When designing a custom battery pack, a topic that will come up between the manufacturer and the customer deals with the fuel gauge. The battery fuel gauge can be found in a range of everyday devices that we use, such as cell phones and computers. The function of the fuel gauge is to inform the customer on how much state of charge (SoC) and state of health (SoH) is left in the battery. The fuel gauge can predict the remaining capacity by measuring the voltage, battery temperature, and current.
Correctly monitoring the battery can allow a customer to determine when the battery pack will need to be replaced. It can also help you evaluate whether there are certain environmental factors or operating conditions that could be negatively impacting the battery's life outside of normal application requirements. The correct measurements can determine whether a more powerful battery will be needed for the application's systems or if certain functions of the application could be adjusted. Then, the correct battery pack can be designed that more aligns to the power needs of the product or equipment.
Electronic Components Featured in a Fuel Gauge
A battery fuel gauge consists of an integrated circuit with components that will perform certain functions based on what is going to be measured. Some batteries do not require the highest amount of accuracy, so they will not need all the same features as a more robust high-accuracy gauge. The following list of components are some found on a Texas Instruments bq26220 fuel gauge:
- An integrated circuit with configured algorithms
- Coulomb counter
- Internal temperature sensors
- Input/output ports
- Analog to digital converters
- High-speed data queue (HDQ) serial interface
- Data storage (RAM, ID ROM, and flash-backed RAM)
- Internal time base
Fuel gauges will also connect to displays outside the battery pack so that the user can see the measurement's readings. This display may consist of an LED color-coded bar or circles that will light up. Under each circle will be numbers that represent the SoC broken down in percentages from 100% to 25% or even lower. There will be a test button on the display to test the batteries SoC. If the battery lights up to 75%, it means it is 75 percent charged.
Correctly monitoring the battery can allow a customer to determine when the battery pack will need to be replaced.
Different Types of Fuel Gauging Options
The fuel gauge for the battery pack can be designed using several different types of options depending on the algorithm that is used. This factor ensures you are getting the desired readings based on the application and the type of battery chemistry that is used.
Types of algorithm options include:
- Voltage Lookup: This method involves the fuel gauge determining the SoC using the battery voltage. This measurement is then compared to the internal lookup table. The internal lookup table consists of an array of data, in this case the voltage ratios, programmed into the IC.
- Coulomb Counting: Coulomb counting method consists of measuring the energy that flows in and out of the battery. This method is often used for high powered devices such as laptops and medical devices.
- Voltage Lookup/Coulomb Counting Combo: This method combines the two previous ones. A reason this option is chosen is to gain all the advantage of each method while lowering the number of accuracies by providing more measurement balance.
Some innovative battery management systems come designed with a learn function that helps to adjust the SoC readings so there will be more accurate measurements. This type of learn function is most often seen with fuel gauges using the voltage lookup option. For BMS, using the Coulomb counting option, the learn function will estimate the battery energy that was delivered to the application during the previous discharge cycle. This feature helps to deal with calibration issues.
Fuel Gauges for Battery Chemistries
Fuel gauging can be used for lead acid batteries, which are most often found in electric vehicles as well as wheelchairs and other motorized mobility devices. The fuel gauges are also common for lithium-based chemistries, including lithium-ion, lithium polymer and lithium-iron phosphate (LiFePo). Nickel-based batteries, such as nickel cadmium (NiCad) and nickel metal hydride (NiMH), can also be designed with fuel gauges.
In addition, fuel gauges can work with single cells as well as battery packs that can contain from 2 up to 4 cells. The type of method option used with battery packs will depend on the chemistry. Almost all types of battery chemistries can use the voltage method with varying accuracies. However, it works well with lead acid batteries that can rest. The Coulomb counting methods works with many battery chemistries yet is a good selection for lithium-based batteries.
Fuel Gauge Calibration
Fuel gauge accuracy is a major issue with battery packs. Inaccuracies are not predictable because there are so many things that can impact the measurements, from internal and external temperatures as well as battery aging. Cell materials can also cause problems and errors. Many battery pack manufacturers can be leery about placing fuel gauges on devices that require battery packs due to these inaccuracies and the dissatisfaction that may occur to their customers.
A customer typically expects the fuel gauge to always be correct when displaying the SoC. If it is inaccurate and the battery constantly needs recharge or no longer adequately holds its charge, a customer may find themselves no longer purchasing that product. In addition, certain applications, such as aerospace, medical, and military equipment, require highly accurate and precise fuel gauge ranges. If the measurements are too misaligned, the errors could cause malfunctions during critical times.
Calibration of the fuel gauge should be done to determine the accuracy of the measurements for a newly inserted battery. When designing fuel gauges, measurement accuracy is determined by taking a fuel gauge that has already been properly configured and measuring the voltage, current and battery temperature. Then, the measurements will be logged during periodic intervals. Another way to determine the fuel gauge’s accuracy is to perform computations across the entire charge and discharge cycles of the battery. These calculations involve determining the past charge of the battery, the true full-charge capacity of the battery, the remaining capacity in the pack, the true SoC of the battery, and the reported SoC error of the fuel gauge. Then the battery's SoC computations can be compared with the fuel gauge's SoC data to determine accuracy and the correct calibration settings. Designing the fuel gauge with impedance tracking features can often eliminate the need for calibration. Impedance tracking is a self-learning algorithm.
Recalibration of the Battery Fuel Gauge
Fuel gauges can go out of calibration during extended use and long periods of time. Since the products are no longer within the control of the manufacturer as well as the company that may be offering the battery pack in its products to consumers and other businesses, establishing recalibration processes can help end users reset the fuel gauge so it can continue to provide the best accurate measurements. Calibration can be accomplished by having the device run down the battery pack until it reaches its low battery settings.
While it is possible to design smart battery packs that self-calibrate during full discharges, this is not always feasible when used in the real world setting due to how much end users will work with the application, how often they will recharge the battery pack before it reaches its low battery state, and if the battery will be fully recharged or just partially recharged. Offering a battery charger with a discharge function included with the battery pack can enable the calibration of the fuel gauge for smart batteries.
How often the fuel gauge will need calibrated will be dependent on the battery pack's usage. For continual use products, calibration can be down every 3 months or when reaching a certain number of battery cycles, such as 40 partial cycles.
When it comes to designing fuel gauges for a custom battery pack, understanding the power needs of the application, the battery pack's operation, and whether a fuel gauge is necessary for the end user can help determine the types of electronic components and functions to place into it. If costs are a big deciding factor, keep in mind that the more functions and higher accuracy measurements designed into the fuel gauge, the higher the costs will be. Also know that for highly accurate fuel gauges, more components will be necessary. The space requirements can impact the amount of space that will be available for the batteries and increase the size of the battery pack as well as the dimensions of the application equipment or device itself.