Introduction
A Battery Management System (BMS) is an electronic device that is installed inside a multi-cell battery pack to ensure safe operation of the battery and monitor its operational state. A BMS safeguards the battery by protecting it from over charging, deep discharging, over current, over temperature, etc. Apart from providing safety, a BMS also increases the operational life of a battery. The most important features of the BMS include
Over charge protection
Deep discharge protection
Over current protection
Short circuit protection
Over temperature protection
Cell voltage balancing
Depending on the complexity of the BMS, it can provide several other features apart from the most important ones discussed above. Most modern BMS will have digital communication ports for communicating real time battery parameters to an external computer system or a master controller.
Need for a BMS
Lithium batteries are particularly sensitive to over charging and extreme temperatures. Prolonged operation of the battery in sub-optimal conditions can result in safety hazards and also reduce the battery life significantly. A BMS is essential to ensure that the battery operates under optimal conditions. All Lithium batteries are made up of large number of individual Lithium cells. Ideally all these cells should have the exact same cell voltage. However, due to mismatch in leakage currents in the cells, the cell voltages begin to diverge over time and the cells acquire unequal voltages. This leads to under-utilization of the pack capacity since the entire pack has to be switched - OFF even if a single cell is over charged or over discharged. A good BMS has the ability to balance out the difference in cell voltages and equalize all the cells. This is called cell balancing. A BMS essentially provides parameter monitoring, safety and long life to the battery.
Building Blocks of a BMS
Regardless of the complexity of design, all the BMS have the following important blocks.
Analogue Front End (AFE): It is responsible for acquiring voltage, current and other analogue data of the battery pack as well as individual cells.
Control Unit: It is responsible for the operational behavior of the BMS. On the basis of the data received from the AFE, the control unit controls balancing, battery switch, and other hardware features.
Power Switch: It is an extremely important component in the BMS that is used to switch - OFF the battery output in case of an abnormal condition. Quality and reliability of the power switch dictates the reliability of the entire BMS
Apart from the three components discussed above, a BMS may have other hardware blocks to carry out secondary functions. A basic functional building block of a BMS is shown below:
Common Types of BMS
There are a variety of BMS commonly available in the market. Cost of the BMS depends primarily on their reliability, features and voltage-current rating. The most basic BMS only provide essential protection features without any communication interface. In India, these are also sometime referred to as "Hardware BMS" since they lack software algorithms. A dedicated hardware chip provides basic level of protection to the battery. More advanced BMS have additional protection features, greater reliability and multiple wired and wireless communication features. These are sometimes referred to as "Smart BMS" or "Software BMS". These are equipped with a micro-controller that may have multiple software algorithm for protection, monitoring and communication.
For small battery packs such as those used in solar street lights, a cost effective non - smart BMS may be used. The BMS will provide the most basic protection features such as over voltage, over load short circuit protection etc. For bigger battery systems or more critical applications such as EVs, a more advanced and reliable BMS should be used. These BMS will typically have communication interfaces such as CAN, RS-485, Bluetooth, etc. to inform the user about battery's internal health and operational parameters. The acquired data can be used to diagnose and prevent premature failure of the battery.
Important Considerations
Beware of Misleading Features
The market today is flooded with BMS that advertise a lot of rather playful features such as Bluetooth, artistic looking monitoring apps, flexibility to alter (read degrade) almost every factory default setting of the BMS and so on. Although we appreciate the pursuit of technological addition, but remember that at it's heart a BMS is a protection and safety device and no compromise should be made in terms of reliability. A good manufacturer would have tuned the BMS to provide optimal performance of the battery after a lot of research and testing. Any attempt to alter these settings would most likely result in degrading the battery performance rather than enhancing it. If the BMS has been carefully designed, there should be no need for manual intervention.
Reliability First
More often than not, low quality power switch (MOSFETs) is the primary cause of failure of the BMS. Apart from having the ability to continuously conduct current, the switch must also have the ability to reliably break the current when required. Most of the switches fail not during conduction but during the instance when they break the current flow. If the nature of load is inductive, the switch undergoes an avalanche during a turn-off event. To handle the avalanche without failing, it must have the ability to withstand Kilo-Watts of power for a fraction of milli-seconds. Not all switches can handle this, especially the cheap ones. Did you know that the random failure rate of leading MOSFET companies such as Infineon, Nexperia, Toshiba etc. can be up to 10,000% lower than inexpensive local brands?
Factory Accuracy is Not Lifetime Accuracy
As a matter of fact all analogue silicon chips such as voltage reference, ADC, amplifiers etc. lose accuracy with time. The loss in accuracy over years could be up to 10 times the initial factory calibrated accuracy. Most inexpensive BMS manufacturers specify the initial accuracy of their device. A reputed manufacturer will always specify its worst case accuracy over the entire life and operating temperature range. As an example, most commonly available BMS will have a cell voltage measurement accuracy of around 10mV, over the years, this could degrade to 50mV or even more. A good quality AFE will have an initial accuracy of a few hundred micro-volts with the lifetime accuracy not degrading below 2mV. Beware!! Unless explicitly mentioned, accuracy figures are only initial accuracy.
Balancing is Not All About Balancing Current
It is a common belief that higher balancing current results stronger balancing performance. For example a 100mA balancing current should be more effective than 50mA current. This however is not true. True balancing is a complex task and depends heavily on the balancing algorithm apart from from the balancing current. Did you know that for a given balancing current, Coulomb-Traced-Balancing or Curve-Traced-Balancing algorithm can be up to 10 times more effective than simple balancing algorithm that most inexpensive BMS deploy? Implementation of these techniques however is not easy and require a lot of software algorithms and experimental data. Do not be misled by over-rated balancing current figures in BMS datasheets. At the end of the day, balancing performance is algorithm dependent. A well documented datasheet will specify how much charge mismatch the BMS can balance in a specified time frame.
X-Series Battery Management System (BMS) is a robust, precise and extremely reliable industrial grade BMS with best-in-class surge current handling and short circuit protection capability. An ultra-fast current response time along with an advanced short circuit detection mechanism ensures safety even under direct shorting of the battery terminals with thick conductor cables. Multi-channel current and voltage sensing located on separate physical silicon chips, dedicated co-processor and instrumentation hardware for short circuit protection, battery- load impedance detector and an extremely robust solid-state relay with clip-bonded MOSFET packaging are some of the key features that makes the X3 BMS unchallenged in terms of reliability and safety.
Other reliability features include isolated communication channels consisting of Iso-CAN and Iso-RS-485. An extremely accurate and precise Analogue Front End delivers 1mV absolute cell voltage measurement accuracy over entire working temperature range and device lifetime. RXN’s proprietary Curve-Traced-Passive-Balancing Technology can balance the cells at all times regardless of battery SOC. This results in up to ten times stronger balancing as compared to conventional passive balancing mechanism.
Find out more at www.rxnelectric.com/x-series-bms
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