Charging Principles and Fast Charging Technology for Lithium Batteries
1. Charging Principles of Lithium Batteries
The charging process of lithium-ion batteries can be divided into three stages: trickle charging (low-voltage pre-charge), constant current charging, and constant voltage charging.
The charging method for lithium batteries is constant current with limited voltage, and it is controlled by an IC chip.
The typical charging process is as follows:
First, the voltage of the battery to be charged is detected. If the voltage is lower than 3V, a pre-charge is initiated. The charging current is set to 1/10 of the specified current, and once the voltage reaches 3V, it enters the standard charging phase.
The standard charging phase involves:
Charging at a constant current set to reach 4.20V, after which it switches to constant voltage charging, maintaining the charging voltage at 4.20V.
During this phase, the charging current gradually decreases until it reaches 1/10 of the specified charging current, at which point the charging process concludes.
The following is the charging curve:
The following is the three stages of lithium battery charging:
(1) Stage 1: Trickle Charging
Trickle charging is used for the initial pre-charge (recovery charge) of completely discharged battery cells.
Trickle charging is applied when the battery voltage is around or below 3V. The trickle charging current is one-tenth of the constant current charge current, which is 0.1C. (For example, if the constant charge current is 1A, the trickle charging current is 100mA).
(2) Stage 2: Constant Current Charging
When the battery voltage rises above the trickle charging threshold, the charging current is increased for constant current charging.
The current for constant current charging is between 0.2C and 1.0C.
The battery voltage gradually increases during constant current charging, with a typical voltage range of 3.0-4.2V for a single cell.
(3) Stage 3: Constant Voltage Charging
When the battery voltage reaches 4.2V, constant current charging ends, and the constant voltage charging stage begins.
The charging current decreases gradually as the charging process continues, based on the saturation level of the battery. When the current decreases to 0.01C, the charging is considered terminated. (C is a representation of current in relation to the nominal capacity of the battery. For example, if the battery has a capacity of 1000mAh, 1C is a charging current of 1000mA.)
2. Lithium Battery Fast Charging Technology and Standards
The BC1.2 standard also defines how each port should enumerate to terminal equipment and protocols for identifying the type of application port.
The three USB BC1.2 port types are SDP, DCP, and CDP.
► The Three Types of BC1.2 Ports
● Standard Downstream Port (SDP)
This type of port has 15kΩ pull-down resistors on the D+ and D- lines. Current limits are as discussed above: 2.5mA when suspended, 100mA when connected, and 500mA when connected and configured for higher power.
● Dedicated Charging Port (DCP)
This type of port does not support any data transmission but can provide more than 1.5A of current. The D+ and D- lines in the port are shorted.
This type of port supports higher charging capabilities for wall chargers and car chargers.
● Charging Downstream Port (CDP)
This port supports both high current charging and full compatibility with USB 2.0 data transfer. The port has 15kΩ pull-down resistors required for D+ and D- communication and internal circuitry for charger detection stage switching.
The internal circuitry allows portable devices to distinguish CDP from other types of ports.
► Charging Process Analysis
From a physical calculation formula perspective, power (P) = voltage (U) * current (I).
Under a fixed battery capacity, power signifies the charging speed.
We can expedite the charging time through the following three methods.
(1) High Voltage Constant Current Mode
In the general charging process for mobile phones, the voltage is first reduced from 220V to a 5V charger voltage,
and then the 5V charger voltage is further reduced to the 4.2V battery voltage.
Throughout the charging process, an increase in voltage generates heat, so both the charger and the phone heat up during charging.
Additionally, higher power consumption not only results in increased heat generation but also causes more significant battery damage.
(2) Low Voltage High Current Mode
Under certain voltage conditions, increasing the current can be achieved by using parallel circuits for shunting.
With a constant voltage, this shunting process divides the load among multiple circuits, reducing the stress on each circuit. When implemented in mobile phones, this approach reduces the load on each circuit.
(3) High Voltage High Current Mode
This method simultaneously increases both the current and voltage.
Following the formula P = UI, we can see that this approach is the most effective way to increase power.
However, raising the voltage also generates more heat, resulting in higher energy consumption. Moreover, voltage and current cannot be increased without limit.