Two charging methods for electric car power batteries

There are two ways to charge the power battery: regular charging and fast charging.

1). Conventional charging method (slow charging method)
The conventional charging method is also called the traditional charging method. It can be charged only by plugging the plug of the car charger into the power socket in the parking lot or at home. Therefore, the charging process is generally completed by the user independently. Its characteristic is that it takes electricity directly from the low-voltage lighting circuit, and the charging power is small. It is powered by a standard power grid of 220V/16A specification. The typical charging time is 8-10h (SOC reaches 95% or more).

This charging method has no special requirements on the power grid, and it can be used as long as the power supply system can meet the lighting requirements. Since charging at home is usually at night or during low power consumption, which is conducive to the effective use of electric energy, the electric power department generally gives electric car users some preferential treatments, such as discounts on charging during low power consumption.

There are three conventional charging methods: constant current charging method, constant voltage charging method and stage charging method.
(1) Constant current charging method
During the charging process, the charging method that keeps the charging current constant is called the constant current charging method. The charging curve is shown in Figure 1.

Constant current charging has the following characteristics.
①During the charging process, the charging current is constant, but the charging voltage is changing (during the charging process, the terminal voltage of the battery continues to rise, in order to ensure the constant charging current, the charging power supply voltage or regulating load should be changed at any time).
②The size of the charging current can be determined according to the charging type and the capacity of the battery.
③Batteries with different terminal voltages can be charged in series.
④Long charging time.

(2) Constant voltage charging method
During the charging process, the charging method in which the voltage applied to both ends of the battery remains unchanged is called the constant-voltage charging method. The charging curve is shown in Figure 2.
Constant voltage charging has the following characteristics.

① During the charging process, the charging voltage remains unchanged (when charging starts, the charging current is very large, and as the electromotive force of the battery continues to rise, the charging current gradually decreases until it is zero).
②Generally, the charging voltage of a single cell battery is 2.5V. If the charging voltage is selected too low, the battery will be undercharged; if the charging voltage is selected too high, the battery will continue to be charged after it is fully charged. The charging at this time is For overcharging.
③At the beginning of constant voltage charging, the electromotive force of the battery is small, so the charging current is very large, which will adversely affect the life of the battery, and it is easy to bend the battery plate and cause the battery to be scrapped; in the middle and late stages of charging, due to the influence of battery polarization, The positive electrode electrical test needs to be higher, and the negative electrode potential becomes lower, so the electromotive force increases and the charging current is too small, resulting in long-term insufficient charging and affecting battery life.
In view of these shortcomings, constant voltage charging is rarely used, and only used when the charging power supply voltage is low and the working current is large.

(3) Stage charging method
This method includes two-stage charging method and three-stage charging method.
① The two-stage charging method adopts a fast charging method that combines constant current and constant voltage. First, charge with constant current to a predetermined voltage value, and then change to constant voltage to complete the remaining charging. Generally, the conversion voltage between the two stages is the first Two-stage constant voltage.
② The three-stage charging method uses constant current charging at the beginning and end of charging, and uses constant voltage charging in the middle. When the current decays to a predetermined value, the second stage is switched to the third stage. This method can minimize the amount of air outflow, but as a fast charging method, it is subject to certain restrictions.
The stage charging method uses constant current charging and constant voltage charging at the same time, and the advantages of these two charging methods can be combined.

2). Fast charging method
The fast charging method is a method that uses a higher charging current to make the battery fully charged in a short period of time. The power of fast oil chargers is very large, generally greater than 30kW, using three-phase four-wire 380V power supply, and its typical charging time is 10~30min. This charging method has a certain impact on battery life, especially ordinary batteries cannot be quickly charged, because receiving a large amount of electricity in a short time will cause the battery to overheat.
Commonly used fast charging methods include pulse fast charging method, Reflex TM fast charging method, variable current intermittent charging method, variable voltage intermittent charging method and amplitude modulation/frequency modulation charging method.

(1) Pulse fast charging method
The method is to first charge the battery with pulse current, then stop charging for a period of time, and then charge the battery with pulse current, and so on, as shown in Figure 3. The intermittent period allows time for the oxygen and hydrogen produced by the battery through chemical reactions to be reconstituted into water, reducing battery polarization, enabling the next round of constant current charging to proceed more smoothly, and increasing the storage energy of the battery. Due to the sufficient reaction time, the amount of outgassing is reduced, and the acceptance rate of the charging current of the battery is improved.

(2) Reflex TM fast charging method
This technology is a patented technology in the United States, and the first main charging object is the nickel-cadmium battery. This charging method alleviates the memory effect problem of the nickel pot battery and greatly reduces the time for fast charging of the battery. A cycle of Reflex TM fast charging includes three phases: forward pulse charging, reverse instantaneous pulse discharge and stop charging maintenance. Compared with pulse charging, a negative pulse is added. This charging method has also begun to be widely used in other types of batteries to increase the charging speed and reduce the polarization of the battery during the charging process.
Figure 4 shows the rapid charging process of Reflex TM. First, use 0.8~1 times the rated capacity current for constant current charging, so that the battery can be charged to 50%~60% of the rated capacity in a short time. When the single cell voltage rises to When 2.4V (for lead-acid batteries) and it starts to bubble, it is automatically controlled by the control circuit of the charger to start pulse fast charging, first stop charging for 25ms (called “pre-stop charging”), and then discharge or reverse charge to make The battery reversely passes a larger pulse current (the pulse depth is generally 1.5 to 2 times the charging current, and the pulse width is 150 to 1000 ms), and then stops charging for 40 ms (called “post stop charging”). The subsequent process: positive pulse charging→front stop charging→negative pulse instantaneous discharge→post stop charging→positive pulse charging…until it is fully charged.

(3) Variable current intermittent charging method
This charging method is based on constant current charging and pulse charging, as shown in Figure 5. Its characteristic is to change the constant current charging section to the voltage limiting variable current intermittent charging section. In the early stage of charging, the method of variable current intermittent charging is adopted to ensure that the charging state is increased. By stopping charging intermittently, the oxygen and hydrogen generated by the chemical reaction of the battery have time to recombine and be absorbed. The polarization of the battery is smaller, so that the next round of constant current charging can be carried out more smoothly, and the storage of the battery is improved. energy.

(4) Variable voltage intermittent charging method
The variable voltage intermittent charging method is formed on the basis of the variable current intermittent charging method, as shown in Figure 6. The difference between the variable voltage intermittent charging method and the variable current intermittent charging method is that the first stage is not an intermittent constant current, but an intermittent constant voltage.
Comparing the above various charging methods, Figure 6 is more in line with the charging curve of the best charging law. In each constant voltage charging stage, due to the constant voltage, the charging current naturally decreases exponentially, and the battery current acceptability rate gradually decreases with the charging process. specialty.

(5) AM/FM charging method
The AM/FM charging method combines the advantages of various charging methods, and the following three control methods are usually used.
① The amplitude of the pulse current is variable, and the frequency of the driving charge/discharge switch (PWM) signal is fixed.
② The amplitude of the pulse current remains unchanged, and the frequency of the PWM signal is adjustable.
③The amplitude of the pulse current remains unchanged, the frequency of the PWM signal is fixed, and the duty cycle of the PWM signal is adjustable.
Figure 7 shows the charging curve of the third charging method. While adjusting the duty cycle of the PWM signal, adding an intermittent charging stop phase can charge more power in a shorter period of time and improve the charging acceptance of the battery. ability.