A BMS collects data, calculates vital statistics, and controls battery function
All batteries work safely and effectively within certain optimum conditions. Using them outside these conditions can shorten productive life and create hazards. For instance, overcharging a battery may both reduce its capacity long-term and be dangerous to users. However, the outside appearance of a battery tells you very little about its condition, making it difficult to monitor the battery’s health.
A battery management system (BMS) solves this problem by monitoring battery vital signs and ensuring safe and reliable battery operation. Even very simple battery-powered electronics use BMS, as evidenced by the ubiquity of the fuel gauge—an indicator that displays a battery’s state of charge or remaining capacity. Other BMS functions can include:
- Temperature sensing and control
- Current monitoring
- Voltage monitoring and balancing
- Charge control
- State-of-health determination
By performing these tasks and communicating information to the user, a BMS maintains a battery in safe operating conditions that promote a long useful battery life.
Let’s dive into the details of BMS function and design. We’ll focus on the three major tasks of a BMS: data collection/input, computation, and output/control.
Aved designs, tests, and manufactures battery packs to fit our customers’ specific needs. Whatever your application, Aved can provide battery packs with the BMS features you need, including thermal management, charge control, voltage balancing, and more. To learn more, contact us or request a quote today.
BMS sensing lines—voltage, current, and temperature
A BMS is a computer. It takes data inputs, performs calculations, and performs tasks or outputs information. The first step of this is the collection of useful information.
A BMS relies on a variety of data inputs. Here, we’ll discuss three of the main statistics commonly monitored by a BMS: voltage, current, and temperature. We’ll also look at circuit diagrams to show how these sensors fit into a BMS. Different BMS have different combinations of sensors depending on their needs.
This conceptual diagram of a BMS for an electric vehicle’s Li-ion battery shows how battery management safeguards a battery. Fundamental to a BMS’s function is the input of data such as current, voltage, and temperature. Image source: Battery Operated Devices and Systems.
Voltage
Used to calculate many other important statistics, voltage is a key vital sign for a battery. Maintaining a battery in optimum voltage conditions is paramount. For this reason, a BMS almost always has at least one voltage sensor, with contacts at each terminal of the battery pack.
A BMS serving a multi-cell battery pack may be designed with voltmeters for each cell in addition to a voltmeter for the whole pack. Multi-cell packs can have different voltages (and therefore different states of charge) in different cells. Measuring local voltage helps the BMS diagnose faults, balance cell charges, and calculate changes in internal resistance.
Current
Another major battery vital statistic, current is often key to understanding how much capacity remains in a battery pack. Capacity is measured in is milliamp-hours (mA-h), so an ammeter is a core part of a BMS.
The precise design of the ammeter may vary depending on the application. Applications where current usage varies greatly (such as an electric vehicle) need high-resolution and high-precision ammeters to detect all the variation. Other applications may need sensors that don’t waste any power. Depending on the need, one of two kinds of ammeters is commonly used in BMS—shunt resistors and Hall effect sensors.
Shunt resistors are very simple ammeters. To measure current, a resistor called a shunt with a precisely-known resistance is placed in the circuit. The voltage across the resistor, which is proportional to current (recall Ohm’s law), is measured. An analog-to-digital converter (ADC) voltmeter transforms the voltage into an electrical signal and transmits it to the BMS. Shunt resistors have the disadvantage of wasting power, which can be undesirable in low-voltage applications.
Hall effect sensors measure the magnetic flux created by a conductor carrying a current. Unlike shunt resistors, they don’t alter the circuit they measure. However, Hall effect sensors are sensitive to external magnetic fields and can be less precise.
Temperature
As we discussed in our article on thermal management[PD1] , temperature can have profound effects on the function of a battery. High temperatures increase voltage but reduce the battery’s cycle life, increase self-discharge, and can lead to hazards like thermal runaway. Cold temperatures cause sharp drops in cell voltage.
One function of a BMS is to manage temperature. To that end, many BMS have thermistors installed. Very similar to a shunt resistor, a thermistor uses a material with a precisely-known resistance and whose resistance varies with temperature. By measuring the voltage across the thermistor, temperature can be determined.
See Our Article on Thermal Management
BMS computation—state of charge and state of health
Voltage, current, temperature, pressure—these are all just numbers. Before a BMS can actually manage its battery, it needs to compute. A BMS transforms the signals it receives from its sensors into useful statistics that can be displayed to the user or compared to optimum operating conditions to make management decisions.
This computation takes place (you guessed it) in some kind of computer. This can simple, with just a circuit board with logic gates. Or it can be very sophisticated, with programmable microcontrollers. BMS circuitry can be purchased with integrated voltage sensing lines, voltage protection options, cutoff mechanisms, and more.
At the most basic level, a BMS compares an observed value with an acceptable value to determine whether the battery is within acceptable operating conditions. In this way, it can detect whether the battery is over-voltage, over- or under-temperature, over-current, or experiencing a short circuit.
But other more complex calculations can be valuable. Let’s look at two of the most common calculations carried out by a BMS—state of charge and state of health.
State of charge
Displaying state-of-charge (SoC) is one of the most common BMS functions. Many electronic devices have fuel gauge displays that show how much battery capacity remains. But calculating SoC is easier said than done. Again, batteries are like black boxes—you can’t look inside to see what’s going on. There are two main methods of calculating SoC.
The first is with the voltage method. All battery chemistries have characteristic voltage curves, which show the change in a cell’s voltage over time as it discharges. Sometimes, the voltage curve can be used to calculate state of charge. This is often impractical, though. Accurate SoC determination with voltage requires an open circuit and a rested battery. This prevents accurate readings during use. Additionally, modern battery chemistries such as lithium-ion have extremely flat voltage curves, impeding SoC calculations.
The second method, sometimes known as coulomb counting, measures current over time to calculate SOC. This method doesn’t suffer from the challenges of the voltage method, but it can lose accuracy as the battery ages and experience self-discharge due to temperature. This requires calibration.
State of health
State of health (SoH) is a parameter describing a battery’s quality or health relative to its peak health. This is measured as a percentage of SoH when the battery was first manufactured.
Different BMS may use various combinations of data points to compute SoH, including battery age, number of discharge cycles, rate of self-discharge, capacity, internal resistance, and past temperature usage.
BMS outputs—communication and control
A BMS gathers information, performs calculations, and finally outputs actions and information. Depending on the need and the level of sophistication, a BMS can take a very active role in controlling a battery pack, including isolation, charge balancing, thermal management, and information output.
FETs are cutoff switches that isolate the battery from the charger or load
Sometimes, charge or discharge conditions become hazardous and a battery needs to be disconnected from the charger or the load. An electrically-controlled switch called a field-effect transistor (FET) allows a BMS to disconnect the battery if needed.
FETs have three terminals—the source, the drain, and the gate. Current flows between the source and the drain. Applying a voltage at the gate modulates the conductivity between the source and the drain. Depending on the layout of FETs in a circuit, a battery pack can be charged and discharged simultaneously.
FETs are electrical switches that can be used in BMS to isolate a battery from the load or the charger in the event of hazardous conditions. By applying a voltage to the FET gate, the conductivity between the source and drain can be opened and closed. Image sources: Wikipedia (left) and Nuts and Volts (right)
Voltage balancing (wasting or shuffling)
Because of internal differences, cells wired in series can acquire different states of charge over time. When a BMS determines that cells are unbalanced, it can balance the charges. There are three ways of doing this: bypass charging, wasting and shuffling.
During a charge cycle, a BMS may detect that one cell is charging faster than the rest. To balance the charges, FET bypass parallel to the cells can be used to temporarily pause charging on one cell while the rest catch up.
This BMS circuit diagram has FET bypasses for each cell. In the event of charge imbalance, the BMS can pause the charging of overcharged cells while the others catch up. Image source: Electronic Design
When one or several cells are overcharged relative to the rest, a simple way for the BMS to balance the charges is to waste the energy. Excess charge is directed through resistors and dissipated as heat. On the one hand, this method is inherently wasteful. On the other hand, it is passive and non-parasitic.
The alternative to wasting is charge displacement or shuffling. SOC can be equalized across cells by shuffling energy from cells with more charge to cells with less charge. On the surface, this can be less wasteful than charge dissipation. But charge displacement is a more complex process, so it may be more cost-effective to use a BMS that wastes charge rather than redistributing it.
Thermal management system control
When a thermal management system (TMS) is present, the BMS may modulate it to control temperature. In response to a high- or low-temperature threshold being crossed, the BMS can switch on a cooling fan, turn on a pump for liquid cooling, or switch on a heater, depending on what kind of TMS is in place.
Outputting information to the user
A BMS can’t do everything. Humans have to make decisions about battery usage, and they need information to do that. Some kind of data output function is present in almost all BMS systems. At the most basic level, a small display showing SoC is commonly used. At the most complex level, a smart BMS can export detailed information on current, voltage, cell and pack SoC, SoH, and more through a data bus to a separate user interface.
A well-designed BMS maintains a battery within optimum operating conditions
A battery management system is a component of many battery packs that collects vital information about battery health and function, performs calculations, communicates with battery users, and controls battery function. Because it ensures that a battery pack remains in optimum operating conditions, a BMS ensures both the safety of users and the longevity of the battery.
Aved can design, test, and manufacture battery packs ideally suited to your needs. We’ll select the right chemistry for your application, properly lay out cells and modules, and design a battery management system to protect your investment. To get started, contact us or request a quote today.