Charging piles are actually equivalent to gas dispensers in gas stations. The popularization and promotion of new energy vehicles has gradually introduced charging piles into people's lives. According to people's actual situation, new energy car owners use fast charging, slow charging and flying cable as the main charging methods. First fly... Keywords: charging pile, new energy vehicle, electric vehicle battery.

Charging piles are actually equivalent to gas pumps in gas stations. The popularization and promotion of new energy vehicles has gradually introduced charging piles into people's lives. According to people's actual situation, new energy car owners use fast charging, slow charging and flying cable as the main charging methods. First of all, flying cable is the most discouraged and most dangerous charging method, and slow charging takes a long time, especially for car owners who do not have private charging piles. The slow charging method puts too much pressure on time, energy and money, and the fast charging method is harmful. There have been rumors about the battery that it needs to be charged using a combination of fast and slow charging.

With the continuous accumulation of electric vehicle users, basic charging problems have gradually been exposed. Charging equipment and companies are not interoperable. Car owners must not only know the location of the charging pile, but also understand the brand and payment method of the charging pile.

How lithium batteries work

To discuss the impact of DC charging on batteries, we must first understand electric vehicle batteries.

Currently, most electric vehicles on the market are lithium-ion batteries. The battery has two poles: the positive electrode is lithium compound and the negative electrode is graphite.

During charging, lithium ions (Li+) are generated on the positive electrode of the battery. The generated lithium ions (Li+) move from the positive electrode to the negative electrode through the electrolyte. The graphite of the negative electrode has a microporous layered structure. The lithium ions reaching the negative electrode (Li+) is embedded in the micropores of the carbon layer. The more lithium ions (Li+) are embedded, the higher the charging capacity; during discharge (that is, the process when we use the battery), the lithium ions (Li+) embedded in the negative carbon layer ) comes out and moves back to the positive electrode. The more lithium ions (Li+) that return to the positive electrode, the higher the discharge capacity. What we usually call battery capacity refers to the discharge capacity. Therefore, under ideal conditions, during the charging and discharging process of lithium batteries, the actual state of movement of lithium ions (Li+) is from positive electrode → negative electrode → positive electrode. As long as the chemical structure of the positive and negative electrode materials does not basically change, the reversibility of battery charging and discharging Well, lithium-ion (Li+) batteries can guarantee long-term cycling.

Fast charging mainly ensures that lithium ions (Li+) are quickly embedded from the positive electrode and quickly embedded in the negative electrode, without causing the deposition of lithium ions (Li+). However, when the current increases, a semipermeable membrane (SEI membrane) on the surface of the electrode negative electrode (graphite) will rupture to a certain extent, causing the electrode material and the electrolyte to react with each other. In addition, the increase in temperature will be accompanied by some side reactions, such as the decomposition of the electrolyte and the formation of sediments on the electrodes, resulting in a reduction in reversibility and a gradual reduction in battery capacity.

The impact of fast charging on batteries

When an electric vehicle is charging, the battery management system BMS on the electric vehicle will estimate the state of charge of the power battery pack (i.e., the remaining battery capacity) and automatically adjust the charging current according to the condition of the battery pack to ensure that the SOC is maintained at a reasonable level. Within the range, it prevents damage to the battery due to overcharging or over-discharging, so there is no need to worry too much about damage to the battery caused by fast charging.

In fact, the lithium ions in the battery are like a swing. When there are no external factors such as friction and gravity, the swing will keep swinging from side to side due to the conservation of energy. But as long as there is an external factor, the lithium-ion swing may not be as high as before. This factor will always exist, and the swing may stop one day.

Therefore, whether the battery pack of an electric vehicle will be overcharged or overdischarged depends largely on the management level of the BMS system. Therefore, when consumers or car companies pursue high battery life, they not only increase battery capacity, but also improve the management and upgrade of the battery management system (BMS).

How to reduce battery damage through fast and slow charging

If you are not confident in the battery management system BMS and have the conditions to build your own charging pile, it is best to use AC slow charging as the main method and DC fast charging as a supplement. Because the input of AC slow charging is alternating current, which is then converted into direct current by the on-board charger to charge the battery pack, the relative input power is small, and the chance of overcharging is very small.

Currently, in the charging pile market, some DC charging piles have built-in overcharge active protection functions. It will actively detect the charging status of the battery pack and intelligently optimize the charging curve. When the charging pile detects that the battery pack is about to be fully charged, the charging pile will automatically reduce the output current and enter the trickle charging stage; when the charging pile detects that the battery pack power has reached a certain value (for example, when the battery pack power reaches 97% ), and after trickle charging exceeds a certain time (such as 10 minutes), the charging pile will actively disconnect charging to prevent the battery pack from overcharging. If you use this kind of charging pile with anti-overcharging function, you don’t need to worry about this problem.

Although users have different charging methods and payment methods when charging at charging piles, alternating charging between different charging piles will basically not have any adverse effects on the battery. As mentioned above, in theory, different charging piles have basically the same charging effect on electric vehicles, and their charging quality is determined by the management level of their own battery management systems.

The promotion time of electric vehicles is still short, and neither the use of vehicles nor the corresponding charging facilities have kept pace with development needs. As the saying goes, if you want to get rich, you must first build roads. If you want electric vehicles to develop and achieve the goals of Made in China 2025 and the 13th Five-Year Plan, the development of charging piles must be at the forefront. (Author: E Chehui)

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