Exploring the difference between fast charge and slow charge according to the principle of lithium battery

At present, there are more and more articles promoting the power battery can be quickly charged. Some manufacturers' propaganda is overdone. Some experts also follow the trend. It can complete the charging of electric vehicles in 30 minutes and 15 minutes. It is even more ridiculous said that the charging task can be completed within 5 minutes. The author expressed concern that propaganda will be unfavorable to the development of the industry. People who engage in engineering understand that “the principle is unreasonable and impossible in engineering”. The principle of lithium-ion battery has told people that they cannot charge quickly.

1. What is fast and slow charging?

Car batteries are positive and negative, can charge the battery must have a positive and negative DC power. What do you mean? The AC power of the grid cannot directly charge the car battery.

However, charging electric vehicles is 220V for civilian use or 380V for factory use. Why is that? This is the use of an "AC/DC" device to turn AC power into DC power. For ordinary users to remember:

1 The power supply for charging the car battery must be a DC power supply. The AC power supply cannot charge the lithium ion battery "directly".

2 Where is the DC power supply? At present, the electricity of the grid is AC. The DC power that charges an electric car is converted by an "AC/DC" device.

3 "AC/DC" is a device that meets the requirements of the standard. If the device is installed in the car, the car and charging interface is the AC charging interface; if the device is installed under the car (ground), the car and charging interface is the AC DC interface.

4 AC to DC (AC / DC) device can be made of high power on the ground (Note: the greater the power, the larger the volume, the heavier the quality; and the car has insulation requirements, volume and weight restrictions, so the car's AC changes Direct current (AC/DC) is low power.

The first level of the concept of "fast and slow." the charging speed of the DC interface is faster than the AC charging interface. In principle, the charging speed of the DC interface is 3 hours to 4 hours, and the AC charging interface is 6 hours to 8 hours. (Note: The grid can guarantee supply, civil 220V, or 380V AC for factory use)

The second level of the concept of "fast and slow." at present, the theoretical DC interface charging speed is 3 hours to 4 hours, compared with the fuel car refueling for 5 minutes, due to the limitations of the principle, some experts put forward the concept of "30 minutes (or 15) charging to 80%" and there is 20% space, charging time is ignored, in other words, the entire process of charging is segmented.

The third level of the concept of "fast and slow." the DC interface can't be fast, it means 3% to 99% of the whole process, completely full; the DC interface can be charged quickly, basically referring to 20% to 80% of the entire space. This is part of the charge (discharge) curve for a lithium-ion battery. Some manufacturers have made some optimizations for this curve. In principle, there is no real innovation.

Second, the DC interface cannot be fast charge is a comprehensive conclusion, can quickly charge is a lot of preconditions

Let's look at an example of everyday life. The wooden barrel is used to store water, and the faucet adds water to the empty wooden barrel. The barrel has a volume (similar to the amount of the car battery), and the maximum water output of the faucet is also certain (similar to the charging interface). analyse as below:

1 To the volume has been determined to add water to the barrel, if you want to fast, you can open the faucet to the maximum.

2 The water output of the faucet is determined. If you want to add water quickly, you can make the barrel volume smaller.

3 To add water to the barrel whose volume has been determined, the water output of the faucet is also determined, and there is an average (rated) speed when adding water. The average speed is the fastest and lowest speed.

The logic analysis of the power battery can be fast charging:

1 car battery capacity has been clear, if you want to charge quickly, the physical practice is to add one (or two) charging interface, the current 12-meter pure electric bus, this is the practice.

2 The specification of the charging interface is the national standard (already clear). If you want to charge quickly, the basic method is to make the car battery capacity small, and the lithium titanate charging is faster than the lithium iron phosphate charging. This is the principle.

Third, the lithium-ion battery cannot be quickly charged in principle.

At present, the battery that can be quickly charged is lithium titanate, but the specific energy of lithium titanate battery is far from the energy of lithium iron phosphate and ternary battery. In other words, the car lithium titanate battery, its cruising range is too short. Running the same mileage, the car lithium titanate battery is also charged more times. The time of the fast-charged lithium-titanium titanate battery is longer than that of a slow-loading lithium iron phosphate or a ternary battery, and there is no advantage for the user. Is it possible to change the electricity, not at all it is also necessary to charge the power exchange, the power-changing technology is more difficult and cost-effective than charging.

Electric vehicle electric energy supplement is slower than fuel vehicle fueling speed and is an objective short board. Everyone is thinking of ways to make up. How to do? The author's point of view, engineering technology (process) must find the principle is the right solution, and then integrate the resources (conditions), do not always use the principle of incorrect ideas, try and try again.

The best way to improve the power battery specific energy, that is, a slow charge, the mileage of the car can exceed one day (two days, three days) of operating mileage. At present, the power battery has been faster than the energy research and development, and everyone's mileage anxiety will slowly disappear.

The page contains the contents of the machine translation.