If electric cars are to rise, this chip cannot be ignored

Source: Content from " ctimes ", thank you.

Henry Ford founded Ford Motor Company in 1903 at a time when few people could afford cars. They were expensive to manufacture, costly to repair, and complicated to operate. At the time, cars were still a luxury for the privileged few. It wasn’t until Ford’s clever use of the internal combustion engine, new manufacturing techniques, and cheap repair designs that he was able to bring a luxury product to the mainstream, forever changing the way the average person travels.

116 years later, people today are facing the same problem again. With soaring oil prices, shrinking resources and increasing environmental pressures, it is no surprise that governments and people around the world are looking for better, cleaner and more efficient alternatives to gasoline. Many European countries have already begun to promote or even legislate against the sale of new non-electric vehicles. This makes the electric vehicle (EV) industry one of the most exciting areas today.

But just like those early cars going back more than a century, producing electric vehicles for the mass market presents a host of challenges. Launching a viable alternative to the internal combustion engine (ICE) is not easy. Breakthroughs in technology continue to open up new possibilities. But translating those breakthroughs into motors, control technologies and batteries, improving and refining models, and encouraging widespread consumer adoption will take more time.

We can first observe what changes will happen in the electric vehicle market in 2019. In this year, it is expected that the battery size of electric vehicles will become smaller and smaller. Although electric vehicles with larger batteries and longer driving ranges bring greater mobility convenience, the market for small battery electric vehicles for urban driving is also increasing. In the future, as the number of charging points increases, urban EVs with shorter driving ranges will become more common.

In addition, the government has been subsidizing the sales price of electric vehicles to stimulate the market. And consumers also expect gas stations to become power supply stations, such as installing fast charging piles near gasoline dispensers. This also means that they must provide drivers with some entertainment during charging, whether it is drinking coffee, watching TV or shopping.

In 2019, electric vehicle chargers will be more strictly managed and required to provide higher quality power and safer functional standards. Chargers will eventually be required to allow load control to ensure that the power grid can meet the high demand for charging. As battery prices fall, higher production and higher energy density also provide safer and longer driving distances.

Lithium-ion batteries have proven to be interesting batteries for electric vehicle manufacturers due to their high charge density and low weight. The characteristics of lithium-ion batteries are large in size and relatively unstable in nature. It is very important that these batteries should not be overcharged or under-discharged in any situation where their voltage and current need to be monitored. The most difficult part of this process is that with so many cells combined together to form an EV battery pack, each cell must be individually monitored for safety and ensure its efficient operation, which requires a special dedicated system called a battery management system (BMS).

Similarly, to get the most power efficiency from the battery pack, we should fully charge and discharge all the batteries at the same time at the same voltage, which again requires the BMS. In addition, the BMS will also take on many other functions.

There are many factors to consider when designing a BMS. The complete set of considerations depends on the exact end application in which the BMS will be used. In addition to BMS for EVs, it can be used anywhere a lithium battery pack is used, such as solar panel arrays, windmills, power walls, etc. Regardless of the application, there are some important factors that should be considered in BMS design.

Discharge control

The main function of the BMS is to maintain the lithium battery within a safe operating voltage. For example, the undervoltage rating of a typical 18650 lithium battery is about 3V. The BMS is responsible for ensuring that all cells in the battery pack do not discharge below 3V.

Charging control

In addition to discharge, the charging process should also be monitored by the BMS. Most batteries tend to be damaged or have a shortened life if they are not charged properly. For lithium battery chargers, there are two stages. The first stage is called constant current (CC), during which the charger outputs a constant current to charge the battery. When the battery is close to full, the second stage is called the constant voltage (CV) stage, during which a constant voltage is supplied to the battery at a very low current. The BMS should ensure that the voltage and current during charging do not exceed the permeable limits to avoid overcharging or fast charging of the battery. Generally speaking, the maximum allowable charging voltage and charging current can be found in the battery's data sheet.

charging

This is the fuel indicator of the EV, it actually tells us the battery capacity percentage of the battery pack. Just like the one in our mobile phones. But it is not as easy as it sounds. The system needs to monitor the voltage and charge and discharge current of the battery pack at all times to predict the capacity of the battery. Once the voltage and current are measured, there are many algorithms that can be used to calculate the capacity of the battery pack. The most commonly used method is the coulomb counting method.

Health status

There are usually two things you need to know about the battery health status. One is the battery state of charge (SOC), which indicates the current battery charge ratio. A fully charged battery has a charge status of 100%. After the battery is used or stored for a long time, the open circuit voltage of the battery will drop, so the percentage value of the charge status will also drop. In addition, the battery health status (SOH) is an indicator of the battery's current output capacity. A brand new battery can achieve a 100% SOH battery health status.

The capacity of a battery depends not only on its voltage and current profile, but also on its age and operating temperature. SOH measurement tells us the age and expected life cycle of a battery, based on its usage history. In this way, we can know how much the mileage of an EV (the distance covered on a full charge) will decrease as the battery ages, and we can also know when the battery pack should be replaced.

Cell Balancing

Another important function of the BMS is to maintain the balance between cells. For example, in a group of 4 cells connected in series, the voltage of all four cells should always be equal. If one cell has a lower or higher voltage than the other, it will affect the entire battery pack, for example, one cell is at 3.5V and the other three are at 4V. During charging, these three cells will reach 4.2V while the other cell has just reached 3.7V. Similarly, this cell will be the first to discharge to 3V. In this way, due to the different condition of this single cell, all other cells in the battery pack will not be able to use their maximum potential, which will affect the efficiency.

To solve this problem, the BMS must implement a method called cell balancing. There are many types of cell balancing techniques, but the most commonly used are active and passive cell balancing. In passive balancing, the idea is that the cell with excess voltage will be forced to discharge through a resistor-like load to reach the voltage value of the other cells. As for during active balancing, the stronger cell will be used to charge the weaker cell to equalize their potential.

Thermal Control

The life and efficiency of a lithium battery pack depends greatly on the operating temperature. Batteries discharge faster in hot climates than at normal room temperature. In addition to this, the high current consumption will further increase the temperature. This needs to be regulated by the thermal system (mainly oil) in the battery pack. This thermal system mainly serves to reduce the temperature, but it should also be able to increase the temperature in cold climates if necessary. The BMS is responsible for measuring the temperature of each cell and controlling the thermal system accordingly to maintain the overall temperature of the battery pack.

Lower system power consumption

The BMS should be active and running continuously even when the car is running, charging, or in standby mode. This situation requires that the BMS circuits must be powered continuously, so the BMS system must consume very little power so as not to consume too much electricity. When an EV is not charged for weeks or months, the BMS and other circuits tend to drain the battery on their own, eventually requiring recharging before the next use. Such problems are still common even on well-known models such as Tesla.

Galvanic Isolation

As a bridge between the battery pack and the engine controller (ECU) of the EV, the BMS must send all the information collected by the BMS to the ECU to be displayed on the dashboard or on the instrument panel. Therefore, the BMS and ECU should communicate continuously through standard protocols such as CAN or LIN bus. The BMS design must also be able to provide galvanic isolation between the battery pack and the ECU.

data record

Since BMS has to store a lot of data, it is important to have a larger memory bank. The health status of the battery can only be calculated when the charging history of the battery is known. Therefore, BMS must track the charging cycles and charging time of the battery pack from the day of installation, which will help engineers provide after-sales service or analyze charging problems of EVs.

Accuracy

When a battery is charged or discharged, the voltage gradually increases or decreases. Unfortunately, the discharge curve (voltage vs. time) of a lithium battery has a flat area, so the change in voltage is very small and must be measured accurately. A well-designed BMS can have an accuracy of ±0.2mV, and the minimum accuracy requires at least 1~2mV. A 16-bit ADC is usually used in this process.

It is worth noting that electric vehicles are gradually moving towards a simplified design, which will drive cost reductions in various aspects, an obvious example being the battery itself. Smaller batteries mean lighter car weight, which will save additional costs for the car chassis, brakes, charging technology, motor power, etc. Smaller capacity electric vehicles will help reduce power consumption in charging technology and make it easier to balance charging needs. Combined with a more efficient BMS system, it will make electric vehicles safer on the road.