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Key considerations for an effective battery-management system design

Feature

By Ralf Hickl, Product Sales Manager Automotive Business Unit, Rutronik 

The demand for batteries is growing at an unprecedented rate, with the global lithium-ion (Li-ion) battery market set to more than double over the next five years, from $35bn in 2020 to $71bn in 2025.

From electric and hybrid vehicles through to portable electronics and industrial applications, Li-ion batteries are taking centre stage across growing swathes of our daily lives. Compared to other energy storage devices, they offer higher energy density and voltage, combined with smaller dimensions, more charge cycles and a longer service life. That is why ensuring that they run as smoothly and efficiently as possible is crucial; this is where battery management systems come in.

Careful consideration

Li-ion batteries are usually used as battery packs consisting of several battery cells. The primary purpose of a battery management system is to ensure that the cells stay within their specified operating range, and are charged and discharged as evenly as possible. Accurate measurements of the charge and discharge currents, cell voltage and temperature are therefore required. All these parameters must be carefully adjusted, both to each other and to the battery cells and the respective application. The battery management system also guarantees the functional safety of the battery, prevents any lasting damage and ensures optimum use of the battery and its long life.

Giving its critical role, designing a battery management system requires careful consideration, starting from the choice of the electronics components.

In a typical battery cell management system, the cells are often connected in series, requiring a resistor and a switching transistor to be connected in parallel for the purposes of balancing. Control of these switches is performed by dedicated balancing ICs that interact via a serial communications bus, whilst a microcontroller oversees the overall cell management system. A bidirectional electronic fuse (e-fuse) permits controlled disconnection of the battery from its charger or load, as required – for example as a result of a fault elsewhere in the system. The ability to sense flowing current is important in these systems, often performed by a small-value shunt resistor, although magnetic sensor ICs may also be used.

Energy cells

The energy storage capability of batteries is usually accomplished with Li-ion cells or Electric Double Layer Capacitors (EDLC), alternatively known as ultracapacitors. Li-ion cells are commonplace in automotive applications, but are also popular in other areas, for example e-cigarettes. The round cells in the common 18650 design (18mm in diameter, 65mm in length) offer the highest energy densities, mechanical stability and efficient assembly options. New versions of this popular cell are now being worked on, including the forthcoming 21700 (21mm in diameter, 70mm in length).

When it comes to ultracapacitors, it is recommended to consider products that exhibit low internal resistances, which means that cooling becomes unnecessary. Consequently, it is possible to reduce footprint whilst significantly reducing costs.

When it comes to e-mobility applications, it is also important to select ultracapacitors that are compliant with the latest standards, including ISO 16750-3 (Road vehicles – Environmental conditions and testing for electrical and electronic equipment – Part 3: Mechanical loads) and SAE 2464 (Electric and Hybrid Electric Vehicle Rechargeable Energy Storage System Safety and Abuse Testing).

Sensing technologies

Protection is important in batteries, and sensing forms an important role here, from calculating residual battery energy and measuring performance through to charging and during short-circuits. In electric motor applications, and others, the measurement must be bi-directional to allow measurement when the motor is operating in a regenerative mode.

Shunts are a simple and popular method for current measurement, although the power dissipated in the shunt increases with the square of the current, requiring a certain mechanical size to dissipate the generated heat. Although shunts are not affected by magnetic fields, they are a non-isolated measurement so, in some cases, the measurement electronics may require galvanic isolation.

Calibration of shunts is simple; however, the accuracy of the measurement is determined by the accuracy of the shunt and all associated tolerances and temperature coefficients.

An alternative method of current measurement is to use magnetic sensors based on the Hall effect. These are popular as they are non-intrusive measurements with no power dissipation. Some types also offer an isolated measurement, saving external components. They do, however, require a greater development effort as they have to be integrated both mechanically and magnetically, which adds complexity to the production process. As they use magnetic fields for measurement, they are also generally susceptible to strong stray magnetic fields.

Balancing the cells

Li-ion cells are affected by production tolerances and ageing effects. Cell-balancing is used to address this by monitoring cell voltages and distributing charging current so that every cell in a series string has the same state of charge.

Balancing can be either passive or active. In passive balancing, a MOSFET connects a resistor in parallel with a cell, causing some of the charging current to bypass the cell and allowing other cells to reach the same state of charge. The main downside to this approach is the power that is dissipated in the MOSFETs and resistors.

Active balancing distributes excess charge from one cell to others in the string using a highly efficient DC/DC converter to minimise dissipation.

Due to their sensitivity to overvoltages, ultracapacitors also require balancing, although they require a special IC to evenly distribute the entire voltage to individual devices.

Microcontrollers

A microcontroller is responsible for control and monitoring, including calculating remaining charge and reporting the corresponding charging time and/or range (in electric vehicles). The microcontroller also monitors all the sub-systems, and interrupts charging or disconnects the load if necessary.

Additionally, microcontrollers operate as diagnostic computers for impedance spectroscopy (DC and AC impedance measurement). As such they are able to provide critical information on the state of charge, the temperature and the general condition of the battery.

The microcontroller is also responsible for system security and for providing functional safety in automotive systems, so powerful multi-core processors that incorporate hardware security modules (HSM) are the most-commonly used.

Isolators

In some cases, it is necessary to isolate signals between the high-voltage and low-voltage side. This is where optocouplers come in. Where high common-mode voltages are present then pulse transformers are a viable alternative.

Interfaces and drivers

CAN is a popular automotive interface and there are a wide variety of transceivers available to operate over two-wire twisted pairs. Some of these have already received pre-approval from multiple automotive OEMs and are suited to data rates up to 5Mbps for the CAN-FD (flexible data rate) standard. They are available in a small TSON8 package (3mm x 3mm) with and without bus wake-up. CAN chokes are an important addition as they ensure signal integrity and remove interference.

Circuit protection devices

There are many types of protection device available, including conventional fuses, pyrotechnical disconnectors and semiconductor switches – all of which have a role in protecting battery circuits. Conventional fuses are one-time devices where the conductor melts in overcurrent conditions – they are low cost, but must be replaced once activated.

Semiconductor switch protection is generally formed from a pair of power transistors connected in parallel. Some of the latest motor control ICs for three-phase BLDC motors can be used as a three-channel electronic fuse. These devices contain three high-side gate drivers and analogue amplifiers that measure current with the use of shunts. Designed for automotive applications, including electric steering and starter generators, they can be used where functional safety is a requirement and often include several protective mechanisms and diagnostic functions.

Pyrotechnical disconnectors are another one-time use product for specialist applications. The power dissipation of a live heating wire in a squib causes a small propellant to explode, thereby separating the connection at a predetermined breaking point within a conductor.

Temperature gauging

Li-ion cells should only be operated within a fairly restricted temperature range. Operation outside this range can reduce the service life of the battery or irreparably damage it. In exceptional circumstances, thermal runaway may occur where all of the stored energy is released in fractions of a second, leading to fire or even an explosion. As a result, the operating temperature of Li-ion cells must be constantly monitored with sensors capable of detecting temperature with minimal lag. Temperature-dependent resistors (thermistors) are simple, robust, easy to use and ideally suited to this application.

An important role

As the global demand for batteries continues to grow, an increasing number of critical applications are becoming reliant on battery technology. Battery management systems have an important role to play when it comes to ensuring that batteries are reliable, efficient and durable. From sensing technologies through to microcontrollers and circuit protection devices, the key electronic components at the heart of a battery management system deserve careful consideration. Keeping up to date with all the technologies available and the latest standards is critical to ensuring that only components truly fit for purpose are selected so that battery management systems are correctly designed.  

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