"Smart battery" is a broad term, used to describe battery packs containing anything from a ROM identity chip to integrated battery-management circuits that report the battery's charge state, monitor and control charging, and supervise the operating environment of the rechargeable batteries.
As the design requirements for modern portable systems have pushed the technology capabilities of rechargeable battery chemistries, smart batteries have become necessary. While materials and manufacturing advancements have historically pushed up cell capacities by about 3 to 5 percent a year, that rate of increase may not be sufficient to meet the demands for longer system run times.
Therefore, smart batteries have become the preferred technique among designers for improving battery-cycle life, delivering higher usable capacities and providing more predictable performance.
For example, battery-management systems for lithium-ion (Li-ion) batteries can inform the system of a cell-imbalance condition that would limit the discharge capacity of the battery pack. In that situation, while the battery-pack voltage would be within the normal operating range, the cell undervoltage limit would be reached, disconnecting the battery from the system load. Without an advance warning, the system could lose valuable information or fail at a critical time. A smart battery could employ a charging technique that limits the problem or adjusts the reported available capacity. The system could inform the user of a worn battery or adjust its operation to the lower available capacity.
Another smart-battery benefit is greater use predictability during charge and discharge. By closely monitoring the charge into and out of the battery, and by compensating that value against the current operating and storage conditions, a greater portion of the available battery capacity can be used during operation. That eliminates the need for a low-battery warning based on voltage and allows full system operation over a greater percentage of the discharge cycle. Reporting the time to full, time to empty and state of charge offers easily understood information and greater convenience to the system users.
Essentially, smart-battery operation can be divided into four functions: identification (ID), capacity monitoring, safety and charge control. The ID function can provide some or all of the information about the battery type: chemistry, capacity, cell manufacturer, pack vendor, pack configuration, battery self-discharge rate, any charge requirements or algorithms, and security ID. The system reads the information from the battery pack and then configures itself accordingly.
The ID function is useful in systems that employ multiple battery chemistries and pack configurations or that use products from multiple battery suppliers. By placing the information within the battery packs, field upgrades are possible by changing the data in the pack. The use of ID also helps limit the unauthorized duplication of the battery pack.
The capacity-monitor function may reside in the system or the battery pack. Most companies prefer that function in the battery pack since it simplifies the system design, although in some very high volume applications it could reside within the system. This function allows the system to report the remaining battery capacity to the user and to make predictions regarding the remaining run time or time to full charge. The added convenience of knowing the battery's state of charge enhances the system operation and improves the user's experience with the equipment.
Safety monitoring is necessary for most modern battery chemistries but is critical for the popular Li-ion and Li-polymer batteries. Safety monitoring for nickel-based chemistry consists of monitoring and reporting battery temperature to the system. In the case of Li-ion, it could consist of monitoring the individual cells within the battery pack. Here, the safety monitor limits the charge and discharge operation of the battery to a fixed set of parameters. That allows optimal capacity while eliminating unsafe operating conditions.
Charge control within the battery pack could consist of circuits monitoring the battery for a variety of charge-termination conditions. Those may be minimum current, peak voltage, delta temperature over delta time, maximum charge capacity or time, or some other physical attribute of the chemistry relating to a fully charged cell. The charge control may inform the system of a full battery condition using some type of interface-digital or analog-or may electrically disconnect the pack from the system. There are advantages and disadvantages to each method.
The digital interfaces are typically one- or two-wire and use a proprietary or industry-standard communications protocol. An analog interface uses voltage steps to represent temperature and battery identification. In general, analog interfaces are inexpensive, but they severely limit the amount of data available from the battery pack.
Smart batteries can be implemented via a variety of circuits and design techniques. For example, standard microcontrollers with internal analog-to-digital converters for building a smart-battery system are available from a number of sources. The firmware-development effort is typically extensive and requires expertise in very low-power design. Residing in the battery pack, this type of circuit could require more than 1 square inch of board area and consume more than 1 mA during operation. For some systems, the size, cost and development time are beyond the acceptable limits.
To address that problem, a number of semiconductor companies provide off-the-shelf battery management circuits that are designed for safety, charge control, battery identification and capacity monitoring.
Advanced integrated products for battery management should address all of the major functions for smart batteries. Implemented on submicron HCMOS processes, advanced battery-management circuits provide a cost-effective approach over standard microcontrollers.