There are increasingly more articles promoting the fast charging capability of power batteries, with some manufacturers exaggerating, and some experts following suit by claiming that electric vehicles can be fully charged in 30 minutes or 15 minutes, and even more ridiculously, claiming that charging can be completed in under 5 minutes. I express concern that such claims will be detrimental to the industry's development. Anyone with an engineering background understands that "if it doesn't work in principle, it won't work in engineering." The principles of lithium-ion batteries have already demonstrated that fast charging is not possible.
I. What are fast and slow charging?
Vehicle batteries all have positive and negative terminals, and only a direct current (DC) power source with positive and negative terminals can charge them. What does this mean? Alternating current (AC) from the power grid cannot directly charge vehicle batteries.
However, electric vehicles are charged using standard 220V household AC power or 380V factory AC power. Why is this? Engineers use AC-to-DC (AC/DC) converters to transform AC power into DC power. For ordinary users, it's important to remember:
① The power source for charging the vehicle battery must be a DC power source; AC power cannot be used to charge lithium-ion batteries "directly".
②Where is the DC power source? Currently, the electricity in the power grid is all AC. The DC power used to charge electric vehicles is converted from AC to DC using an AC-to-DC converter.
③ "AC to DC conversion" is a device that meets standard requirements. If this device is installed on the vehicle, the vehicle and charging interface will be an AC charging interface; if this device is installed under the vehicle (on the ground), the vehicle and charging interface will be an AC to DC interface.
④ AC to DC converters can be made to have high power on the ground (Note: the higher the power, the larger the size and the heavier the weight; however, automobiles have insulation requirements and are subject to size and weight limitations, so the AC to DC converters on vehicles are all low power).
The first level of the "fast and slow" concept. DC charging is faster than AC charging. In principle, DC charging takes 3 to 4 hours, while AC charging takes 6 to 8 hours. (Note: This assumes the power grid can guarantee a supply of 220V for residential use or 380V for factory use).
The second level concerns the concept of "fast and slow." Currently, the theoretical charging speed of a DC interface is 3 to 4 hours, compared to the 5 minutes it takes to refuel a gasoline car. Due to limitations in principle, some experts have proposed a concept of "charging to 80% in 30 minutes (or 15 minutes)," leaving a 20% margin for error, with the charging time being ignored. In other words, the entire charging process is segmented.
The third level of the "fast and slow" concept. The inability of a DC interface to fast charge refers to the entire process from 3% to 99% for a full charge; the ability of a DC interface to fast charge generally refers to the entire range from 20% to 80%. This is part of the lithium-ion battery charging (discharging) curve. Some manufacturers have only made some optimizations to this curve. In principle, there is no substantial innovation.
Second, the conclusion that DC interfaces cannot fast charge is based on a combination of factors; fast charging capability depends on many prerequisites.
Let's look at an example from everyday life. A wooden bucket is used to store water, and a faucet is used to add water to an empty bucket. The bucket has a capacity (similar to the rated capacity of a car battery), and the faucet also has a fixed maximum water flow (similar to a charging port). The analysis is as follows:
① To add water to a wooden bucket with a fixed volume, you can turn the tap on to the maximum if you want to speed it up.
②Once the water flow rate of the faucet is determined, if you want to add water quickly, you can make the volume of the wooden bucket smaller.
③ To add water to a wooden bucket with a fixed volume, and given that the water flow from the tap is also fixed, there is an average (rated) speed for adding water. This average speed is derived from the maximum and minimum speeds.
Logical analysis of the fast charging capability of power batteries:
① The capacity of the vehicle battery has been determined. If fast charging is required, the physical solution is to add one (or two) more charging ports. This is the current approach for 12-meter pure electric buses.
② The specifications of the charging interface are national standards (which have been clearly defined). If fast charging is desired, the most basic approach is to reduce the capacity of the vehicle battery. Lithium titanate charges faster than lithium iron phosphate, which is based on this principle.
Third, lithium-ion batteries cannot be fast-charged in principle.
Currently, lithium titanate batteries are the only type that can be fast-charged. However, the energy density of lithium titanate batteries is far lower than that of lithium iron phosphate and ternary lithium batteries. In other words, the driving range of lithium titanate batteries in vehicles is too short. To travel the same distance, lithium titanate batteries require more charging cycles. Multiple fast charges of a lithium titanate battery take longer than a single slow charge of a lithium iron phosphate or ternary lithium battery, offering no advantage to the user. Is battery swapping an option? No. Battery swapping still requires charging, and the technology is more difficult and costly than charging.
The slower refueling speed of electric vehicles compared to refueling gasoline vehicles is a significant drawback. Everyone is trying to find ways to address this. How? In my opinion, engineering technology (processes) must find solutions with sound underlying principles, and then effectively integrate resources (conditions). We shouldn't keep trying and trying with ideas that are fundamentally flawed.
The best solution is to increase the specific energy of the power battery, meaning that a single slow charge should allow the car to achieve a range equivalent to more than one day (two or three days) of driving range. Currently, research and development on the specific energy of power batteries is progressing relatively quickly, and range anxiety will gradually disappear.
