Battery Management System (BMS) : The Manager
If you have ever wondered:
- Why don’t smartphones fast-charge till 100%?
- What happens to the battery connected to solar panels or wind turbines on capturing excess power?
- What happens to a smartphone’s battery when left connected to the charger after full charge?
The answer is Battery Management System (BMS).
What is a Battery Management System?
I can think of only one phrase to describe it — battery’s personal caretaker.
Formally, it is a built-in electronic system that manages the working of a rechargeable battery. It could be a simple switch (or a fuse) that detects the voltage and disconnects the battery from the charging source after full charge or a complicated system involving multiple electronic components that prevent energy wastage.
Additionally, it maps the battery’s current chemical state to an electric signal which is used to create the battery’s digital icon on the screen.
Note — The system is not used for non-rechargeable batteries because they are not recharged. Hence, charge control and management mechanism is not required.
Tasks
While a battery management system performs multiple functions, it typically has one role to play — to protect the battery by:
- Increasing its longevity, and,
- Preventing it from being destroyed
All other tasks it performs supplements the primary role!
- Monitoring physical quantities to compute battery characteristics (refer Fig 2)
- Constantly senses physical quantities like voltage, current and temperature.
- Computes battery characteristics (viz. state of charge and depth of discharge) to determine its condition based on the detected quantities.
2. Manages cooling the battery
- The heat generated during operation is cooled either using natural air or a cooling fluid.
- Tracks coolant flow (air or a cooling liquid).
- Ensures that battery functions within the prescribed operating temperature
- Batteries are operated in a “safe” temperature range to prevent unwanted damage. I’ve described more about the operating temperature of the battery here.
3. Protects the battery from
- Over-voltage and over-current during charging cycle
- Power fluctuations during charging and discharging cycle
- Temperature variations (refer point 2)
- Leakage and accidental short circuits
4. Recovers wasted energy back to the battery (optional)
The charging mechanism flowchart shown in Fig 2 is very simple and acts as an abstraction.
Replace the charging algorithm block with any of the other tasks to understand the working of the entire system.
While the flowchart looks simple, it is easy to forget that each block has multiple tasks. For instance, Fig 3 shows a flowchart of a commonly used charging algorithm — constant voltage and constant current [3].
Note — The owner retains all the rights of Fig 3.
Battery Balancing Techniques in Battery Management System
Different systems with different topologies are used for different applications. Most heavy duty applications like electric vehicles and battery backups for storing the harvested energy from renewable sources require multiple battery packs.
Battery packs are basically a collection of batteries uniquely arranged (in a series-parallel combination) to provide the desired output. Even when the batteries used in a battery pack are of same type, no two batteries are identical.
This discrepancy arises due to variations in manufacturing conditions. For example, during production, if a battery is exposed to a higher temperature than other units then its chemical composition changes ever so slightly.
The process is similar to cooking. Even if you cook the same meal with the same ingredients using the same cooking apparatus, the end result will never be identical because regardless of what you do there will always be an external factor beyond your control that induces a marginal change — the farming conditions or the storage conditions.
Likewise, any two batteries of the same type may have different characteristics (viz. voltages and capacities) even if they are rated same on the packaging cover. The slight difference in voltages may cause lower-voltage battery to over-charge or/and over-discharge. This may not only damage the individual battery but also affect the operation of the battery pack.
To prevent such consequences, a technique called battery balancing is used. As the name suggests, it balances the pack by ensuring that the individual battery having the lowest capacity is neither over-charged nor over-discharged.
The two types are:
- Passive Balancing — Charge and voltage regulators are examples of a passive balancing technique. It is passive because the excess energy (charge) is dissipated as heat and not redirected back into the pack.
- Commonly used in toys, rooftop solar panels or prototypes for educational purposes.
- Though not advisable, it is still used for a complex application like harvesting solar energy using rooftop solar panels because it is simple and economical.
2. Active Balancing — The excess charge is redirected back to charge the battery with lowest capacity.
- Used in complex systems like smartphones, hybrid automobiles, electric vehicles, wind turbines and laptops.
- Due to its functionality, it has a complex design and is expensive.
- Lithium based batteries demand active balancing in its BMS to constantly manage, monitor and protect the battery for best possible results.
For the geeks — Most active systems are application-specific designs. A system used in an electric vehicle cannot be used for harvesting wind energy! Thus, various topologies like centralized, distributed and modular are used accordingly [2]. Visit here to know more about how to select a BMS.
Battery Management and Software
With advancements in computing, battery management issues are increasingly being addressed using embedded systems and software. Artificial Intelligence (AI) and Machine learning (ML) are some of the powerful modern day computing tools used to tackle a variety of problems and battery management system is no stranger.
Machine learning techniques like genetic algorithms are being used to better estimate characteristics like state of charge which directly indicate the health and current condition of the battery.
Companies like ION Energy focus on deploying AI and ML techniques to model, track and estimate parameters thereby improving (application-specific) battery performance. Additionally, companies are adopting Software as a Service (SaaS) model that generally uses a cloud-based platform to remotely track all the necessary quantities.
Conclusion
As materials change and batteries continue to improve, the complexity of battery management system will also increase.
However, the trade-off of such a complex system is better protection of battery, improved performance and longevity. Furthermore, the inclusion of software will enable plug-and-play based solutions that works on most general-purpose battery management chips.
While the price may be an issue initially, according to the demand-supply equilibrium, it will eventually decrease as the technology becomes mainstream.
Thank you everyone for your time.
References
[1] Wikipedia Contributors, “Battery Management System”, Wikipedia, The Free Encyclopedia, June, 2020
Originally published at https://www.energyio.tech on July 14, 2020.
