Lithium Battery Pack System Experiment.3.7v 18650 lithium battery

　　In practical application, two sets of 36V8A. h lithium manganese battery pack protection boards were designed and implemented in parallel and 10 sections in series for a certain brand of electric bicycle production factory. The single lithium battery protection chip used Japan Seiko's S28241. The protection board is mainly composed of the main circuit, control circuit, shunt discharge branch, filtering, optocoupler isolation, and level adjustment circuit. Its basic structure is shown in Figure 7. The discharge branch current is selected to be around 800mA, and a 510 Ω resistor is used in series and parallel to form a resistance network.

　　The debugging work is mainly divided into two parts: voltage testing and current testing. Voltage testing includes two steps: charging performance detection for overvoltage, equalizing charging, and discharging performance detection for undervoltage. You can choose to use a battery simulation power supply instead of the actual battery pack for testing, as multiple batteries are connected in series, the testing cost of this solution is relatively high. You can also use the assembled battery pack for direct testing to cycle charge and discharge the battery pack, observe whether the protection device operates normally during overvoltage and undervoltage, record the real-time voltage of each battery during overcharging protection, and judge the performance of balanced charging. But this plan takes a long time to test at once. When testing the charging performance of the battery pack, a 3-digit semi precision voltmeter was used to monitor the charging voltage of 10 batteries. It can be seen that each battery is within the normal working voltage range, and the difference between the individual cells is very small. During the charging process, the voltage deviation is less than 100mV, the full charging voltage is 4.2V, and the voltage deviation is less than 50mV. The current testing section includes two steps: overcurrent detection and short circuit detection. Overcurrent detection can be achieved by connecting an ammeter in series between the resistive load and the power circuit, slowly reducing the load. When the current increases to the overcurrent value, check if the ammeter indicates a current cutoff. Short circuit detection can directly short circuit the positive and negative poles of the battery pack to observe the status of the ammeter. On the premise of ensuring that the device is intact and the circuit is soldered correctly, current testing can also be directly carried out through the status of the power indicator light on the protection board.

　　In practical use, considering that external interference may cause unstable battery voltage, which can cause extremely short periods of overvoltage or undervoltage, leading to misjudgment of the battery protection circuit. Therefore, the protection chip is equipped with corresponding delay logic, and if necessary, a delay circuit can be added to the protection board, which will effectively reduce the possibility of external interference causing misoperation of the protection circuit. Due to the fact that when the battery pack is not working, the switching devices on the protection board are in a disconnected state, the static loss is almost 0. When the system is working, the main loss is the on state loss on the two MOS transistors in the main circuit. When the equalizing circuit is working in a charging state, the resistance heat loss in the shunt branch is relatively large, but the time is short, and the overall dynamic loss is at an acceptable level during the normal working cycle of the battery pack.

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