Figure 1: Battery pack selection guide based on function and series battery section configuration
When choosing a lithium-ion battery pack for a portable application, whether it's a smart watch or an electric bike, the task is not as daunting as it seems. Lithium-ion battery packs have three basic functions: protection, monitoring, and fuel gauge. These three functions are typically handled by a single device; however, some devices are capable of implementing multiple functions with a more integrated solution. In fact, a battery pack consisting of the right components and these three features will improve system performance, provide more protection, and enable longer run times. The first part of this article will delve into battery protection. Along with the information given below, you must take a look at the selection guide in Figure 1 to help you choose the right device based on the size of the battery, where 1S means 1 = 1 series, 2S = 2 series, etc. Wait.
protection
Battery pack devices contain a variety of features; however, protection is their primary function and should be a primary consideration when selecting a device. There are two different types of protection:
• Voltage protection consists of internal comparators that continuously measure the voltage of individual battery sections and compare them to overvoltage (OV) and undervoltage (UV) thresholds. Voltage protection is considered a first-class protection and is a must-have protection feature of any lithium-ion battery pack.
Current protection works like voltage protection, but it has a sense resistor that calculates the current flowing through the battery pack and compares this current value to the current threshold to detect overcurrent or short circuit conditions based on polarity. If the battery current exceeds the threshold, then a charge (CHG) field effect transistor (FET) or discharge (DSG) FET is turned on to disconnect the battery and prevent further damage.
Protection functions are not always limited to voltage and current aspects; for example, some high battery section applications require Open Circuit Detection (OCD). This type of protection confirmation monitor is always connected to each battery. Battery monitor devices such as the bq76925, equipped with OCD, are very common power tool applications.
When choosing a protection feature for your battery pack, consider:
· An independent protector (only hardware protection is provided).
• A monitor with hardware protection.
• Fuel gauge with firmware protection.
- A combination of the above three.
Choosing the right component for your battery pack with the required protection will depend on your application requirements.
For redundancy purposes, a simple protector such as an OV protector provides the last protection function. Applications with secondary protection include laptops, electric bikes, electric scooters, and electrician tools. In some single-cell devices, such as smart watches and mobile phones, there is no need for a monitor because there is only one battery. In these single-cell applications, a fuel gauge with built-in firmware protection is a good choice. If you have a soft spot for firmware protection, then a simple fuel gauge paired with a single-cell battery protector, such as the bq29700, is more appropriate.
There are a few points to note between hardware and firmware protection. Both types of protection are valid, but when choosing any of these types of protection, you must weigh and make trade-offs. Firmware-based protection is common in fuel gauges or MCUs, while hardware protection is often used in stand-alone protectors or monitors, but several fuel gauges also have hardware protection. The main difference between the two is that the hardware protection actually uses internal circuitry (such as a comparator) to compare internal registers or save the threshold value of electrically erasable programmable read only memory (EEPROM) to view Has it failed? Firmware-based protection uses the internal algorithm of the fuel gauge to compare the measured values ​​of the battery section with the thresholds written in the code. Sometimes there are two types of protection to achieve redundancy.
A lithium-ion battery pack requires protection to keep the application running longer while maintaining battery safety. A good protection solution can help your system run by preventing worse conditions such as reduced battery performance or explosions. The following sections will discuss in depth the monitors and fuel gauges and their role in the battery pack. Describes the other two components required to create a safe and healthy battery pack: a diagnostic monitor that collects diagnostic information for each battery in the battery pack, and a monitor that provides protection, as well as a fuel gauge that intelligently calculates battery state of charge and health.
Monitor and fuel gauge
Monitor (Analog Front End (AFE))
Since the protection function is the primary function of all battery pack electronics, general battery pack devices are equipped with certain levels of protection. For example, a battery monitor (Analog Front End [AFE]) is designed to implement the following features, or a mix of features:
• Measure single cell voltage, battery current, and battery temperature (measured by a separate temperature sensor).
• Drive an integrated LDO that powers other components, such as a fuel gauge or microcontroller (MCU).
• Drive a battery balancing network to ensure that each battery in the battery pack remains consistent while charging.
• Drive the charge (CHG) and discharge (DSG) FETs to protect the battery pack.
· Provides hybrid hardware protection including voltage and current protection.
As you can see, the monitor provides key features for the battery pack. These features make the monitor extremely important for the battery pack; however, it is important to note that not all applications require a separate monitor. Some applications with a lower number of battery segments (less than 5 batteries) do not use a monitor; instead, an all-in-one fuel gauge (protector, monitor, and fuel gauge) that includes monitoring functions such as the bq40z50-R1 is used. In addition, all monitors require some form of host, MCU, or fuel gauge to control them and receive communication information such as battery section measurements.
Fuel gauge
Now let's talk about what the fuel gauge is and how to choose a fuel gauge. A fuel gauge means to measure. In other words, its job is to estimate or determine the magnitude, amount, or size of certain parameters. In a battery pack, the fuel gauge calculates two things: state of charge (SOC) and state of health (SOH). The SOC is the amount of power remaining in the battery to support the system's operation, while the SOH is a measure of battery life and determines if it needs to be replaced. Then, the fuel gauge will know when to prompt the system to pay attention to these values, so that the user can take the correct action. In selecting the battery timing, you can take any of the following 3 ways, you can choose:
· An all-in-one fuel gauge (equipped with a monitor (AFE), and some types of protection)
• A discrete fuel gauge that does not perform any monitoring or protection functions.
· An MCU.
A discrete fuel gauge is a device with special firmware that performs charge metering activities. Choosing an MCU instead of a fuel gauge will depend on your experience with the development of fuel gauge algorithms, or whether you prefer to use accurate, ready-to-use, easy-to-use devices. TI has a large portfolio of products that support different algorithms, each with its own level of complexity and precision. For example, our Impedance Track algorithm provides very precise accuracy with an error rate of 1%.
The last thing you need to know is the role that a discrete fuel gauge plays on the monitor within the battery pack. In this case, the fuel gauge is the brain that controls the monitor and performs operations such as setting a protection threshold or driving a protection FET.
All battery packs have three basic functions: protection, monitoring and fuel gauge. The battery pack can contain a single-chip solution (such as the bq40z50-R1) or a multi-chip solution; both have their own advantages and disadvantages. The protection function in the battery pack may be a stand-alone device or included in a fuel gauge or monitor. Hardware and firmware protection work together for redundancy purposes. The monitor relies on the host to communicate, and a battery pack requires a fuel gauge or MCU with a certain fuel metering algorithm.
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