Basic knowledge of lithium battery and principle of protective plate
1. Wide use of batteries
The purpose of developing high technology is to make it better serve mankind. Since its appearance in 1990, lithium-ion battery has developed rapidly because of its excellent performance, and is widely used in society. Lithium ion batteries have quickly occupied many fields with incomparable advantages over other batteries, such as well-known mobile phones, laptops, small cameras and so on, and more and more countries use the battery for military purposes. The application shows that lithium ion battery is an ideal small green power supply.
2. Main components of lithium ion battery
Positive electrode -- the active material is cobalt lithium oxide
Diaphragm ---- a special composite membrane
Negative electrode - the active material is carbon
3. Superior performance of lithium ion battery
High operating voltage
Greater specific energy
Long cycle life
Low self discharge rate
No memory effect
4. lithium battery type and capacity selection
First, calculate the continuous current that the battery needs to provide according to its own motor power (the actual power is required, and the general riding speed will correspond to a corresponding actual power). For example, if the motor has a continuous current of 20a (1000W motor at 48V), the battery can provide 20A current for a long time and the temperature rise is very low (even if the temperature outside is 35 degrees in summer, the battery temperature should be controlled below 50 degrees). In addition, if the current is 20A at 48V, the current will continue to reach about 50A after the overpressure is doubled (96V, such as ecpu3). If you prefer to use it under overpressure for a long time, please choose a battery that can continuously provide 50A current (or pay attention to temperature rise). The battery continuous current here is not the merchant's nominal battery discharge capacity. The merchant's nominal several C (or tens or hundreds of Amperes) is the battery discharge capacity. If the battery is discharged under this current, the battery heating is very serious. If the heat dissipation is not done well, the battery life will be very short. (the battery environment of our electric vehicles is that the batteries are piled up and discharged, basically leaving no gap. The package is very tight, not to mention how to do a good job of forced air cooling and heat dissipation). Our use environment is very bad. The battery discharge current needs to be derated. The evaluation of battery discharge current capacity is to see the corresponding temperature rise of the battery under this current.
The only principle discussed here is actually the temperature rise of the battery during use (high temperature is the life enemy of lithium battery). It is best to control the battery temperature below 50 degrees. (preferably between 20-30 degrees). This also means that if it is a capacity lithium battery (controlled to discharge below 0.5c), it needs more than 40ah to provide a continuous discharge current of 20a (of course, it mainly depends on the internal resistance of the battery). If it is a power lithium battery, it is normal to discharge continuously according to 1C. Even for A123 ultra-low internal resistance power lithium battery, it is usually best to discharge at 1C (it is better not to exceed 2C. In fact, 2C discharge can only be discharged for half an hour, so it is not of great use value). The choice of capacity depends on the storage space of the car, personal expenditure budget, expected range of activities of the car and other factors. (power lithium battery is generally required for small capacity)】
5. Screening and assembly of batteries
The big taboo of using lithium battery in series is that the battery self discharge is seriously unbalanced. As long as everyone is the same, it doesn't matter. The problem is that this state is an urgent and unstable state. The self discharge of a good battery is very small, and the self discharge of a bad battery is very large. The self discharge is not small, and the state is generally from good to bad. This process is unstable. Therefore, it is necessary to screen out the batteries with large self discharge and leave only the battery matching group with small self discharge (generally, the self discharge of qualified products is small, which has been measured by the manufacturer, and the problem is that many unqualified products flow into the market).
On the basis of small self discharge, select series with similar capacity. Even if the capacity is not close, it will not affect the battery life, but it will affect the available capacity of the whole battery. For example, if 15 capacities are 20Ah and only one is 18ah, the total capacity of this battery can only be 18ah. At the end of use, the battery runs out of power, the protection board needs to be protected, and the voltage of the whole group of batteries is still relatively high (because the other 15 batteries have normal voltage and electricity). Therefore, the discharge protection voltage of the whole battery can see whether the capacity of the whole battery is consistent (provided that each cell must be fully charged when the whole battery is fully charged). In short, the unbalanced capacity does not affect the battery life, but the whole group capacity, so try to select the assembly with similar capacity.
When assembling the battery, good ohmic contact resistance between electrodes must be achieved. The smaller the contact resistance between the wire and the electrode, the better. Otherwise, the larger the contact resistance, the electrode will generate heat, which will be transmitted to the interior of the cell along the electrode, affecting the battery life. Of course, the performance of large assembly resistance is that the voltage drop of the battery pack is large under the same discharge current. (the voltage drop is partly the internal resistance of the cell and partly the assembled contact resistance and wire resistance)
6. Selection of battery protection board battery management system(BMS) and matters for charging and discharging
(the data is for lithium iron phosphate battery. The principle of ordinary 3.7V battery is the same, but the data are different)
The purpose of the protection board is to protect the battery from overcharge and overdischarge, prevent large current from damaging the battery, and balance the battery voltage when it is fully charged (the equalization capacity is generally small, so it is difficult to balance if there is a cell protection board with large self discharge, and there are protection boards that are balanced in any state, that is, balance from the beginning of charging, as if there are few).
For the service life of the battery pack, it is recommended that the battery charging voltage should not exceed 3.6V at any time, which means that the protection action voltage of the protection plate is not higher than 3.6V, and the equalization voltage is recommended to be 3.4v-3.5v (3.4V of each cell has been charged with more than 99%, which means that in the static state, the voltage will rise during high current charging). The battery discharge protection voltage is generally more than 2.5V (more than 2V is not a big problem. Generally, it is rarely used. There is no power at all, so this requirement is not high).
The maximum voltage recommended by the charger (the last step of charging can be the maximum constant voltage charging mode) is 3.5 * the number of strings, for example, about 56V for 16 strings. At ordinary times, the charging can be cut off at an average of 3.4V per section (basically full), so that the battery life is guaranteed. However, because the protection board has not started to balance, if the battery cell has large self discharge, the capacity of the whole group will gradually decrease over time. Therefore, it is also necessary to charge each battery to 3.5v-3.6v on a regular basis (such as every week) and keep it for several hours (as long as the average is greater than the equilibrium starting voltage). The greater the self discharge, the longer the equalization takes. The cells with excessive self discharge are difficult to balance and need to be removed. Therefore, when selecting the protection board, try to select the 3.6V overvoltage protection and start the equalization at about 3.5V. (in the market, most of them are overvoltage protection above 3.8V and startup equalization above 3.6V). In fact, selecting the appropriate balanced starting voltage is more important than the protection voltage, because the maximum voltage can be limited by adjusting the maximum voltage of the charger (that is, the protection board does not have the opportunity to do high-voltage protection at ordinary times), but if the balanced voltage is high, the battery pack will not have the opportunity to balance (unless the charging voltage is greater than the balanced voltage, but this will affect the battery life), and the self discharge capacity of the battery will slowly decrease (an ideal cell with self discharge of 0 does not exist).
Continuous discharge current capability of the protection board. This is the worst thing to comment on. Because it is meaningless to talk about the current limiting capacity of the protection plate alone. For example, for a 75nf75 tube, you let it continue to pass 50A current (at this time, the heating power is about 30W, and at least two plates at the same port are connected in series for 60W). As long as there are heat sinks 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 protective plate can last 50A. Because most of our protective plates are placed in the battery box, which is very close to or even close to the battery. So such a high temperature will heat up the heat conducting battery. The problem is that high temperature is the sworn enemy of the battery.
Therefore, the use environment of the protection board determines how to select the current limit (rather than the current capacity of the protection board itself). If the protective plate is taken out of the battery box, almost any protective plate with heat sink can handle 50A continuous current or even higher (at this time, only the ability of the protective plate is considered, and there is no need to worry about the damage caused by temperature rise to the battery cell). Next, the author talks about the common environment, which is in the same confined space as the battery. At this time, the maximum heating power of the protection board should be controlled below 10W (if it is a small protection board, it needs less than 5W, and the 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 that the maximum temperature of the whole board should not exceed 60 degrees when the current is continuous (it is best below 50 degrees). In theory, the lower the temperature of the protective plate, the better, and the less impact on the cell.
Because the charging MOS of the same port plate is connected in series, the heating of the same port plate is twice that of the different port plate. For the same heating, only the number of tubes is 4 times higher (on the premise of the same model MOS). Let's calculate that if the current lasts for 50a, the internal resistance of MOS is 2 milliohm (five 75nf75 tubes are needed to obtain the equivalent internal resistance), and the heating power is 50 * 50 * 0.002 = 5W. At this time, it is OK (in fact, the MOS current capacity of 2 milliohm internal resistance is above 100A, but the heating is large). If it is the same port board, four 2 milliohm internal resistance MOS are required (each 2 parallel internal resistance is 1 milliohm, and then connected in series, the total internal resistance is equal to 2 milliohm. If 75 tubes are used, the total number is 20). If the allowable heating power of 100A continuous current is 10W, a tube with an internal resistance of 1 milliohm is required (of course, the same equivalent internal resistance can be obtained in parallel through MOS). If the number of dissimilar plates is still 4 times, if the maximum allowable heating power of 100A continuous current is still 5W, only 0.5 milliohm tubes can be used, which requires 4 times the number of MOS relative to 50A continuous current to generate the same heat). Therefore, when using the protection board, in order to reduce the temperature, select the board with small internal resistance. If the internal resistance has been determined, please try to make the board and the outside heat dissipation better. Select the protection board. Don't listen to the continuous current capacity fooled by the seller. Just ask the total internal resistance of the discharge circuit of the protection board, and you can calculate it yourself (ask what type of pipe is used and how many quantities are used, and check the internal resistance calculation yourself). I feel that if it is really discharged under the seller's nominal continuous current, the temperature rise of the protection plate should be relatively high. Therefore, it is best to reduce the amount and select the protective plate. (for 50A duration, you need 30A. If 50A duration is needed, you'd better buy 80A nominal duration). For users using 48V ecpu, it is recommended that the total internal resistance of the protection board should not be greater than 2 milliohm.
Difference between common port battery protection board and seperated port battery protection board: the commono port protection board is the same port for charging and discharging, and both charging and discharging are protected.
The seperated port protection board is independent of the charging port and discharge port. The charging port only protects overcharge when charging. If it is discharged from the charging port, it does not protect (but it can discharge completely, but the current capacity of the charging port is generally small). The discharge port is used to protect over discharge during discharge. If charging from the discharge port, it does not protect over charge (therefore, the reverse charging of ecpu can be fully used for the different port board. And the reverse charging energy is absolutely less than that used, so there is no need to worry about overcharging the battery due to the reverse charging. Unless it is a few kilometers downhill immediately after it is fully charged, it is possible to overcharge the battery by starting EABS reverse charging all the time. This basically does not exist.) However, it is absolutely not allowed to charge from the discharge port for normal use unless you have been monitoring the charging voltage (for example, for temporary roadside emergency high current charging, you can charge from the discharge port and continue to ride without charging, so you don't have to worry about overcharging)
Calculate the maximum continuous current of your motor, select a battery with appropriate capacity or power to meet this continuous current, and the temperature rise can be controlled. The smaller the internal resistance of the protection plate, the better. In fact, the over-current protection of the protection plate only needs short-circuit protection and other abnormal use protection (never limit the current required by the controller or motor through the current limit of the protection plate) If your motor needs 50A current and you do not use the protection board to limit the current for 40a, it will cause frequent protection, and the controller will be easily damaged if it is suddenly powered off.
7. Voltage standard analysis of lithium ion battery
Open circuit voltage: refers to the voltage of lithium-ion battery in non working state. At this time, no current flows. When fully charged, the potential difference between the positive and negative electrodes of the battery is usually about 3.7V, and the high can reach 3.8V;
Corresponding to the open circuit voltage is the working voltage, that is, the voltage of the lithium-ion battery in the working state. At this time, there is a current flowing. Because the internal resistance when the current flows is to be overcome, the working voltage is always lower than the voltage when it is fully charged;
Termination voltage: that is, the battery should not continue to discharge after being placed at a certain voltage value, which is determined by the structure of lithium-ion battery. Generally, due to the action of protective plate, the voltage of battery at the end of discharge is about 2.95v;
Standard voltage: in principle, the standard voltage is also called rated voltage, which refers to the standard value of the potential difference caused by the chemical reaction between the positive and negative materials of the battery. The rated voltage of lithium-ion battery is 3.7V. It can be seen that the standard voltage is the standard working voltage;
From the voltage of the above four lithium-ion batteries, the voltage of the lithium-ion battery involved in the working state includes standard voltage and working voltage. In the non working state, the voltage of the lithium-ion battery is in the open circuit voltage and termination voltage, because the chemical reaction of the lithium-ion battery can be reused. Therefore, when the voltage of the lithium-ion battery is in the termination voltage, The battery needs to be charged. If it is not charged for a long time, the service life of the battery will be reduced or even scrapped. Therefore, the lithium-ion battery must be used together with the battery protection board and the battery management system.