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Interpretation of the key points and difficulties of bridge law;
(A) the detection principle of the bridge method
The working principle of bridge method is that BMS calculates the insulation resistance of positive electrode/car body and negative electrode/car body by detecting the change of partial pressure between high voltage positive electrode and high voltage negative electrode. The detection principle is as follows:
1. Close switch S 1 and switch S2:BMS detects V 1 voltage, V2;
2. Close switch S 1 and open switch S2:BMS detects the voltage of v 1';
3. Turn off the switch S 1 and turn on the switch S2:BMS detects the voltage of V2;
4. According to the above three steps, the total voltage U of the battery, the voltage dividing resistances of the positive and negative bridge arms and their ratios are known, and three equations U=aV 1+bV2 can be listed.
5. According to this equation, we can get the resistance value of positive electrode/shell =Rp, and the resistance value of negative electrode/shell =Rn.
The two resistance values are the insulation values we usually read on the whole vehicle, and the above is the detection principle of the bridge method.
(B) Design difficulties of bridge method
The stability and reliability of the bridge method need to pay attention to the following points (the above four voltage values V 1, V2, V 1', V2' are hereinafter collectively referred to as V 1, V2, and supplementary discussion is welcome):
1. Selection of voltage dividing ratio and ADC:
In order to take into account the cost, insulation detection will sacrifice some accuracy (sampling with 12-bit ADC, or even directly sampling with ADC in single chip microcomputer). At this time, it is necessary to select the voltage dividing ratio of the resistor (R 1/R2 or R4/R3).
The resistance voltage division ratio is too large, and the sampling resolution is not enough, so it is impossible to achieve high accuracy;
When the resistance voltage division ratio is too small, the sampling is out of range and the whole voltage range cannot be sampled.
2. The influence of parasitic capacitance:
As we all know, the actual existence of parasitic capacitance on the whole vehicle (generally in the order of several hundred nanofarads, and some are much larger than this order).
Because parasitic capacitance will lead to V 1, it will take some time for the voltage value of V2 to stabilize, and several problems will occur at this time:
BMS can't accurately judge the stable sampling point of V 1, the voltage of V2, and the voltage of V 1, V2 is not a real voltage division due to the unstable capacitor voltage or capacitor leakage, so the calculated insulation value is inaccurate, which is one of the main reasons for the unstable insulation of some cars in previous years, and it is much better now;
BMS waits for the voltage to stabilize for a long time, which leads to a long insulation detection time, which may not meet the time requirements of FTTI in terms of functional safety;
The value of parasitic capacitance will change with the weather and the aging of the vehicle. At this time, to ensure that the design still meets the sampling accuracy and time targets in the previous period, higher requirements are put forward for the stability and adaptability of the algorithm, and the main hardware circuits and software filtering should be considered.
3. Influence of voltage V 1 and V2 real-time sampling synchronization.
Theoretically, the higher the real-time performance of V 1 and V2, the better the accuracy and stability of insulation sampling. Unfortunately, this is just a theory, and obviously it can't be completely synchronized. For the sake of understanding, I will assume a very extreme real vehicle working condition to illustrate the impact of synchronous real-time performance:
The first stage: slam on the accelerator on a steep slope. At this time, BMS just detected the V1'in step 2;
The second stage: slam the brake pedal down the steep slope, at which time BMS just detected V2 in the third step;
You can think about this scene first and its influence on insulation detection. When you step on the accelerator pedal, the battery pack will release a lot of external current. Due to the DCR+ polarization internal resistance of lithium battery, the high voltage of the battery pack will drop sharply (determined by the current, generally 50~ 100V, and the actual output voltage of the battery is 350V and 400V). When the brake pedal is depressed, the high voltage of the battery pack will instantly rise to 450V due to the braking energy recovery of the whole vehicle, and the battery pack will be charged with a large current. Then the problem is coming. V 1' is detected by 350V partial pressure, and V2' is detected by 450V partial pressure. It is not suitable to calculate insulation with this set of voltages, but the error of insulation value is large. In the worst case, there may be false alarm and omission of insulation, which will lead to the corresponding fault strategy of the whole vehicle.