Anatomy of Battery Management Systems
Recent improvements in battery technology have surrounded us with ‘mobile’ devices. From cordless drills to electric vehicles, batteries are enabling a battery-powered revolution of the 21st century. Increased power density and reduced cost of production make battery power a viable alternative to traditional energy solutions like fossil fuels and pave way for completely new designs which were impossible to develop in the past. However, novel battery technology is not without its drawbacks. High power density, unstable chemistry, and common proximity of volatile chemicals to customer’s nether regions have created a new class of supervisory devices— battery management systems (BMS). In this article, we will dissect the architecture of a BMS device, its functions, and modern implementations of the technology.
What is a Battery Management System?
A battery management system… Manages batteries… Thank you for coming to my TED Talk. The broad umbrella term of ‘management’ can be confusing because it can mean different things depending the design. Let’s look at a LiPo /LiIon battery architecture and identify potential failure modes. From these we can derive core functions of a BMS and compare them to modern implementations of the technology.
Lithium Battery Theory (Summarized. I promise)
Lithium batteries are composed of layers of cathodes and anodes immersed in an electrolyte. Each cathode-anode layer pair is separated by an insulator so that layers can be compacted into a small form factor. In cylindrical batteries the layers are rolled together (like 18650 cells). This is the most efficient configuration for energy density. Some batteries take on a more a flat, rectangular shape. This format is often called a prismatic pouch. Prismatic pouches are found in the majority of modern mobile devices such as phones or laptops, while cylindrical cells are used in electric vehicles, power tools, and system where energy density is more important than size and weight.
Without going into too much detail into the electrochemistry of batteries, during charging (energy accumulation) the cathode undergoes oxidation loosing some negatively charged electrons. To compensate for this change, an equal number of positively charged ions dissolves into the electrolyte and travels toward the anode, where it is intercalated (deposited) into the graphite layer of the anode. The electron flow is from anode to cathode, while the current flows, by definition, is in the opposite direction. During discharge, lithium ions are de-intercalated (removed) from the anode and start flowing towards the cathode, while the electron flows from cathode to anode. Theoretically, these processes should be symmetric, meaning the movement should be the identical in both directions. Unfortunately, practical material limitations restrict how these reactions can occur. The important factors to monitor here are rate of electron flow (charge and discharge current), maximum and MINIMUM potential difference between anode and cathode, and temperature range. Violating any of these parameters will cause damage to the battery and make if fail sooner. Did I mention that LiPo batteries have a limited life-span? Unfortunately, modern LiPo batteries are still disposable components which must be replaced once they reach a certain cycle-count. Practically, the battery will degrade with each charge-discharge cycle. The most noticeable differences will be present in battery capacity and in-series resistance causing shorter operating time and lower voltage.
This short analysis of LiPo chemistry should give us a brief list of things we need to have in our BMS to monitor the health of the battery.
- Over- and under-voltage protection
- Over- and under-current protection
- Capacity monitoring
Ideally, these parameters should be monitored for each battery cell. Now let’s look at battery packs — multiple cells connected together.
Series vs Parallel
Batteries can be arranged either in series, parallel, or both. Putting cells in series increases the voltage the pack can provide, while connecting them in parallel increases the current. More complicated battery packs will use combinations of series and parallel connections to create a battery pack that satisfies system requirements (For a crazy battery architecture check out this Tesla Model 3 battery teardown).
Batteries in parallel are demarked with the letter S. For example, 2 cells in series would be described as 2S (three cells? 3S). Parallel cells use the letter P (2 parallel cells? 2P). A battery pack with 8 cells in series and 2 cells in parallel would be a 8S2P pack. In this configuration, we have 16 batteries total with every other battery is connected in parallel for a total of 8 parallel connections and 8 series connections. One benefit of the parallel configuration is self balancing, meaning cells in parallel will actively channel current between the parallel units until both cells present the same voltage. Due to manufacturing imperfections between cells, in-series configuration can have uneven voltages between cells. With time cells will start to charge/discharge unevenly. This will result in different levels of charge and reduced overall capacity of the pack. In series battery capacity is limited by the smallest capacity of an individual cell in the pack. This is why cells in-series require balancing by the BMS (this process can be active or passive). If you’re interested in this behavior, I recommend reading this article.
Pack architecture brings several new challenges to our BMS:
- Individual cell voltage monitoring
- Cell voltage balancing. Active or Passive
- Thermal management — Even though temperature can be measured for individual cells, most systems will implement thermistors that monitor several batteries at once.
Several nice to have functions make battery packs more versatile and user friendly. These functions include:
- SMBus communication — I2C protocol used to communicate with and control battery packs.
- Fuel Gauge technology — Theoretically, battery capacity can be calculated from pack voltage but in practice this is much more difficult. Fuel Gauges implement algorithms which account for battery chemistry, age, pack architecture to provide an accurate measurement of battery capacity as well as ‘time left’ to complete discharge.
Even more optional:
- Ship Mode — Useful mode for manufacturers. Batteries can be ‘sealed’ and set to ship mode which ensures higher safety during transportation. Usually ship mode is turned off after connecting the battery pack to a charger or host device.
- Authentication — Another useful feature for manufacturers. Through digital ID and tracking systems, the manufacturer can identify counterfeit packs.
Functionality of Battery Management
At this point we should have a pretty thorough list of BMS functionality. Let’s reiterate the most important ones:
- Over- and under-voltage protection —OV and UV protection prevents cell voltage from reaching levels harmful to the battery cell. Over-voltage protection acts similar to a Schottky diode connected to ground. When the threshold voltage is reached, the protection starts shorting the cell through a bleed resistor to bring the voltage down and prevent overcharging. On the other hand, under-voltage protection will actively prevent discharge of either the pack or a single cell.
- Over-current protection — OC protection monitors current draw during charge and discharge. If a threshold is reached the appropriate circuit is disabled.
- Active or passive cell balancing — Passive cell balancing works similar to OV protection. To ensure even distribution of charge, a new voltage threshold is selected by the BMS. The BMS will iteratively short individual cells through a bleed resistor to bring voltage of all cells to the same level. Active cell balancing can distribute the charge from overcharged batteries to undercharged ones during charge or discharge cycles. This is a more energy efficient approach to balancing because it redistributes the excess energy instead of dissipating it.
- Thermal protection — As batteries charge and discharge, the internal resistance will cause the battery pack to heat up. Excessive heat can damage the battery and cause runaway thermal behavior. Thermal shutdown can prevent damage caused by heat to the battery pack and extend the lifecycle of a battery pack. A similar mechanism can be implemented for low temperatures. Although low temperatures are not as dangerous, they can have a large effect on the chemical behavior of the cell.
- Capacity monitoring — This can be implemented through voltage monitoring or more complex Fuel Gauge algorithms. The goal is to measure remaining capacity of the battery and adjust safety thresholds accordingly.
In theory, most of these functions can be implemented with discrete components. For example, a primitive BMS could use a Schottky diode for over-voltage protection, a resetable PTC fuse to prevent over-current, and a comparator circuit to detect under-voltage. If you look at older BMS designs you will find this architecture.
Modern BMS systems are built on specialized ICs which integrate functional, system groups into one chip. The newer the design of the BMS IC, the fewer external components are needed. Two commonly used BMS ICs are the Renesas’ ISL94202/3 and TI’s BQ40Z50/60/80. However, most modern BMS controllers have a similar architecture. Let’s look at the ISL94202 and compare it to our initial circuit.
One of the biggest differences is that a single ISL94202 IC can monitor 4–8 battery cells, provide the necessary voltage and current protection, as well as thermal monitoring. In addition, all parameters of the BMS can be defined through the SMBus Interface. Therefore, the same design can be used for different battery chemistries (Li-ion CoO2, Li-ion Mn2O4 and Li-ion FePO4), cell configurations, and system architectures. The integrated design drastically reduces the size and bill of materials. The chip itself has become very popular in the DIY community kickstarting a number of homemade battery management systems (SBMS100, Keenlab BMS).
The BQ40 Family of BMS ICs is one of the most advanced on the market. The controller comes with all the standard protection features, dedicated charging profiles, more accurate fuel gauge technology, but most importantly it integrates the cell balancing circuit, drastically reducing the external component count. The entire circuit for a 4S battery pack controller can be implemented in a ~300mm² PCB, while maintaining the flexibility of programmable BMS with a wide assortment of features. If we compare it with the first battery management system, we can see the massive technological advancement between the two in both system integration, feature enhancement, but also overall simplicity.
Conclusion
Battery management systems may seem like black magic at first glance but in reality they all are relatively simple circuits with several key features. As the technology progresses, these systems gain additional functions but the basic theory of operation remains the same. More advanced BMS ICs will integrate external components and reduce component count while making the system more accurate, safe, and reliable.