Battery Management System BMS

Recall that Battery pack (电池组/电池包) is made up of several modules (电池模块). Modules is made up of several cells (电池). BMS is the ‘brain’ of battery pack. BMS is linked to VCU.

A battery management system (BMS) 电池管理系统 is any electronic system that manages a rechargeable battery to operate within safe range.

It is usually made up of two or more PCBs inside the battery pack: one Main Board and several Slave Boards. The function of each board is shown in the diagram below:

In brief, BMS does the following:

First BMS measures the Voltage, Current and Temperature.

Then BSM estimates the SoC, SoH, SoP.

Upper and lower limit are set up to prevent battery from over charge and over discharge.

EV, typically limit the charge to 85% and discharge to 25% to prolong battery life.

Battery manufacturers will specify the upper & lower limit of charging / discharging.

When a battery is over charged (over upper limit), it gets hot & stressed, and may cause fire/explosion. This is an issue on safety.

When a battery is over discharged (over lower limit), it suffers from irreversible capacity losses. This is an issue on battery-life longevity.

To reduce stress & prolong battery life, every EV BMS has a Battery Protection Circuit (Lithium-ion protector).

Every Battery Protection Circuit has a Discharge MOSFET (switch) & a Charge MOSFET (switch).

During charging, if the Battery Protection Circuit detects that the voltage across the cells is reaching the upper limit, it will stop the charging by opening the Charge MOSFET chip.

During discharging, if the Battery Protection Circuit detects that the voltage across the cells is reaching the lower limit, it will stop the discharge by opening the Discharge MOSFET chip.

BMS is a complicated topic, because there is a lot of confusing, non-standardized terminology & abbreviation in the market. You usually see 'BCU' as Master and 'BMU' as Slave. But the diagrams below prove against this. Abbreviation is often confusing if we don't know the full definition and function, for example, BCU can mean differently to different customers.

The table below shows the different terms that I came across when I was doing BMS research:

This article use terms ‘Master’ and ‘Slave’ to avoid confusion.

BMS has 3 types of topology:

An example of Distributed Topology:

An example of Modular Topology:

Different types of BMS topology are adopted by the automotive OEMs:

Slave Board measures Cell Voltage & Cell Temperature.

Slave Board also performs Cell Balancing. Cell balancing 电池均衡 is a technique, in which voltage levels of every individual cell connected in series to form a battery pack, is maintained to be equal to achieve the maximum efficiency (maximising the capacity) of the battery pack.

A fresh battery pack will always start off with balanced (same SoC) cells. But as the battery pack is put into use (charging/discharging), the cells get unbalanced (each cell has different SoC).

Such cell imbalance after use is caused by:

·      Different self-discharge rate in each cell.

·      Internal resistance (IR) variation. As cell ages, its IR also changes. Inside a battery pack, not all cells will have the same IR.

·      Different thermal gradient in battery pack. Temperature difference will cause one cell to charge/discharge faster than the other.

Master Board measures Pack Current & Pack Voltage.

The Pack Current is measured by a current sensor.

There is no voltage sensor for Pack.

The Pack voltage is calculated from summing up individual Cell Voltage (measured by Slave Board).

Some customers separate the High Voltage Monitor (HVM) from the BMS Master Board.

HVM may be called High Voltage Management, or IVU (Inspecting Voltage Unit).

HVM measures the Battery Pack’s current.

The green lines shown above are connected to the Insulation Monitor 绝缘 检测 circuit (part of BMS Master Board).

Insulation Monitor (绝缘监测): Isolation or Insulation Monitoring/Detection, Ground/Earth Fault Detection, 绝缘监控/检测, 绝缘故障-监测, 接地故障检测, 隔离故障检测.

BMS continuously monitors the isolation resistance between the Battery and chassis, for deterioration of insulation and potentially dangerous levels of leakage current 漏电流.

This is to ensure that the Battery is electrically disconnected from any touchable conductive object (eg: chassis).

Minimum isolation resistance is 100Ω/V (for DC circuit) and 500Ω/V (for AC circuit).

Therefore, a 350 V Battery(DC) should maintain a resistance above 350x100Ω between the Battery and the chassis. This ensures that current will be limited to approximately 10 mA which is uncomfortable but not generally life threatening.

If isolation resistance falls below specified safety level, BMS Master Board will command High Voltage Board to cut off the high-voltage circuit by opening the relay inside the BDU. High Voltage Board will send off an alarm too.

Note: ground fault is not equal to short circuit.

Another function of BMS is HVIL (High Voltage Interlock Loop) 高压互锁回路 Monitor.

HVIL is a low voltage loop that routes through every high voltage (HV) component (eg: OBC, BJB, MCU, DC/DC Converter, Battery Pack), HV connector and MSD in the vehicle.

The BMS monitors the continuity of this loop by producing a constant current and measuring the resistance. If there is an open circuit (change in resistance eg: loose connection), BMS will send out an HVIL alert and disconnect the electrical supply from the battery power.

Future trend of BMS:

Battery Pack manufacturer faces two challenges: high cost of Li-On battery & lack of space.

·      Li-On battery technology has almost reached the peak. There is no new development on Li-Ion battery. In fact, battery is the major cost of battery pack. 

·      For BEV to have more battery power, more Li-On battries are needed. However, the size of battery pack is almost fixed in size. There is a limit on how many batteries you can squeeze inside the battery pack.

What Battery Pack manufacturers can do now is to reduce the cost of other parts, for example BMS. Flexible Printed Circuit (FPC) is used to replace the cabling of traditional BMS. The circuitry of Master Board & Slave Board can be replaced by chips. These chips can be mounted on the FPC. With this solution, it helps to save cost on cables and PCB, and save space. Thus, you will see less connectors and cables in future BMS solution.

This is my last article on Electric Vehicle. BMS is the most difficult topic to understand in Electric Vehicle. I hope you have enjoyed reading this.