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14 Dec 2018, 9:39

BMS Signal Transformers - Everything you need to know

1. Introduction

Battery Management Systems (BMS) connect to high-energy battery packs and manage the charging, discharging of the pack, as well as monitoring essential safety factors including temperature, state of charge and the state of health of the pack. Providing additional application protection, the BMS is able to connect the battery and disconnect it from the load or charging source as required. This application note provides an overview of the key features of battery monitoring ICs typically specified in BMS. Background information on battery cell chemistries as they relate to the requirements for communications in high voltage BMS is included. An application example will be used to outline the technology benefits Bourns’ transformers deliver to meet these specifications.

2. Overview of Lithium Ion

Figure 1: Forecasted Growth in Li-Ion Sales (Avicennes)

Market research and consulting firm Avicennes[1] predicted that the use of Lithium Ion (Li-Ion) battery cells for automotive and energy storage applications will continue to see significant growth through 2025 with compound annual growth rates up to 30% forecasted in the transport sector in China.

Table 1 provides an overview of the most popular chemistries by energy density, cell voltage and charge rate for 48V and higher voltage battery packs. These next-generation packs match the power density required to drive new electronics and motor designs. The latest battery cell developments in different chemistries deliver the increased power energy over longer periods of time necessary for full electric battery power.

Cathode

Anode

Energy Density

Cell Voltage

Charge Rate

NMC

Graphite

150-220Wh/Kg

3.6-3.7

1C Max

LFP

Graphite

100-120Wh/Kg

3.2-3.3

1C Max

NMC

LTO

50-80Wh/Kg

1.8-2.5

5C Max

LMO

LTO

100-150Wh/Kg

2.4-2.6

3C Max

Table 1: Summary of Most Common Li-Ion Chemistries for Battery Applications


There are several factors to consider when choosing the chemistry for a battery-powered application. As can be seen in Table 1, Lithium Nickel Manganese Cobalt (NMC) with Graphite has the highest energy density among the commonly-used chemistries. This is advantageous for heavy loads such as consumer energy storage or plug-in electric vehicles. The disadvantage, however, of this chemistry is it creates a higher risk of lithium plating on the anodes, which can reduce battery life and can lead to thermal runaway (fire or explosion). These harmful conditions become worse with today’s faster-charging connectors.

Lithium Titanate (LTO) has a lower energy density than NMC and does not suffer from the problem of cracking graphite, thereby, improving estimated battery life. The lower internal resistance of LTO facilitates faster charging rates making this battery chemistry beneficial for plug-in electric vehicles. The downside is the higher cost for heavier battery packs as more cells are needed to provide the necessary energy in kilowatt hours (KWH).

Lithium chemistries have very narrow operating temperature ranges typically from 20C to 40C. Operating outside these temperatures leads to a loss of capacity and a shorter lifespan. Elevated temperatures can also cause further degradation and a thermal runaway condition. A paper by Nasa[2], which studied the protection within 18650 cells found that the interrupt devices in all the cells connected in series and parallel were not as effective as single cells in preventing thermal runaway during fault conditions. This study illustrates the strong need for a Battery Management System when multiple cells are.

3.Overview of Battery Management ICs and Transformers

Figure 4 Block Diagram of BMS IC

A typical battery monitor IC (shown in Figure 4) measures cell voltage, pack temperature and performs cell balancing. In some models, there is also a current sense input port for shunt-based current measurement. Including this feature makes sense in 48V systems that do not have to deal with hazardous voltage levels, and that use a limited number of battery cells and, hence, monitoring ICs.

Conversely, it does not add a lot of value to integrate a current sense function into an IC for high voltage battery packs. These packs require only one current sensing chip and several hundred monitoring ICs to monitor the individual cells in the pack. For example, the Nissan Leaf has a working voltage of 360V and energy of 24KWh (NMC technology). This means that the structure of the pack will be 100S13P (13 strings of 100 battery cells assuming 5Ah each). Or a simpler way to put it; if each monitoring IC can check 10 cells, then at least 130 monitoring ICs will be needed. Another consideration in high voltage battery packs is that the BMS IC module or board needs to be located on top of the shunt resistor, which may pose a mechanical design challenge.

BMS High Voltage Communications

The BMS typically has two ports for isolated communications allowing battery monitoring modules to be daisy-chained throughout the battery pack. The source and sink currents of the serial port drivers are balanced enabling the IC to drive a transformer without saturating it. The transformer with a rated working voltage of several hundred volts provides the necessary protection of the communications line from any hazardous voltage coming from the battery pack. Furthermore, the drivers on the IC encode a four-line serial peripheral protocol into the differential signal needed for isolated communication from board to board.

Serial Peripheral Interface (SPI) is an interface bus commonly used to send data where one device or “master” transmits a clock pulse and control bit to a series of slaves. On each clock pulse, the slave either reads a command from the master or if the control bit is inverse, transmits its data on the data line. In this way, a central battery controller IC (master) can interrogate each monitoring IC (slave) in turn and hence retrieve necessary voltage and temperature information from the whole pack. In addition, the transformer and integrated common mode choke filter out common mode noise from the daisy-chained network.

 

Figure 5 BMS Transformer with Centre Tap Capacitor and Resistor. Top Right Image of SPI Signal

Although BMS ICs have balanced currents on their I/O pins, most manufacturers recommend a centre-tapped transformer. These have been found to improve CMNR  if a filter capacitor and termination resistor is used as shown in Figure 5.

Bourns BMS Transformer Safety Features

Figure 6: Bourns 2 Channel BMS Transformer SM91501AL

The windings inside the Bourns Model SM91501AL transformer uses enamelled fully-insulated wire (FIW) that passes the dielectric strength (Hi-POT) test of 4.3KV (1mA, 60 Seconds). Per Table 2N of IEC60950, the minimum creepage distance for material group I, pollution Degree 2 of functional insulation for a working voltage of 1600V is 8.00mm. The Bourns SM91501AL transformer datasheet shows a minimum 10mm creepage distance. This is because the actual tracking distance over the surface of the transformer and chokes have been measured as 10.4mm.

The replacement test for IEC60950 (IEC62368-1), which becomes mandatory June 2019 for audio/video, information technology and communication equipment will recognize FIW in the future. The use of FIW may qualify the device as having reinforced insulation with a lower working voltage (depending on the standard) of approximately 800V. This would allow the device to meet UL approval requirements and enable its use in additional applications such as consumer energy storage, which mandates reinforced insulation.

Recommended Electrical Characteristics

The recommended primary inductance values by some IC manufacturers will depend on the voltage of the communication signals, the pulse widths and the frequency. Bourns designed its SM91501AL transformer with a primary inductance span between 150uH and 450uH over an operating temperature range of -40C to +125C. The inductance is directly proportional to the permeability of the core. The permeability of the ferrite core of a transformer is temperature-dependent and tends to increase with temperature, therefore, the primary inductance will drift up towards 450uH at the upper end of the temperature range. This is why there is a big variation in the inductance value as specified on the datasheet.

The noise immunity of the BMS IC and transformer can be evaluated using a bulk current injection (BCI) test. The BCI test injects current into the twisted-pair lines at set levels over a frequency range of 1MHz to 400MHz with the bit error rate being measured. A 40mA BCI test level is sufficient for most industrial applications. The 200mA test level what is typically used for automotive testing. Bourns Models SM91501AL and SM91502AL have been evaluated by BMS IC manufacturers for automotive applications and have successfully passed requirements for BCI.

Figure 7: SM91501AL on Bourns BMS Demonstration Board

Summary and Conclusions

The demand for Li-Ion battery power is predicted to grow at a CAGR of 20-30 per cent over the next eight years. Battery Management Systems with reliable isolated communications are expected to be an important part of the safety and security of the battery system. An effective BMS is an essential design element that will help increase the lifespan of Li-Ion cells and contribute to enhanced safe operation for end users.

Bourns engineered its latest SM91501AL and SM91502AL BMS transformers with the higher working voltages of 1600V and 1000V respectively making them ideal solutions for automotive, industrial and consumer battery applications.

Bourns P/N

Description

BMS IC 

SM91501AL

2 Channel BMS Transformer 1600V DC

LTC68111

Analog Devices, Inc. (ADI) has successfully tested a Bourns transformer with one of its BMS ICs.

In addition, Bourns Models SM91501AL and SM91502AL have been tested with NXP33771 and by some companies (in mass production status) and found them to function well on board, passing the necessary BCI tests.

References

[1] Avicennes, Pillot Christophe, Rechargeable Battery Market 2017-2025 The Battery Show, Hannover May 15, 2018

[2] Judith Jeevarajan, Safety Limitations Associated with Commercial 18650 Lithium-ion Cells Lithium Mobile Power and Battery Safety 2010

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14 Dec 2018, 9:39