Principle of 3.7V lithium battery protection board-analysis of primary and voltage standards of lithium battery

Wide range of uses of batteries

The purpose of developing high technology is to make it better serve humanity. Since its introduction in 1990, lithium-ion batteries have increased due to their excellent performance and have been widely used in society. Lithium-ion batteries quickly occupied many fields with incomparable advantages over other batteries, such as well-known mobile phones, notebook computers, small video cameras, etc. More and more countries use this battery for military purposes. The application shows that the lithium-ion battery is an ideal small green power source.

Second, the main components of lithium-ion batteries

(1) Battery cover

(2) Positive electrode-active material is lithium cobalt oxide

(3) Diaphragm-a special composite membrane

(4) Negative electrode-the active material is carbon

(5) Organic electrolyte

(6) Battery case

Third, the superior performance of lithium-ion batteries

(1) High working voltage

(2) Larger specific energy

(3) Long cycle life

(4) Low self-discharge rate

(5) No memory effect

(6) No pollution

Four, lithium battery type and capacity selection

First, calculate the continuous current that the battery needs to provide based on the power of your motor (requires actual power, and generally, the riding speed corresponds to a corresponding real power). For example, suppose the engine has a continuous current of 20a (1000w motor at 48v). In that case, the battery needs to provide a 20a current for a long time. The temperature rise is shallow (even if the temperature is 35 degrees outside in summer, the battery temperature is best controlled below 50 degrees). In addition, if the current is 20a at 48v, the overpressure doubles (96v, such as CPU 3), and the continuous current will reach about 50a. If you like to use over-voltage for a long time, please choose a battery that can continuously provide 50a current (still pay attention to the temperature rise). The continuous current of the storm here is not the nominal battery discharge capacity of the merchant. The merchant claims that a few C (or hundreds of amperes) is the battery discharge capacity, and if it is discharged at this current, the battery will generate severe heat. If the heat is not adequately dissipated, the battery life will be concise. (And the battery environment of our electric vehicles is that the batteries are piled up and discharged. Basically, no gaps are left, and the packaging is very tight, let alone how to force air cooling to dissipate heat). Our use environment is very harsh. The battery discharge current needs to be derated for use. Evaluating the battery discharge current capability is to see how much the battery's corresponding temperature rise is at this current.

The only principle discussed here is the temperature rise of the battery during use (high temperature is the deadly enemy of lithium battery life). It is best to control the battery temperature below 50 degrees. (Between 20-30 degrees is best). This also means that if it is a capacity type lithium battery (discharged below 0.5C), a continuous discharge current of 20a requires a capacity of more than 40ah (of course, the most crucial thing depends on the battery's internal resistance). If it is a power-type lithium battery, it is customary to discharge continuously according to 1C. Even the A123 ultra-low internal resistance power type lithium battery is usually best to remove at 1C (no more than 2C is better, 2C discharge can only be used for half an hour, and it is not very useful). The choice of capacity depends on the size of the car storage space, personal expenditure budget, and the expected range of car activities. (Small ability generally requires power type lithium battery)

5. Screening and assembly of batteries

The big taboo of using lithium batteries in series is the severe imbalance of battery self-discharge. As long as everyone is equally unbalanced, it's okay. The problem is that this state is abruptly unstable. A good battery has a small self-discharge, a bad storm has a large self-discharge, and a condition where the self-discharge is not small or not is generally changed from good to bad. State, this process is unstable. Therefore, it is necessary to screen out the batteries with large self-discharge and leave only the battery with small self-discharge (generally, the self-discharge of qualified products is small, and the manufacturer has measured it, and the problem is that many unqualified products flow into the market).

Based on small self-discharge, select series with similar capacity. Even if the power is not identical, it will not affect the battery life, but it will affect the functional ability of the entire battery pack. For example, 15 batteries have a capacity of 20ah, and only one battery is 18ah, so the total capacity of this group of batteries can only be 18ah. At the end of the use, the battery will be dead, and the protection board will be protected. The voltage of the entire battery is still relatively high (because the voltage of the other 15 batteries is standard, and there is still electricity). Therefore, the discharge protection voltage of the entire battery pack can tell whether the capacity of the whole battery pack is the same (provided that each battery cell must be fully charged when the entire battery pack is fully charged). In short, the unbalanced capacity does not affect the battery life but only affects the entire group's ability, so try to choose an assembly with a similar degree.

The assembled battery must achieve good ohmic contact resistance between the electrodes. The smaller the contact resistance between the wire and the electrode, the better; otherwise, the electrode with a significant contact resistance will heat up. This heat will be transferred to the inside of the battery along the electrode and affect the battery life. Of course, the manifestation of the considerable assembly resistance is the significant voltage drop of the battery pack under the same discharge current. (Part of the voltage drop is the internal resistance of the cell, and part is the assembled contact resistance and wire resistance)

Six, protection board selection and charging and discharging use matters

(The data is for the lithium iron phosphate battery, the principle of the ordinary 3.7v battery is the same, but the information is different)

The purpose of the protection board is to protect the battery from overcharging and over-discharging, preventing high current from damaging the storm and balancing the battery voltage when the battery is fully charged (the balancing ability is generally relatively small, so if there is a self-discharged battery protection board, it is exceptionally It is challenging to balance, and there are also protection boards that balance in any state, that is, compensation is performed from the beginning of charging, which seems to be very rare).

For the life of the battery pack, it is recommended that the battery charging voltage not exceed 3.6v at any time, which means that the protective action voltage of the protection board is not higher than 3.6v, and the balanced voltage is recommended to be 3.4v-3.5v (each cell 3.4v has been charged more than 99 % Battery, refers to the static state, the voltage will increase when charging with high current). The battery discharge protection voltage is generally above 2.5v (above 2v is not a big problem, generally there is little chance to use it entirely out of power, so this requirement is not high).

The recommended maximum voltage of the charger (the last step of charging can be the highest constant voltage charging mode) is 3.5*, the number of strings, such as about 56v for 16 rows. Usually, charging can be cut off at an average of 3.4v per cell (basically fully charged) to guarantee the battery life. Still, because the protection board has not yet begun to balance if the battery core has a large self-discharge, it will behave as a whole group over time; the capacity gradually decreases. Therefore, it is necessary to regularly charge each battery to 3.5v-3.6v (such as every week) and keep it for a few hours (as long as the average is greater than the equalization starting voltage), the greater the self-discharge, the longer the equalization will take. The self-discharge Oversized batteries are difficult to balance and need to be eliminated. So when choosing a protection board, try to choose 3.6v overvoltage protection and start the equalization around 3.5v. (Most of the overvoltage protection on the market is above 3.8v, and the equilibrium is formed above 3.6v). Choosing a suitable balanced starting voltage is more important than the protection voltage because the maximum voltage can be adjusted by adjusting the maximum voltage limit of the charger (that is, the protection board usually has no chance to do high-voltage protection). Still, suppose the balanced voltage is high. In that case, the battery pack has no chance to balance (unless The charging voltage is greater than the equilibrium voltage, but this affects the battery life), the cell will gradually decrease due to the self-discharge capacity (the ideal cell with a self-discharge of 0 does not exist).

The continuous discharge current capability of the protection board. This is the worst thing to comment on. Because the current limiting ability of the protection board is meaningless. For example, if you let a 75nf75 tube continue to pass 50a current (at this time, the heating power is about 30w, at least two 60w in series with the same port board), as long as there is a heat sink enough to dissipate heat, there is no problem. It can be kept at 50a or even higher without burning the tube. But you can't say that this protection board can last 50a current because most of everyone's protective panels are placed in the battery box very close to the battery or even close. Therefore, such a high temperature will heat the battery and heat up. The problem is that high temperature is the deadly enemy of the storm.

Therefore, the use environment of the protection board determines how to choose the current limit (not the current capacity of the protection board itself). Suppose the protection board is taken out of the battery box. In that case, almost any protection board with a heat sink can handle a continuous current of 50a or even higher (at this time, only the protection board capacity is considered, and there is no need to worry about the temperature rise causing damage to the battery cell). Next, the author talks about the environment that everyone usually uses, in the same confined space as the battery. At this time, the maximum heating power of the protection board is best controlled below 10w (if it is a small protection board, it needs 5w or less, and a large-volume protection board can be more than 10w because it has good heat dissipation and the temperature will not be too high). As for how much is appropriate, it is recommended to continue. The maximum temperature of the entire board does not exceed 60 degrees when current is applied (50 degrees is best). Theoretically, the lower the temperature of the protection board, the better, and the less it will affect the cells.

Because the same port board is connected in series with the charging electric mos, the heat generation of the same situation is double that of the different port board. For the same heat generation, only the number of tubes is four times higher (under the premise of the same model of mos). Let's calculate, if 50a continuous current, then the mos internal resistance is two milliohms (5 75nf75 tubes are needed to get this equivalent internal resistance), and the heating power is 50*50*0.002=5w. At this time, it is possible (in fact, the mos current capacity of 2 milliohms internal resistance is more than 100a, it is no problem, but the heat is large). If it is the same port board, 4 2 milliohm internal resistance mos are needed (each two parallel internal resistance is one milliohm, and then connected in series, the total internal resistance is equal to 2 million 75 tubes are used, the total number is 20). Suppose the 100a continuous current allows the heating power to be 10w. In that case, a line with an internal resistance of 1 milliohm is required (of course, the exact equivalent internal resistance can be obtained by MOS parallel connection). If the number of different ports is still four times, if the 100a continuous current still allows the maximum 5w Heating power, then only 0.5 milliohm tube can be used, which requires four times the amount of mos compared to 50a continuous current to generate the same amount of heat). Therefore, when using the protection board, choose a board with negligible internal resistance to reduce the temperature. If the internal resistance has been determined, please let the board and the outside heat dissipate better. Choose the protection board and don't listen to the seller's continuous current capacity. Just ask the total internal resistance of the discharge circuit of the protection board and calculate it by yourself (ask what type of tube is used, how much quantity is used, and check the internal resistance calculation by yourself). The author feels that if it is discharged under the seller's nominal continuous current, the temperature rise of the protection board should be relatively high. Therefore, it is best to select a protection board with derating. (Say 50a continuous, you can use 30a, you need 50a constant, it is best to buy 80a nominal continuous). For users who use a 48v CPU, it is recommended that the total internal resistance of the protection board is not more than two milliohms.

The difference between the same port board and the different port board: the same port board is the same line for charging and discharging, and both charging and discharging are protected.

The different port board is independent of the charging and discharging lines. The charging port only protects from overcharging when charging and does not protect if it is removed from the charging port (but it can discharge completely, but the current capacity of the charging port is generally relatively small). The discharge port protects against over-discharge during discharge. If charging from the discharge port, over-charge is not covered (so the reverse charging of the CPU is entirely usable for the different port board. And the reverse charge is more minor than the energy used, so Don't worry about overcharging the battery due to reverse charging. Unless you go out with full payment, it's a few kilometers downhill immediately. If you keep starting eabs reverse charging, it's possible to overcharge the battery, which doesn't exist), but regular use of charging Never charge from the discharge port, unless you constantly monitor the charging voltage (such as temporary roadside emergency high-current charging, you can trust from the discharge port, and continue to ride without being fully charged, don't worry about overcharging)

Calculate the maximum continuous current of your motor, select a battery with a suitable capacity or power that can meet this constant current, and the temperature rise is controlled. The internal resistance of the protection board is as small as possible. The over-current protection of the protection board only needs short-circuits protection and other abnormal use protection (don't try to limit the current required by the controller or motor by limiting the draft of the protection board). Because if your engine needs 50a current, you do not use the protection board to determine the current 40a, which will cause frequent protection. The sudden power failure of the controller will easily damage the controller.

Seven, voltage standard analysis of lithium-ion batteries

(1) Open circuit voltage: refers to the voltage of a lithium-ion battery in a non-working state. At this time, there is no current flowing. When the battery is fully charged, the potential difference between the positive and negative electrodes of the battery is usually around 3.7V, and the high can reach 3.8V;

(2) Corresponding to the open-circuit voltage is the working voltage, that is, the voltage of the lithium-ion battery in the active state. At this time, there is current flowing. Because the internal resistance when the current flows is to be overcome, the operating voltage is always lower than the total voltage at the time of electricity;

(3) Termination voltage: that is, the battery should not continue to be discharged after being placed at a specific voltage value, which is determined by the structure of the lithium-ion battery, usually due to the protective plate, the battery voltage when the discharge is terminated is about 2.95V;

(4) Standard voltage: In principle, the standard voltage is also called the rated voltage, which refers to the expected value of the potential difference caused by the chemical reaction of the positive and negative materials of the battery. The rated voltage of the lithium-ion battery is 3.7V. It can be seen that the standard voltage is Standard working voltage;

Judging from the voltage of the four lithium-ion batteries mentioned above, the voltage of the lithium-ion battery involved in the working state has standard voltage and working voltage. In the non-working condition, the voltage of the lithium-ion battery is between the open-circuit voltage and the end voltage because of the lithium-ion battery. The chemical reaction of the ion battery can be used repeatedly. Therefore, when the voltage of the lithium-ion battery is at the termination voltage, the battery must be charged. If the battery is not charged for a long time, the battery's life will be reduced or even scrapped.