BMS hardware – function introduction and design suggestions

Maybe you have never explored the inside of the battery, but have you ever wondered what keeps the battery healthy and safe? The answer is BMS.

Whether it is top 10 electric motorcycle manufacturers in the world or ordinary lithium battery producers, they will not ignore BMS hardware. In the BMS hardware design process, there are some key points that we need to pay attention to and dig deeper, such as the design of temperature detection, voltage detection, power balance, current detection, etc.

This article mainly explains the functional principles and design suggestions of temperature detection, voltage detection and power balance of BMS hardware.

The role of BMS hardware

The function of the battery management system (Battery Management System, BMS) is to intelligently manage and maintain each battery unit, prevent the battery from over-discharging and over-charging, and prolong the service life of the battery; and monitor the state of the battery in real time to make it in the best working status.

BMS hardware function introduction

The battery management system (BMS) in electric vehicles is a very important component. Among them, the BMS hardware mainly includes temperature detection, voltage detection and equalization functions. The following is a detailed introduction of BMS hardware functions:

Temperature detection

Temperature detection is a basic function of BMS hardware.

According to different devices on the board, temperature detection can be divided into battery temperature detection, equalization resistance temperature detection and MOS temperature detection.

At the same time, according to different working states of the battery, it can be divided into temperature detection in discharge, charge and static (no discharge or charge) modes.

Finally, according to different temperature thresholds, it can be divided into high temperature alarm, low temperature alarm and normal state.

Voltage detection

BMS voltage detection can generally be divided into B+ voltage detection, P+ voltage detection and single-string battery voltage detection. Here we mainly introduce the single-string battery detection.

By detecting the voltage of a single string of cells, it can be known whether any cell is in an over-discharge or over-charge state. Once abnormality is confirmed, stop charging or discharging immediately.

Then balance the battery cells, one is to ensure the safety of the battery, and the other is to extend the service life of the battery.

At the same time, the voltage detection can also judge the connection status between the BMS board and each battery cell, that is, the disconnection detection.

Equalization circuit

The third important function of BMS is power balance.

When the battery is charging, if one of the cells reaches full charge first, the charging will stop.

Otherwise, the cell may expand, leak or even run away from heat. At this time, the other cells connected in series will not be able to be charged. The overall energy utilization rate of the battery will be reduced.

When the battery is discharged, if one of the cells reaches the over-discharge state first, the discharge will stop. Otherwise, the cell will be damaged by over-discharge and cannot be recovered.

The equalization function of BMS is to solve the above two problems.

BMS hardware design suggestions

Design of temperature detection function

The temperature detection circuit is very simple, and the resistance value of the NTC device will decrease as the temperature increases. Therefore, different voltage values can be collected at different temperatures, and finally the current temperature value can be obtained through formula calculation.

The formula of NTC resistance and temperature is as follows: Rt = R *EXP(B*(1/T1-1/T2))

According to the formula, the temperature detection section of the BMS hardware can be designed according to the actual situation. The specific design suggestions for the temperature detection function of the BMS hardware are as follows:

1) Selection of resistance value

● It is recommended that the resistance value of R1 = the resistance value of NTC at normal temperature. The advantage is that the measurement range of high and low temperature is not much different.

● If some special scenes are more sensitive to high or low temperature, R1 can be adjusted appropriately;

● High resistance value can be selected for high temperature precision requirements, and low resistance value can be selected for low temperature precision requirements.

2) Selection of NTC resistance B value

As the B value is larger, the slope of the relationship graph between its resistance value and temperature is larger.

● For scenes with high temperature accuracy but narrow temperature range, NTC with larger B value can be selected;

● For a wide temperature range, especially for high-temperature measurement scenarios, it is recommended to choose an NTC with a small B value.

Voltage detection function design

At present, most of the voltage detection functions of BMS hardware are implemented by AFE chips.

The following figure is the AFE chip hardware design diagram:

The role of R2~R21:

One is to set a fixed filter cut-off frequency to match the filter sampling circuit and time inside the AFE to achieve high precision;

The second is hot swap protection.

Consider the leakage current problem of the AFE channel. If the leakage current is large, the series resistance must not be too large.

Otherwise there would be a voltage drop here large enough to affect sampling accuracy.

However, if it is too small, it will reduce the protection ability. It can be selected according to the specification. Generally, the recommended value is 20R~1KR.

Balance function design

For the selection of the balancing resistor, its resistance value, package and maximum power need to be considered. Because electric energy will be converted into heat energy and consumed during passive balancing.

The smaller the resistance value, the faster the equalization speed, but the heat is also large, and the maximum working power of the resistor cannot be exceeded; the larger the package, the faster the heat dissipation, but the larger the PCB area occupied.

Taking lithium iron phosphate battery as an example, the maximum voltage of a cell is 3.65V, and the balancing resistor is 51R.

Then the maximum power P=U*U/R=3.65*3.65/51≈0.27W
Therefore, the 1210 package and above are all satisfied (1210 is 1/3W, 1812 is 1/2W, 2010 is 3/4W).

Summary

Whether it is temperature detection, voltage detection or power balance, the design of BMS hardware should be carefully considered and adjusted according to different scenarios and needs.

For example, in a high temperature environment, BMS hardware design needs to consider how to prevent the battery from overheating and how to make the battery life longer; in a low temperature environment, BMS hardware design needs to consider how to avoid battery overcooling and how to maintain battery performance.

In addition, the BMS hardware design also needs to consider the capacity and output current of the battery to meet the needs of different application scenarios. For instance, the bike battery voltage differs from other batteries.

BMS hardware design needs to consider many aspects in order to achieve optimization.