Practical system experiment of lithium battery protection board

by:CTECHi     2021-07-16

With the progress of society and the continuous development of the electric industry, lithium battery protection boards are now commonly used, so the practical system experiment of lithium battery protection boards is imminent. In practical application, the Chinese Chuangfa designed and realized 2 sets of 36V8A·h lithium manganate power battery protection board in parallel and 10 in series in response to the needs of a certain brand of electric bicycle manufacturer. The single-cell lithium battery protection chip is made from Japan. Seiko's S28241, and the protection board is mainly composed of main circuit, control circuit, shunt discharge branch, filtering, optocoupler isolation and level conditioning circuit, and its basic structure is shown in the figure. The current of the discharge branch is selected to be around 800mA, and a resistor network is formed by using 510Ω resistors in series and parallel.

The debugging work is mainly divided into two parts: voltage test and current test. The voltage test includes two steps: charge performance detection over voltage, equalization charge and discharge performance detection under voltage. You can choose to use a battery analog power supply to replace the actual battery pack for testing. Because multiple batteries are connected in series, the test cost of this solution is relatively high. You can also use the assembled battery pack to test directly, charge and discharge the battery pack in cycles, observe whether the protection device operates normally during overvoltage and undervoltage, record the real-time voltage of each battery during overcharge protection, and judge the performance of balanced charging. But this scheme takes a long time to test once. When testing the charging performance of the battery pack, a 3-digit half-precision voltmeter is used to monitor the charging voltage of 10 batteries. It can be seen that each battery is within the normal operating voltage range, and the difference between the cells is very small. The voltage deviation is less than 100mV, the full charge voltage is 4.2V, and the voltage deviation is less than 50mV. The current test part includes two steps: over-current detection and short-circuit detection. Over-current detection can connect an ammeter in series between the resistance load and the power circuit to slowly reduce the load. When the current increases to the over-current value, see if the ammeter indicates a current cut. Short-circuit detection can directly short-circuit the positive and negative poles of the battery pack to observe the status of the ammeter. Under the premise that the device is intact and the circuit welding is correct, the current test can also be carried out directly through the status of the power indicator on the protection board.

In actual use, considering that external interference may cause the battery voltage to be unstable, this will cause the voltage to be over-voltage or under-voltage for a very short time, which will lead to the wrong judgment of the battery protection circuit. The chip is equipped with corresponding delay logic. If necessary, a delay circuit can be added to the protection board, which will effectively reduce the possibility of malfunction of the protection circuit caused by external interference. Since the switching devices on the protection board are in an off state when the battery pack is not working, the static loss is almost zero. When the system is working, the main loss is the on-state loss of the two MOS transistors in the main circuit. When the equalizing circuit is working in the charging state, the resistance heat loss in the shunt branch is larger, but the time is shorter, and the overall dynamic loss is in the battery The group is at an acceptable level during the normal working period.

   After testing, the design of the protection circuit can meet the protection needs of series lithium battery packs. The protection function is complete, and it can reliably protect against overcharge and overdischarge, and at the same time realize the balanced charging function.

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