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The temperature sampling circuit of the cell is almost the same. The external NTC resistance is measured by ADC, and the resistance value is converted into the temperature value, so as to obtain an analog quantity close to the real temperature of the cell. In fact, many people think that BMS hardware has no technical content, but they just take the integrated IC over and over again. As a practitioner of BMS hardware, it is also very weak in this situation. The reasons for refutation are always not enough, even psychological, but at the same time they want to kill the person who said this; But the reality is that most people who say such words are either leaders or leaders. Well, I can bear it.

In fact, you don't have to think so much. It's useless to think about it. Just design the BMS well. Take the temperature sample. It looks simple. There are some things to do to get into it.

Picture 1 Schematic diagram of temperature sampling circuit

Here is how to select the appropriate NTC.

The temperature sampling accuracy of BMS includes two parts, one is the sampling accuracy of the circuit itself, and the other is the accuracy of NTC.

NTC is not mentioned separately in QCT-897, but the actual sampling accuracy requirements include NTC, and the accuracy of NTC has a great impact on the overall temperature sampling accuracy; Many main engine manufacturers only put forward an overall temperature accuracy requirement, but we need to know the hidden rules. We should take the initiative to ask the main engine manufacturer how NTC is selected, because it is likely to be selected by others.

The sampling accuracy of the circuit itself also includes the pull-up resistance accuracy, ADC accuracy, reference voltage source accuracy and power supply accuracy, which is what we need to pay attention to in circuit design.

So, what kind of NTC do you choose? The following figure is the selection table on the official website of Ishizuka. I try to introduce the two key characteristic parameters.

Picture 2 NTC selection table of Shizuka (from its official website)

R25 resistance value

The resistance value of the thermistor is defined in the Japanese industrial standard JIS C 5602 as follows: under the specified ambient temperature, the resistance body is heated by the detection current itself, and the change of resistance value caused by the change of resistance value relative to the total detection error can be ignored. The DC resistance value of the thermistor measured by the detection electric power can be ignored. Also called zero power resistance value, R25 corresponds to the ambient temperature of 25 ℃.

Its accuracy represents the range of resistance change at this ambient temperature. From the perspective of practical application, the commonly used accuracy is ± 1%, and higher accuracy is rarely seen.

R25 resistance values are usually 10K, 47K and 100K. Basically, most of them are 10K, while others are relatively few. In reality, they are selected in this way; But if we choose it ourselves, which resistance value should I choose?

In fact, it depends on how much resistance we want NTC to be in. In the range of - 40 ℃ - 125 ℃, the corresponding resistance range of R25 with different resistance values is also different; In short, the larger R25 is, the greater its resistance at other temperature points is (refer to the figure below).

Picture 3 Ishizuka KT series RT table (from its official website)

R25 is not as big as possible. One possible effect is shown in the figure below. If the internal resistance of ADC is small, the resistance value may be close to the NTC resistance value at low temperature, which will affect the temperature sampling accuracy.

Photo 4 The small internal resistance may cause sampling deviation

B value

B value is a physical quantity used to express the sensitivity of NTC to temperature change, that is, the change rate of resistance value.

Refer to the figure below, the larger the value of B, the greater the change rate of resistance, and the more sensitive to the change of temperature.

Figure 5 Schematic diagram of the difference between the size of B value

The calculation method of B value is to take two temperature points as reference points. Generally, the manufacturer is 25 ℃ and 85 ℃, also marked as B25/85. Its calculation formula is as follows.

In practical use, the B value of many NTC contacted is 3435 ± 1%, which can be used as a reference value for selection; In addition, the larger the B value is, the larger the R25 is. It is difficult to flexibly match the two.

summary

When you design the circuit, you will have a desired resistance range; In this range, the temperature points should be scattered as far as possible, and the resistance distance between the two temperature points should be large, so that we can improve the accuracy of the resolution. So we can lock the NTC model we want according to the R25 and B values (i.e. RT table).

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