Overview of high-voltage circuits involved in BMS

I have been traveling to the north for a few days on business, and I haven’t been back to the north during this season for a long time. I see familiar poplars and willows everywhere, and the temperature is cool in the morning and evening. I feel homesick.

The core part of the BMS hardware circuit is the high-voltage circuit, which is also the difficulty and key point in BMS design. When the voltage increases, it is not just a simple increase in the mathematical sense, but actually brings many considerations and requirements.

So, what is high voltage?

In the national standard 18384 , the concept of Class B operating circuit is defined . This is the actual upper voltage limit of power batteries that can be encountered currently, which is also the high voltage defined in this field.

The high-voltage functional circuits involved in BMS include the following types, which are listed below.

1. High voltage detection

This function is total voltage sampling, which is the most basic and important function. Its purpose is to provide the most basic data for basic battery status monitoring.

The principle of total voltage sampling is basically the same among all manufacturers: use resistors to divide the high voltage of the battery pack, collect the divided voltage value on a certain resistor, and then convert it into a total voltage value.

There is also another detection method: using the cell voltage value for accumulation, this value is more accurate.

These two methods are generally used at the same time to achieve redundant detection.

2. Insulation detection

This function is used to detect the insulation status between the positive and negative electrodes of the battery pack and the vehicle body ground. The purpose is to ensure human safety.

Under normal circumstances, the high-voltage power battery is completely insulated from the vehicle body ground. However, due to aging, harsh use environment and other issues, the positive or negative electrodes of the power battery may form a leakage circuit to the vehicle body ground, causing the vehicle body to be electrified and endangering personal safety.

The method of neutral detection is also recommended in the national standard GB/T 18384.1 (as shown below): simply speaking, it is calculated by artificially increasing the parallel resistance of known resistance and using a series of equations; it should be noted that the battery pack is The effect of the Y capacitance of the vehicle body on the detection time.

The above method is also called the bridge method, and there is another method called the pulse injection method, which will be discussed in detail later when I have the opportunity.

3. Relay adhesion detection

The high-voltage relay is a switch used to control battery charging and discharging. This adhesion detection function is to identify whether there is a fault in the relay, resulting in uncontrollability (cannot be closed or cannot be opened).

The detection method is also similar (the picture below comes from Ke Lie's patent). The on or off state is identified by detecting the voltage before and after the relay; usually, the total voltage detection and adhesion detection functions are combined. However, as the number of relays increases, the detection circuit and diagnostic logic become more and more complex, and the detection of negative relay adhesion has always been a difficulty.

In addition, the relay drive circuit has a protection circuit that specifically shared its low-voltage part (see link at the end of the article).

4. High voltage interlock

The high-voltage interlock function has been written in detail in an article before (see link at the end of the article), so I won’t go into details here.

5. Total current detection

You can tell what the total current detection function is just by looking at the name. There are two main ways to implement it, one is to use a Hall sensor, and the other is to use a shunt.

Both have their own characteristics. Among them, Hall sensors are divided into several types, such as flux gates, which are also commonly seen and used at present; shunts mainly consider the accuracy and usage impact caused by temperature drift and heating.

There are also considerations of functional safety, how to select detection methods, and how to design hardware architecture.

6. High voltage power-on pre-charging

The high-voltage precharge function has been specifically written about before (see link at the end of the article), so I won’t go into details.

Summarize :

There hasn’t been much introduction to high-voltage content recently. Let’s start this article by briefly introducing all the high-voltage functional circuits involved. Don’t look at what I say is simple. After each function is unfolded in detail, there are a lot of troublesome things. Later, It is necessary to summarize the knowledge points involved in high-voltage circuits.

Previous link:

A brief analysis of the high voltage interlock (HVIL ) circuit in BMS

Relay drive protection circuit (Part 1)

Relay drive protection circuit (Part 2)

Let’s talk about the precharge circuit and precharge resistor (Part 1)

Let’s talk about the precharge circuit and precharge resistor (Part 2)