Why are There Three Charging Stages in Lithium Battery?
Lithium-ion batteries are the most common choice for cash portable electronic products. Compared with other types of batteries, lithium-ion batteries are light in weight and have no memory effect. Compared with nickel-metal hydride batteries, lithium-ion batteries have twice the energy density and 6-8 times lower self-discharge rate.
When using lithium-ion batteries for application design, the most important thing is to understand its characteristics in the charging and discharging process to ensure the safety of the application and ensure the optimization of the use time.
At present, the industry has formed a three-stage strategy for charging lithium-ion batteries: precharge, constant current charging, and constant voltage charging.
Why are there 3 stages?
The figure below shows the relationship between the capacity, cycle life and charging voltage of lithium-ion batteries. The vertical axis is the battery capacity and the horizontal axis is the number of cycle lives. It can be seen that the higher the charging cut-off voltage, the shorter the cycle life and the faster the capacity drop.
The figure below shows the relationship between the capacity, cycle life and discharge current of lithium-ion batteries. The vertical axis is the battery capacity and the horizontal axis is the number of cycle lives. It can be seen that the larger the charging rate, the faster the capacity attenuation speed.
Chemical properties of lithium-ion batteries
During the charging process, under the action of the external electric field applied by the charger, Li+ protrudes from the positive electrode LiCoO2 into the electrolyte and moves to the negative electrode, and then enters the negative electrode composed of graphite, where the LiC compound is formed. If the charging speed is too fast, it will make it too late for Li+ to enter the negative electrode grid, and Li+ will gather in the electrolyte near the negative electrode. These Li+ close to the negative electrode are likely to capture an electron from the negative electrode and become a metal Li. Continuous lithium metal generation will accumulate near the negative electrode and grow into dendritic crystals, commonly known as dendrites.
In another case, as the negative electrode is filled to a higher and higher degree, there are fewer and fewer spaces left in the LiC lattice, and the chance of Li+ moving from the positive electrode to find the space is getting smaller and smaller, and the time required is getting longer and longer. If the charging speed remains the same, it is also possible to form a local accumulation of Li+ on the surface of the negative electrode.
Therefore, the charging current must be gradually reduced in the second half of charging. The growth of dendrites will eventually pierce the diaphragm between the positive and negative stages, forming a short circuit.
It is conceivable that the faster the charging speed, the more dangerous it is, the higher the voltage at which the charging terminates, the more dangerous it is, and the longer the charging time, the more dangerous it is. If you can’t imagine what’s going on inside the battery, please think of this battery as a soap bubble. The process of blowing gas into the soap bubble is equivalent to the process of charging the battery. If you blow too fast, the expansion speed of the water film formed by the soapy water can’t keep up with the speed at which the gas is blown in, the soap bubble can easily break.
Precharge occurs when the battery voltage is relatively low. For most lithium-ion batteries, this voltage is usually defined as below 2.9V~3V, and the charging current at this time is generally allowed to be below C/10.
Most of the constant current charging current is set at about 1C (determined by the attenuation of the capacity to 80% of the initial capacity after 500 use cycles).
In the constant voltage charging stage, the current will gradually decrease. After it drops to a certain extent (usually C/10), we can think that the battery is fully charged and the charging process will be cut off.
The last stage is called the supplementary stage, which is actually a combination of the constant current stage and the constant voltage stage. It is a compensation measure to compensate for the decrease in capacitance caused by the self-discharge of the battery and the consumption of other loads connected to it. This is done to ensure that the battery (and the system composed of it) is always in the state of being as fully charged as possible when it is separated from the charging equipment.
In addition, the temperature of the battery will have a significant impact on the charging strategy. Due to the different characteristics of the materials that make up the battery at different temperatures, the capacity and suitable charging voltage of the battery have also undergone huge changes.