BMS (battery management system) lesson 3 – BMS function list and sampling requirements

Time:2021-10-8

BMS function list

In personal habits, BMS functions are divided into three parts

  • Basic functions of BMS: V / I / T sampling, protection function (overvoltage, overcurrent, overtemperature, insulation resistance), relay drive, status sampling, relay adhesion detection, can communication;

  • Core functions of BMS: cell balance, SOP (power), SOE (energy), SOC (state of charge), SOH (Health);

  • BMS application related: collision signal detection, AC / DC charging, charger state detection, thermal state, heating / cooling demand, precharge, wake-up / sleep, communication with VCU;

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The residual capacity estimation of BMS is the core content of BMS and has always been a difficulty in the industry. Firstly, it is an estimated value, which is calculated by algorithm according to battery voltage, current, discharge rate, temperature and other factors, which requires your system to collect accurate and fast enough to ensure the accuracy of the final result. However, this is related to the processing speed of your main control chip, the accuracy of AFE, the scheme selection of collecting current, and the accuracy of temperature sensor. There are also many factors related to the size of the sampling frequency from the overall consideration of the system. The selection of high-speed and high-precision chips is bound to increase the cost. The faster the sampling frequency, the greater the system load. Therefore, under the current technical conditions, we all refer to our own specific projects to weigh various factors.

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According to the V / I / T measurement value, estimate the internal resistance capacity, then estimate the SOC and SOH, synthesize the SOC and SOH to obtain the SOP available power, and finally react to the user, that is, the remaining driving mileage;


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General industry requirements for estimation accuracy are as follows:

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Data sampling requirements

Data acquisition delay / synchronization problem

BMS A / D conversion 100 μ s

Communication network 2 + 8-byte question and answer frame ~ 0.3ms

BMS resource coordination and scheduling

Problems caused by

  • Fast charging safety issues

  • Poor equalization effect

  • Accuracy of state estimation algorithm

proposal

  • Select the sampling frequency (5 ~ 10 Fmax) according to the signal characteristics

  • Different physical quantities, different sampling frequencies (I highest, t lowest)

  • Information reporting needs systematic consideration to avoid false positives

Data acquisition accuracy

  • Current acquisition accuracy determines SOC accuracy

  • Voltage accuracy affects SOC and safety

  • Voltage accuracy determines the equalization effect

proposal

  • The higher the accuracy, the better

  • Avoid high costs

  • Over reliance on accuracy reduces system reliability

  • Appropriate accuracy needs to be fully demonstrated and verified

V / I / T accuracy requirements

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The provisions in the national standard qct897 are as follows. The general enterprise standards will be a little higher than this

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