Several parameters that need to be determined when choosing a lithium battery protection board
The lithium battery protection board is used when the lithium battery is used, and is used to prevent the battery from overcharging and short-circuiting and other fault protection circuit boards. Since lithium batteries are sensitive to charge and discharge voltage, and faults such as battery short-circuits may bring certain dangers, lithium batteries need to be equipped with a suitable protection board when in use.
Lithium batteries are also different according to different voltage levels. Commonly used lithium batteries, such as lithium iron phosphate batteries with a rated voltage of 3.2V, polymer batteries with a rated voltage of 3.7V, and some mobile phone batteries that support fast charging also have 3.8V or 3.85 V rated voltage. In addition to being used as overcharge and discharge protection, the lithium battery protection board also needs to perform discharge overload protection. Therefore, when choosing a lithium battery protection board, two main parameters need to be determined, voltage and discharge current.
Determination of the basic parameters of the protection board
Determine battery voltage
When the lithium battery is in use, it will be combined in series and parallel according to the working voltage of the load and the required capacity, and the single lithium battery is assembled into a lithium battery pack, such as 48V, 60V, etc. which are common in electric vehicles. Therefore, when selecting the protection board, first determine the type of single battery and whether the voltage is 3.2V or 3.7V or other voltages, and then determine the voltage of the entire group, or the battery pack is composed of several strings of single batteries. The management chip of part of the protection board reserves the battery voltage selection terminal, and the rated voltage of the single battery can be set by the high and low level of the port.
Determine the discharge current
The maximum current that a battery can output or the continuous output voltage is related to factors such as battery capacity and battery type. For example, the common 18650 lithium battery is also divided into different discharge currents. Batteries with low discharge current are generally used in non-power applications, such as notebook battery packs. This type of battery has a small discharge current and a large monomer capacity, which is suitable for long-term low-current discharge occasions. This type of battery is called a capacity battery.
Corresponding to the capacity type battery is called the power type battery. The characteristic of this type of battery is that it can support a larger discharge current. It is generally used in power load situations, such as hand drills and electric vehicles. Because the current of the motor load is very large when starting, in order to ensure the current supply when starting, the battery needs to be able to withstand high current discharge. This kind of battery is called a power battery, and its single capacity is smaller than that of a capacity battery.
Regardless of whether it is a capacity battery or a power battery, its discharge current is generally based on the capacity, so the battery discharge current is generally represented by C, which is the discharge rate. For example, for a 3C battery, its discharge current is 3 times the capacity value. If the capacity is 2000mAh, the continuous discharge current it supports is 6A.
Although the discharge current of the battery has a certain relationship with the type and capacity of the battery, the determination of the final discharge current is related to the load itself. According to the current when the load is working, the protection board with the appropriate current is selected, and the protection board is usually selected according to twice the discharge current.
Determination of other parameters
Battery voltage and discharge current, these two are the two more important parameters for choosing a protection board. After determining these two parameters, you can basically choose a suitable protection board. Since some protection boards will also integrate other functions, other parameters sometimes need to be considered when choosing.
The equalization function generally appears in multiple series of protection boards. When the battery is used in series, if the battery voltage is not consistent, during charging, the higher voltage is easy to overcharge; when discharging, the lower voltage is easy to overdischarge. When assembling lithium battery packs, the consistency of capacity and voltage is generally selected and determined by means such as capacity division, but even the determined batteries cannot be completely consistent. After long-term use, voltage imbalance will occur. The problem. The balance function of the protection board is to make the battery achieve voltage balance during use.
For multiple strings of lithium batteries, a tap is drawn in the middle of every two strings and connected to the protection board. Therefore, the protection board can detect the voltage of each string of batteries. The simpler method of battery balancing is to discharge through resistance, which is controlled by each string of switching tubes. Which string of voltage is too high will turn on the switching tubes, and reduce the voltage by discharging. There is a certainty about this equalization method, because it is through the way of discharge, it will cause a certain amount of energy loss.
There is also an active balancing method. The battery string with higher voltage is controlled by the switch tube to charge the battery pack with lower voltage, thereby reducing the voltage, so that the voltage of the battery pack with low voltage will also be increased. Although this balancing method will also bring some energy loss, it is better to replenish it through charging and discharging than passive balancing.
Communication and metering functions
Although charging and discharging management and short-circuit protection are the main functions of the protection package, when the battery is in use, in order to facilitate the setting or statistics of battery usage, some protection boards integrate a battery management system, that is, BMS. The protection board integrated with BMS can detect the basic parameters of the battery, such as the voltage of each string of batteries, charge and discharge current, battery status, battery temperature, etc., and the statistical data can be packaged and transmitted through the communication interface, which is convenient for collection or remote transmission . Part of the protection board, especially the protection board of electric vehicles, will also integrate a Bluetooth module. The current battery parameters can be viewed through the mobile phone APP, and the working conditions can be set.
Charge and discharge interface
For most protection boards, charging and discharging are carried out through the same interface, and the charging and discharging protection currents of this protection board are the same. However, in some applications, the protection current of charging and discharging may be different. For example, the charging current is not high, but the discharge of large current is required. In this case, if the protection board with the same port is used, the cost will be higher. When choosing a split-port protection board, the charging current can be selected to be relatively small, and the discharging current can be selected to be relatively large, so that it can be lower in terms of cost. Of course, the port protection board may not be as convenient to use as the port due to the separate charging and discharging ports. This also needs to be selected according to the actual usage.
The above are several parameters that need to be understood and determined when choosing a lithium battery protection board. Of course, these are several relatively important parameters. As the application of lithium batteries becomes more and more widespread, the battery protection board will also integrate more functions. When choosing to use it, you should also choose according to the actual application environment.