Smart batteries for consumer systems

Electronic News, March 2, 1998 by David Heacock

Microelectronics

Recent articles, papers, and product press releases have announced new innovations in battery management circuits and in the concept of standardized batteries. Why all of this attention to battery management? Answering this question requires a closer examination of today's portable equipment marketplace.

With the push toward Pentium-class processing power in portable systems, designers must effectively manage the system power to provide an acceptable system run-time, which is typically more than three hours. The market is also demanding smaller form factors and lighter system weights. Responding to these demands, battery manufacturers have made significant improvements in battery technology. Unfortunately, these improvements are not as tolerant of over-charge and over-discharge as earlier NiMH designs. In fact, over-charge can be a potential safety hazard in some battery chemistries.

Now, for the first time, battery manufacturers must rely on semiconductor products to ensure that potentially hazardous conditions do not occur. Using these newer technologies requires electronic circuits that limit over-charge and over-discharge. These circuits effectively manage charge cycles so as not to stress the chemistry, and can provide state-of-charge indication throughout the use cycle. Benchmarq Microelectronics recognizes the need for these circuits, and offers low-cost embedded controllers with on-chip analog-to-digital converters and references, PWM circuits with inputs to allow current and voltage regulation, low-cost fast charge control, and cell supervisor circuits that allow designers to implement cost-effective battery systems.

By addressing the power-conversion issues associated with recharging the battery and providing multiple full-charge detection techniques, functionality can be maximized while minimizing costs. These circuits must provide reference tolerances typically better than one percent over the operating range, must be capable of discerning millivolt changes in battery voltage during charge and discharge, and must sense voltage levels in the micro-volt range across the series sense resistor for current measurement and battery capacity monitoring. In the past, this type of battery management system typically required multiple ICs and components (sometimes greater than 100 components) and cost between $10 and $25 to implement. Only the top-tier manufacturers had the engineering capability to provide these workable solutions. However, recently introduced battery management ICs have reduced the part count, and cut this cost in half for the complete PCB solution that includes LEDs and a push-button display switch. In the area of standardization, Intel and Duracell--reacting to the large forecast for portable equipment--began promoting a group of specifications called Smart Battery Systems (SBS) to standardize the battery management interface, data specification, and physical pack size. SBS utilizes an I2C-like bus called System Management Bus (SMBus) for communication between the various subsystems. Firmware developers such as Phoenix Technologies and SystemSoft are developing BIOS code based on SBS that allows succeeding generations of products to utilize the same interface while taking advantage of advances in battery chemistry and capacities.

SBS-compliant systems allow for multiple battery chemistries, since the charge control information is stored in the battery pack, and not in the host system. The SBS-compliant battery would simply notify the host of its desired charge criteria, automatically reconfiguring the charge controller. This methodology allows system designers to transparently support multiple battery technologies without extensive re-design. While custom interfaces and battery management schemes are still available and widely used, SBS is the first push for a unified industry standard for battery management. "Smart batteries" (battery packs with state-of-charge indicators) have highlighted what many larger OEMs have done for years, which is to manage system power more effectively and provide meaningful run-time and length-of-use information to users by accurately measuring battery capacity and charge/discharge current. While SBS has made headway into portable computing, it has not penetrated the mobile communications market. Recently, some battery management circuits have been introduced to this segment that meet the cost and accuracy requirements. With these new parts, displaying accurate talk time and standby time estimates is a real possibility.

For small handheld terminals and personal data organizers, rechargeable alkaline batteries, such as Rayovac's Renewal cells, enable designers to achieve the benefits of rechargeability at a lower cost to the consumer than primary alkaline. Compared to primary alkaline cells, rechargeable alkaline batteries typically provide a 10-to-one improvement in cumulative capacity at 100 percent depth-of-discharge and a 25-milliamp discharge rate. This chemistry benefits from no "memory" effect typical of NiCd systems and has negligible self-discharge. This makes this chemistry ideal for many consumer applications.