC/ABS in weather resistance and flame retardancy.

 

In addition, some products also partially use AL6063 aluminum alloy, which can increase strength and improve thermal conductivity.

 

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