Can lithium batteries be fast charged safely?

A lithium battery can be charged as fast as 1C, whereas a lead acid battery should be kept below 0.3C. This means a 10AH lithium battery can typically be charged at 10A while a 10AH lead acid battery can be charged at 3A.

How fast can it be called fast charging? There is no standard literature that gives specific values. For the time being, we will refer to the numerical thresholds mentioned in the most well-known subsidy policies. As can be seen from the 2017 subsidy standards for new energy buses, the entry level for fast charging is 3C. In fact, in the subsidy standards for passenger cars, there is no mention of fast charging requirements.

Current-Rate——"A C rate is a measure of the rate at which a battery is discharged relative to its maximum capacity. A 1C rate means that the discharge current will discharge the entire battery in 1 hour. "

From the promotional materials of general passenger cars, it can be seen that everyone generally believes that a 30-minute full charge of 80% can already be used as a gimmick for fast charging, so let's just think that the 1.6C of passenger cars can be the entry-level fast charging reference value. According to this idea, 15 minutes of publicity is filled with 80%, which is equivalent to 3.2C.

Common fast charging methods

There are many charging methods for lithium batteries. For the requirements of fast charging, the main methods include pulse charging, Reflex charging, and smart charging. The applicable charging methods of different battery types are not exactly the same, and there is no specific distinction in the method section.

1. PULSE CHARGING
This is a pulse charging method from the literature. The pulse stage is set after the charging reaches the upper limit voltage of 4.2V, and it continues above 4. 2V. Not to mention the rationality of its specific parameter settings for the time being, there are differences between different types of batteries. Let's pay attention to the pulse implementation process.

It mainly includes three stages: precharge, constant current charging and pulse charging. During the constant current charging process, the battery is charged with a constant current, and part of the energy is transferred to the inside of the battery. When the battery voltage rises to the upper limit voltage (4.2V), enter the pulse charging mode: use a pulse current of 1 C to intermittently charge the battery. Within a constant charging time Tc, the battery voltage will continue to increase, and the voltage will slowly decrease when the charging stops. When the battery voltage drops to the upper limit voltage (4. 2V), charge the battery with the same current value, start the next charging cycle, and so on until the battery is fully charged.

During the pulse charging process, the battery voltage drop speed will gradually slow down, and the stop time T0 will become longer. When the constant current charging duty cycle is as low as 5% to 10%, the battery is considered to be fully charged and the charging is terminated. Compared with conventional charging methods, pulse charging can charge at a larger current, and the concentration polarization and ohmic polarization of the battery will be eliminated during the stop-charging period, making the next round of charging more smoothly, fast charging speed, small temperature changes, and small impact on battery life, so it is currently widely used. But its disadvantages are obvious: a power supply with limited current function is required, which increases the cost of pulse charging.

2. INTERMITTENT CHARGING METHOD
The intermittent charging method of lithium batteries includes the intermittent charging method of variable current and the intermittent charging method of variable voltage.

1) Variable current intermittent charging method

It is characterized by changing constant current charging to voltage-limiting variable current intermittent charging. In the first stage of the variable current intermittent charging method, a larger current value is first used to charge the battery, and the charging is stopped when the battery voltage reaches the cut-off voltage V0. At this time, the battery voltage drops sharply. After maintaining the stop-charging time for a period of time, the reduced charging current is used to continue charging. When the battery voltage rises to the cut-off voltage V0 again, charging is stopped, and the charging current will be reduced by the set cut-off current value after several times (generally about 3 to 4 times). Then enter the constant voltage charging stage, charge the battery at a constant voltage until the charging current is reduced to the lower limit, and the charging ends.

In the main charging stage of the variable current intermittent charging method, under the condition of limiting the charging voltage, the intermittent method of gradually decreasing the current is used to increase the charging current, which speeds up the charging process and shortens the charging time. However, this charging mode circuit is more complex and expensive, and it is generally only considered for high-power fast charging.

2) Variable voltage intermittent charging

On the basis of the variable current intermittent charging method, someone has studied the variable voltage intermittent charging method. The difference between the two lies in the first stage of the charging process, which replaces intermittent constant current with intermittent constant voltage.

3. REFLEX FAST CHARGING METHOD
Reflex fast charging method, also known as reflex charging method or "hiccup" charging method. Each working cycle of this method includes three stages: forward charging, reverse instantaneous discharge, and stop charging. It solves the phenomenon of battery polarization to a large extent and speeds up the charging speed. However, reverse discharge will shorten the life of lithium batteries.

4. SMART CHARGING METHOD

Intelligent charging is currently a more advanced charging method. The main principle is to apply du/dt and di/dt control technologies to determine the state of charge of the battery by checking the increment of the battery voltage and current, and dynamically track the acceptable charging current of the battery, so that the charging current is always near the maximum acceptable charging curve of the battery. Such intelligent methods generally combine advanced algorithm technologies such as neural networks and fuzzy control to realize automatic system optimization.