Lithium battery management
1 Overview of lithium battery characteristics
Lithium ion batteries (lithium batteries) are rechargeable batteries that use lithium alloy metal oxides as anode materials, graphite as cathode materials, and non-aqueous electrolytes. Lithium-ion batteries have the following advantages: light weight, its energy density is twice that of nickel-cadmium batteries; self-discharge is 6-8 times less than nickel-cadmium batteries; there is no memory effect; the unit voltage is about 3.6V, which is a relatively high level. Usually can meet the needs of most applications. These characteristics allow lithium-ion batteries to be widely used in portable electronic products. The following figure shows several mobile phone lithium battery samples.
However, because of its characteristics, lithium batteries are particularly active, and they are easy to burn if they are not careful. The temperature rise and volume expansion caused by combustion will cause great damage to the surrounding environment and even the human body, and even lead to many disasters. Care must be taken during the production, storage and transportation of lithium batteries; in addition, there must be a lot of research on its protection and charging and discharging.
2 Lithium battery charging strategy
The charging process of the lithium battery can be regarded as the process of inflating (filling) the
balloon : at the beginning, the balloon can be inflated at a faster speed. As the balloon expands larger and larger, the inflation speed should be appropriately reduced. If you charge too fast, If the inflation speed of the balloon cannot keep up with the inflation speed, the balloon will explode with a "bang";
During the charging process, under the action of the external electric field applied by the charger, Li+ escapes from the positive electrode LiCoO2, enters the electrolyte and moves to the negative electrode, and sequentially enters the negative electrode composed of graphite, where LiC compounds are formed.
If the charging speed is too fast, it will make it too late for Li+ to enter the negative grid, and Li+ will accumulate in the electrolyte near the negative electrode. These Li+ near the negative electrode are likely to trap an electron from the negative electrode to become metallic Li. Continuous metal lithium production will accumulate near the negative electrode and grow into dendritic crystals, commonly known as dendrites. In another case, as the negative electrode becomes more and more full, the LiC lattice leaves fewer and fewer spaces, and the Li+ moving from the positive electrode has a smaller chance of finding spaces, and the time required is getting longer. The longer, if the charging speed does not change, 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 during 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, the higher the charging termination voltage, the more dangerous, the longer the charging time, the more dangerous.
Based on the understanding of the characteristics of lithium-ion batteries, the industry has formed a three-stage strategy for charging lithium-ion batteries: precharge, constant current charging, and constant voltage charging. The meaning of pre-charging is to adjust the state of the battery so that it can be charged with high current quickly; the role of constant current charging is to quickly store electrical energy in the battery; the constant voltage charging stage is the final adjustment stage , It maximizes the capacity of the battery, but its process is completely in accordance with the needs of the battery itself, unlike the constant current charging stage that has a strong external pressure (electric field force) on the battery. Any behavior that violates the characteristics of the battery itself, especially the operation that exceeds the battery’s ability to accept excessive current or exceeds the battery’s overcharge voltage, will have an irreversible impact on the life of the battery. Therefore, any comprehensive charging management plan must be strictly followed. Design.
For an over-discharged lithium battery, a pre-charging process is required in the early stage of charging. At this time, the charging voltage is less than a minimum value. At this time, the charging current must be small. Slowly make the battery enter a charging state that can accept high-current charging. . The pre-charging process also has the function of releasing the battery over-discharge protection. After successful normal use, try to prevent the battery from entering this state, because the battery often in this state will cause battery damage.
After the battery precharge exceeds a certain threshold, it will enter the fast charging stage. At this stage, the battery can be charged with constant current (CC, constant Current). The charging current at this time can generally be determined by the battery capacity, which is generally between 0.5C and 1C. The lower the internal resistance of the battery, the more high current can be used for charging.
When the voltage of the battery reaches the rated voltage of the battery, the charging process must be switched from the constant current (CC) mode to the constant voltage charging (CV, constant voltage) mode. In this mode, the charging current will gradually decrease; The charging voltage needs to be well controlled to prevent the danger of overcharging the battery; generally this value is between 4.1V and 4.4V.
When the charging current reaches the cut-off current, the charging will be cut off. At this time, the battery can be considered as fully charged; continuous charging of the fully charged battery is not allowed, which will shorten the battery life; only the battery voltage drops to a certain value Value, the charging process will be restarted;
3 Temperature characteristics of lithium battery
The above figure shows the effect of temperature on the capacity and voltage of lithium-ion batteries;
many switch-mode charging devices can adopt different charging strategies at different temperatures in accordance with the specifications established by JEITA. They generally divide the practical temperature range into 5 intervals ( Cold, cool, normal temperature, warm, hot), and implement different charging parameters in different intervals. In the cold and hot zone, charging is prohibited; in the cool and warm zone, it is recommended to reduce one or both of the charging current and the charging voltage; in the normal temperature zone, the charging voltage and the charging current are kept at Normal value. See the figure below for details:
4 Lithium battery charging timer
Normally designed lithium-ion battery charging management system can fully charge the battery within the normal charging time, but if it encounters some unexpected conditions, such as the battery leakage behavior that exceeds the specification limit (short circuit is also considered to be this type) or With a load participating in the system (this often happens when the charging circuit, battery, and system are connected in series), the conditions for the end of the charging process may never be met, and we will never see the prompt message that the charging is complete. In this situation, the charging safety timer is an indispensable configuration. It will limit the longest charging time. Once the time is up, no matter what the battery is in, the charging process will automatically end. Only restart the system to make the charging operation again. get on.
5 Path management
The so-called path management is not actually a problem of charging management, it is a problem of system design. For a battery-powered system, whether it is necessary to keep the load in working condition while charging, which has a huge impact on the design of the system. Consider the path management problem, that is, how to let the electric energy enter the battery, and how to supply power from the battery to the load, consider whether to let the external power supply directly supply power to the load, consider whether the external power supply is not enough to meet the load demand, charging demand or the combination of the two When necessary, the question of how to do it; the
path management is generally divided into two connection methods: series connection and parallel connection;
series connection is that the charging device, battery and load are connected one by one, so from the outside The current of the power supply must pass through the charging management device and the battery before it can enter the load; the series connection is very simple, as long as there is enough power in the battery, the load can work directly, and the cost is low, so it is widely used. Its only drawback is that when the battery power does not meet the needs of the load, the load cannot enter the working state; in actual project applications, when the battery voltage is too low, the system will not turn on even if there is an external power input;
The parallel connection mode can automatically distribute the input current between the load and the battery and give priority to meeting the demand of the load, while sending as much energy as possible into the battery; a further approach is when the input power cannot meet the demand of the load , Automatically add the battery to the ranks of power supply, and together with the external power supply to supply power to the load to ensure that the load can work normally. The specific application is shown in the figure below:
6 Linear charging and switching charging
In any case, only the voltage formed by the external power source is higher than the battery voltage, the current can enter the battery to achieve the charging goal, so the output voltage of almost all charging power adapters is higher than the highest battery voltage. But we usually cannot connect the external power supply directly to the battery for charging, which will result in uncontrollable current, which is why there are charging management devices.
Obviously, the MOSFET between VIN and BATT inside the IC constitutes the adjustment element between the external power supply and the battery (this is very similar to a linear regulator). It is also a very pleasing task while completing the adjustment task. An annoying heat source-the power consumption formed by the voltage difference between VIN and the battery voltage multiplied by the charging current flowing through it is converted into heat here. This is a waste and may be a disaster for some systems. It is especially serious when the battery voltage is low and the power supply voltage is high. In order to solve this problem, the simplest way is to reduce the input voltage (it is the best practice to make the input voltage slightly higher than the battery voltage) and reduce the charging current, but lowering the input voltage may cause the problem of battery dissatisfaction (this practice is often Unable to implement, maybe one day there will be an automatically adjusted power supply to meet this need), reducing the charging current will cause the problem of too long charging time.
Another way to solve efficiency and thermal problems is to abandon linear constant current and constant voltage sources and switch to switching conversion circuits;
7 Overvoltage, overcurrent protection and undervoltage protection
The entire charging link can be shown as follows. The power source terminal is generally the most peripheral interface part of the entire system. For the wired charging external cable is repeatedly plugged and unplugged here and the wireless charging enters and exits, the current is on and off here, although most The external power supply is all designed with 5V, but the actual voltage may be very different, especially during the plugging operation and the current change, the high voltage impact higher than the set voltage is unavoidable. Therefore, it is necessary to add protective measures such as overvoltage and overcurrent in the circuit after the power source. When any kind of accident occurs, it can cut off the connection between the external power supply and the internal system within a certain time to ensure the system's Safety;
In addition, battery overvoltage and overcurrent protection should be added to the battery input terminal to prevent unpredictable damage to the battery caused by external bad input;