The primary role of a BMS is to protect and communicate the status of the battery.
Batteries are widely used in many applications, such as electric vehicles with different categories (battery EVs, hybrid vehicles and fuel-cell EVs), as well as energy storage for various purposes, such as grid stability, peak shaving and renewable-energy time shifting. In these applications, lead-acid, nickel metal hydride (NiMH) and lithium-ion (Li-ion) batteries are commonly used. The proper management of these battery packs is a highly important task that requires both hardware and software components. This task is typically implemented by a battery management system (BMS), which IEEE Standard 1491 defines as “a permanently installed system for measuring, storing and reporting battery operating.”
This article proposes a global overview of the different chemistries used in power batteries and the main purpose of implementing a BMS.
Principle & types of power batteries
The first distinction that can be made between batteries is that there are primary and secondary batteries. Primary batteries are non-chargeable, whereas secondary batteries are rechargeable. Each battery system is characterized by its chemistry, and there are many varieties of power batteries on the market. Because the focus of this article is mainly on secondary batteries, the most important characteristics of the most used cells are summarized in Table 1.
| Ni-Cd battery | Ni-MH Battery | Li-ion battery | |
| Average operating voltage (V) | 1.2 | 1.2 | 3.6 |
| Mass energy density (Wh/kg) | 50–80 | 70–95 | 118–250 |
| Volume energy density (Wh/L) | 50–150 | 140–300 | 250–693 |
| Mass power density (W/kg) | 200 | 200–300 | 200–430 |
| Volumetric power density (W/L) | 200 | 300 | 800 |
| Self-discharge (month) @ 20°C | 10% | 20% | <5% |
| Operating temperature (°C) | −20~50 | −20~60 | −20~60 |
| Cycle life | >800 | >800 | >1,000 |
| Environmental impact | Heavy cadmium pollution | Heavy metal pollution | Relatively low |
| Safety | Good | Good | Medium |
| Production cost | Lowest | Low | High |
Table 1: Characteristics of the most used cells
Nickel cadmium (NiCd) batteries have been developed for over a century. They are known for being relatively cheap and robust and have been widely adopted for their high capacity, easy maintenance and low cost. The average cell voltage is about 1.2 V. These characteristics make NiCd batteries very popular for power tools. The energy density and specific energy are relatively low, which are drawbacks to NiCd batteries. In addition, NiCd batteries suffer from the so-called “memory effect.” Finally, the use of cadmium results in serious environmental problems.
Unlike NiCd batteries, NiMH batteries, introduced in 1990, have a higher energy density and specific energy. NiMH batteries have been widely used in applications like notebook computers, cellphones and shavers. They also bring improvements when it comes to the memory effect and metal pollution. When it comes to drawbacks compared with NiCd batteries, NiM suffers from a higher self-discharge rate, is less robust to overcharging and has a more complex charging process.
Compared with nickel-based batteries, Li-ion batteries support a higher C-rate, higher energy density and longer cycling lifetime. In addition, Li-ion cells offer the advantage of a high average operating voltage of 3.6 V. Li-ion batteries also have considerably lower self-discharge rates than nickel-based batteries. They also do not suffer from the memory effect and are less capable of delivering large currents, expressed in C-rate, than nickel-based batteries. Over-discharging Li-ion batteries leads to a decrease in cycle life. Without further precautions, overcharging Li-ion batteries leads to dangerous situations and may even cause a fire or an explosion of the battery. Hence, it can be generally stated that overcharging and over-discharging of Li-ion batteries is not allowed.
Based on the used cathode, LiFePO4, LiMn2O4, NCM and Li-ion batteries have varieties that propose different performance levels when it comes to charge rate, safety, cost, charge, discharge and environmental impact. Applications include notebook computers, cellphones and EV batteries.
The performance of power batteries is essential for market acceptance. For example, when it comes to EVs, energy density is a key factor. Li-ion batteries have higher energy density, power density and lifetime and are more promising for the future compared with nickel-based batteries. There is ongoing research to improve different aspects of the batteries, namely on cost, charge rate and safety.
There are many other aspects to consider when choosing a battery, and it is best to be advised by an FAE or an expert. You can always reach out to your local BMS and battery expert at Arrow Electronics to help you out.
Hazards to be protected from
BMSes are made to regulate and monitor the charge and discharge of batteries. There are several characteristics to be monitored, including temperature, current, voltage, battery type, isolation in high-voltage systems, state of charge (SOC), state of health (SOH) and extreme high-current flow. All of those monitored values are necessary for the tasks of a BMS. In principle, a BMS is suited to maximize SOC, optimize SOH and protect the battery against deep discharge and overvoltage by keeping the values inside the given window, as shown in Figure 1.
Figure 1: Operating window for a Li-ion cell (NMC)
Over- and undervoltage protection (cell balancing)
In a multi-cell battery, the cell with the lowest charge determines the capacity of the entire system. As shown in Figure 1, the battery will suffer irreversible damage if the voltage drops below or rises higher than the threshold voltage for which the battery is designed. In case of a lower voltage, the anode copper dissolves. In case of a higher voltage, lithium plating will occur, and if the voltage rises even more, the cell will start outgassing and ignite.
Cell balancing is normally performed by an integrated circuit (IC) with high-precision analog-to-digital converters. The main types of cell balancing are active and passive balancing. In active balancing, a higher charge of a single cell can be transmitted to another single cell, while in passive balancing, the charge is dissipated with the help of a resistor. The individual cell controllers can perform specific, particularly energy-saving housekeeping functions, such as periodic cell measurements and condition analysis necessary for functional safety, independent of the main BMS controller. Safety functions for signaling over- or undervoltage are triggered autonomously.
Over-discharge protection/low-voltage cutoff
Over-discharge protection, also known as low-voltage cutoff, is an important safety feature that many, and normally all Li-ion, battery packs have. It is meant as a protection against a voltage drop below a certain level.
The consequences of a deeply discharged battery are diverse, but in nearly all cases, it leads to irreversible damage. For example, reduced life-cycle performance or even thermal runaway can then lead to fire.
Hence, different cell chemistries have different safety operating areas. In general, we use the IC to determine the safe operating range and provide the necessary protection for the cell/pack in the application.
Short-circuit protection
Overcurrent protection is needed when a short-circuit occurs on the battery. This leads to extreme discharge behavior; hence, there is a high-current flow, the battery heats up rapidly and a thermal runaway event occurs.
There are three ways to protect the battery: thermal cutoff, pyro fuse and circuit breaker. BMS manufacturers can use one or all features in one system, depending on the required safety level. The thermal cutoff kicks in when the battery pack reaches a certain temperature level. In high-voltage systems like EVs, this feature is normally activated by a digital processor, whereas in low-voltage applications, it is possible to implement this protection to be triggered by itself based on a predefined threshold.
In environments where humans can be harmed, protection against fire and explosion is especially important. Therefore, a digitally triggered pyro fuse comes into play. The fuse is connected to the high-voltage path on either minus or positive or on both. The fuse gets triggered as the last defense line to prevent significant damage to the battery.
In special environments like trucks, where service continuity is important, we tend to use more elaborate and expensive solutions. A circuit breaker based on back-to-back SiC MOSFETs is one possible way to protect the battery pack system from damage in case of a short-circuit. The drawback of this solution is the price and size. The functionality is the same as a pyro fuse, but it can be turned on after the event.
Overcurrent protection
As explained earlier, the cells get balanced with the help of a current flow. Depending on the charge capabilities of the battery, this current flow is in the range of 100 mA and 500 mA. The overcurrent protection is a specific current limit that the balancing IC shall not exceed. Mostly, this limit can be set individually and helps to protect the battery against irreversible damage, fire or explosion.
The current consumption depends on the ambient temperature, which should be considered when defining thresholds. Furthermore, it is necessary that the current-level limit is set below the real battery current-draw level. Normally, the level is charged with an additional safety factor of 2 to 3. In case of small current fluctuations, false triggering of the BMS overcurrent protection can occur. To prevent the system from doing this, some BMSes have a feature called hysteresis and digital filters.
Thermal runaway protection
Batteries can support temperatures as high as 60°C based on the used chemistry. The temperature of a hot cell can spread to the neighboring cells and the entire battery pack can heat up in no time. The heat triggers a chain reaction that can set the whole battery pack on fire through different chemical reactions that can release inflammable gas.
The thermal runaway protection gets triggered when the predefined temperature threshold is met. It shuts down the battery and prevents the battery from going into the thermal runaway.
The primary role of a BMS is to protect and communicate the status of the battery. The variety of hazards to be protected from is huge. The safe operating area in green (Figure 1) illustrates the limited conditions in which a battery can be used. We need to make sure that the battery pack is not used outside of the design operating range. Complex and safe mechanisms are necessary to keep a battery in this area and to make it safe for humans. The software portion used in a BMS is a huge part of the design and needs to be considered in the early project stage. There’s a cost associated with the choice of IC, architecture, software and battery packs, and without deep knowledge, it’s hard to make the “best” choice. Don’t hesitate to reach out to Arrow if you need any advice or support.
Article written by Ulrich Lentz, he is a technology field application engineer at Arrow Electronics.
References
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- 6 Bergveld, H.J., Kruijt, W.S., & Notten, P.H.L. (2002.) “Battery Management Systems”