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Design Articles

Smart Battery Management Considerations for Portable Applications

New battery technologies and leading-edge applications are constantly pushing the limits of existing battery management systems. Systems that can currently support the latest trends may end up being outdated and insufficient to support future applications.

By Ravi Pragasam, Senior Manager, Fusion Product Marketing, Actel Corporation

Portable applications need the support of battery management systems to ensure that the productivity of batteries is maintained and to deliver the best power profile over the batteries’ lifetime. In most applications today, batteries need to be replaced often and a system that can offer a means to have efficiently managed so as to prolong its life can offer several benefits. In addition to a lower overall cost since the consumer will not have to continually purchase new batteries, prolonging battery life means fewer battery replacements, which in turn means less waste.

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Increasingly, consumers acknowledge the need for smart battery management from a cost and environmental impact perspective. Laptops, cell phones and handheld devices like cordless drills and vacuum cleaners require longer run times, longer lifetimes and more end-user confidence in the battery information. And, proper handling of the battery results in the longest possible life for that battery. One of the biggest challenges for typical management schemes has been the ability to recognize and support batteries of different technologies and chemistries.

Traditionally, many designers believed battery optimization could be achieved using a standardized smart battery that includes all the necessary electronics to monitor itself and communicate to the greater system. However, today, the smart battery concept defines interfaces, a data set and behaviors of the smart battery, battery selector, smart charger and host elements in a smart battery system. A smart battery management system generally contains an analog monitoring chip whereby the voltage, current and temperature of the battery cell need to be measured; a digital controller chip, which is used to issue the right commands to implement the re-charging function; various discrete diodes, transistors, passive components; and a redundant safety monitor chip.

Smart battery management systems typically use off-the-shelf components to support a wide variety of battery types (Figure 1). However, new battery technologies and leading-edge applications are constantly pushing the limits of existing battery management systems. Systems that can currently support the latest trends may end up being outdated and insufficient to support future applications. The answer to this could be the use of a flexible and programmable platform that can be repurposed to support next-generation applications without any major re-haul of the design.

Certainly, when choosing a smart battery management approach, a number of factors should be considered, including cost; other required external hardware, such as temperature sensors and stable oscillators; level of standardization or, conversely, flexibility and programmability; development support; and the power consumption of the monitoring circuitry.

Smart Battery Management Functions

figure 1
Figure 1: A typical Smart Battery implementation using off-the-shelf components
A smart battery management controller must be able to measure the voltage of each individual cell within the battery pack and should be able to sense the status of the application. An off-the shelf smart battery management system could use independent voltage, current and temperature monitoring components to monitor the various physical parameters to gauge the status of the cell. Part of the function of the analog blocks within the system is to sense the application status and take the required action.

The system also has the responsibility to ensure that it places the application in a stand-by or sleep mode when the application is not being used to conserve power and extend battery life. For example, if the portable application is in a stand-by or shutdown mode, then the battery management system needs to make sure that the correct amount of charge is available for the operation of the application without any loss of content.

Another function of a smart battery management system is to ensure that when the application is placed in a stand-by or sleep mode, the current state of the application is saved someplace so that when the application wakes up from its sleep state, there is no loss of state and the application starts up where it left off. Off-the-shelf battery management systems use external memory devices to store the contents of the system when it is placed in a sleep mode.

The battery management system can also be used to scale voltages based on the different inputs to the system. To implement the different functions above, an off-the-shelf system can use voltage scalars that can handle a wide variety of voltages.

One of the greatest advantages of a smart battery management system is the power-management possibilities it offers to a system engineer. Using information from a smart battery management system, you could cater sensitivity to the reported battery state. If you know a battery is empty, you could design the system to apply the full charge current. Once the battery fills up, the system could increase the sensitivity to identify the end-of-charge point very closely.

A Traditional Approach to Smart Battery Management

A traditional smart battery management approach would be to use discrete off-the-shelf components to implement the application.

Typically, an analog-to-digital converter (ADC) performs the task of converting the analog functions into a digital format. The physical parameters that are measured such as current, voltage and temperature are converted into a digital form and are then processed by a microcontroller to enable a decision based on the system status. If the voltage is not within a specified range or needs to be maintained within a specific range, this information is then relayed back to the battery management system to ensure the correct operation of the portable application.

An off-the-shelf ADC can be used for this purpose with a resolution of 12 bits and an accuracy of about one percent or less. The same process is in place when the current is being measured or the temperature is being measured. There are available off-the-shelf current and temperature monitors that can be used to provide the appropriate current and temperature measurement to the ADC, which in turn performs the conversion and provides the information for further processing by the battery management system.

After processing, the decision, which is in digital format, is then converted back into an analog format that is used to control a physical parameter on the outside like turning the charge capacitor on or switching the system to back-up. When there is a need to save the application state before going into a sleep mode, the microcontroller saves the current state of the application in a memory device before shutting down. When the system is awakened, the state is retrieved from the memory device and loaded back into the system so that upon wake-up the system starts up from where it left off.

Once the information about the amount of charge is obtained, the next phase of the application is to decide the action that needs to be implemented that is based on a pre-determined application. The decision could be to place the application in a sleep mode since there is no activity and the charge needs to be conserved. Or the decision could be that there needs to be a shutdown process since there is an emergency and the application has exceeded some pre-determined limits. Or the decision could be to supply more charge to the system as the power has been drained in the system. Lastly, the decision could be to switch the system to a back-up system since the generic system has now been depleted of its power. Off-the-shelf microcontrollers are generally used to implement these decision-making steps that are then converted back to an analog format and output to the system.

When the individual functions for the smart battery system are implemented using discrete components, it could add several devices based on the complexity of the application. As additional functionality gets added in and the system grows in device count, the system design becomes more and more complicated. But this still does not address any feature changes or functions that could be added in the future to make the system scalable. Microcontrollers offer some feature integration, offering built-in analog inputs, ADC and DAC, clock circuits and a CPU core to make decisions. But when it comes to scalability, a microcontroller may not have the programmability and flexibility that is needed to support this requirement

Using PSCs for Smart Battery Management

Another approach to solve this problem is to use a platform that offers integration along with flexibility and scalability. Programmable system chips (PSCs) are becoming more and more ubiquitous, and with their ability to offer programmability and flexibility, they are achieving greater prominence in applications that have a need for upgrading in the future. An ideal platform would be one whose features include both analog and digital functionality along with the ability to add intelligence in the form of a soft processor core, it may support all the features needed for a smart battery management system.

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Figure 2: Fusion device architecture overview (AFS600)

One such option is the Fusion PSC as shown in Figure 2 that offers an analog block with multiple analog inputs that interface with a 12-bit ADC with the optional pre-scalars for voltage scaling. The analog block also includes several monitoring functions such as voltage, current and temperature. The PSC also includes several clock features to implement wake-up and stand-by features. With the addition of an optional soft processor core, intelligence can be achieved that can be used to prompt the system to transition into a sleep mode wake-up from a sleep mode when needed.

Leveraging a PSC solution provides the required programmability and flexibility to keep pace with changing battery technologies and emerging applications, but also enables overall board space reduction. With the ability to integrate several discrete components and functionality into a single chip, along with the battery management function, designers can realize tremendous board, power and cost savings that can’t be achieved using a discrete approach.

Summary

When selecting a solution to implement a smart battery management system, it should be ensured that the system is able to offer the consumer the ability to prolong the battery life of the portable application while providing the option to be upgraded for future enhancements with no major design revisions and with less additional cost. While traditional off-the-shelf components offer an adequate solution, newer technologies may offer higher integration and flexibility and need to be considered as a viable solution. Smart battery management will continue to play an important role in portable applications and, with new options available, designers can easily implement a highly efficient solution.

Actel Corporation
Mountain View, CA
(650) 318-4200
www.actel.com

This article originally appeared in the July, 2008 issue of Portable Design. Reprinted with permission.

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