Design Feature: March 2, 1995

A smart-battery-management philosophy and a surge of battery-management products now provide you with powerful means to optimize battery performance. Smart-battery technology produces accurate information about the state of a battery and enables optimum charge control. One implementation of this technology is a standardized smart battery that includes all the necessary electronics to monitor itself and communicate to its host (see box, "What's a smart battery?"). However, you can also team up many available ICs and batteries to tailor the battery's level of intelligence to your particular system.

The need for smart-battery technology stems from the introduction of new battery types, each with its own stringent requirements for charging. In many cases, battery manufacturers won't supply these batteries-nickel-metal hydride (NiMH) and lithium-ion (Li-ion) batteries in particular-without mandating the use of an approved charge-control scheme.

NiMH batteries are more sensitive to overcharge than their NiCd relations. High heat resulting from a high-rate overcharge is most damaging to an NiMH battery's capacity and cycle life. Thus, fast charging an NiMH battery requires tight control of charging characteristics and accurate feedback about the state of the battery.

Although not widely available, Li-ion batteries mandate tight battery management for safety purposes alone. Li-ion batteries are simply dangerous if not charged properly. As one manufacturer said, "without battery management, Li-ion batteries wouldn't exist in the marketplace."

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The concept and underlying technology of a smart battery is not new. In 1989, without much fanfare, Sanyo Energy USA introduced the SI101, a fast-charge control module for NiCd and NiMH batteries ($6.89 for 10,000 of the modules, $2.93 for just the IC). The company has integrated the module into battery packs for a wide variety of OEM customers. Many top-tier computer manufacturers are on this list, and others have developed their own battery-management schemes and ICs.

However, recent industry developments have put the spotlight on smart batteries and battery management. In an effort to standardize smart batteries and the way they communicate, Duracell and Intel introduced the System- Management Bus and Smart-Battery Data Specification last year. They hope their efforts, which included widespread industry input, will lead to a standard power-management bus for portable equipment and a standard smart-battery hardware configuration and data set (see box, "Smart-Battery and System-Management Bus Specifications"). The standards push is not without controversy (see box, "The debate over smart-battery standards,").

This year, Duracell will introduce its smart-rechargeable batteries as the first products to comply with these specs. However, many companies have designed intelligent-battery schemes of their own and perfected the underlying technology necessary to bestow battery intelligence, such as gas-gauge and charge-control techniques (see box, "For free information..."). Since 1991, Benchmarq Microelectronics has designed six gas-gauge ICs. In addition to Sanyo's module, Energizer Power Systems and National Semiconductor have teamed to develop an intelligent-battery chip set. And Rayovac and Benchmarq Microelectronics have cooperated on the design of an IC to control charging of Rayovac's Renewal line of reusable alkaline batteries. Rayovac plans to offer a full rechargeable system comprising four AA cells, the bq2901 IC, and a wall-cube adaptor for an OEM price of less than $6.

Finally, software vendors are getting involved. SystemSoft and Phoenix Technologies offer software that makes some of the battery data that an intelligent battery supplies available to a computer end user. The goal of such products is to let the user make changes in power-management software. The software would indicate what affect these changes would have on battery capacity.

The advantages of a high IQ

The advantages of an intelligent battery or intelligent battery-management system are clear: longer run times, longer lifetimes, and more end-user confidence in the battery information. Batteries that can deliver accurate information about their state of charge let you use all of that available charge more fully. Shorter charge times, which must be commensurate with controlled charging, result in longer run times. And, proper handling of the battery results in the longest possible life for that battery.

Depending on the specific implementation, other advantages include a management scheme that can recognize and handle batteries of different chemistries. The Duracell/Intel spec and many of the battery-management products can currently deal with numerous battery chemistries, including NiCd, NiMH, Li-ion, and lead acid. In addition, many ICs tailored specifically for Li-ion batteries will appear this year.

One of the greatest advantages of smart batteries or systems is the power-management possibilities they offer to a system engineer. These batteries provide a wealth of information that you can use to develop a proprietary power-management scheme, regardless of whether you use a standard battery or communication protocol. Dave Heacock of Benchmarq Microelectronics suggests adaptive charge control as one such technique. Using information from an intelligent battery, you could design a system that caters its 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.

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Smart-battery qualities

In some cases, a truly self-contained smart battery may be the right choice for your product. However, you have other choices. Although the Duracell/Intel specification's goal is standardization, the spec has flexibility and contains many implementation layers from which you can pick and choose. For example, a high-end computer may include all smart-battery electronics in the battery pack, but the same computer manufacturer could also produce a cheaper line with a slightly modified battery pack.

While standardization is under debate, you can choose how much intelligence to design in and where to locate it (see box , "Looking ahead,"). A certain level of intelligence may suit one product but be overkill for another. The requirements of the notebook-computer user differs greatly from the occasional cellular-phone user, for example.

Obviously, much of what you design depends on the product's battery chemistry or on the desire to handle multiple chem-istries. Remember: Every battery has a unique personality profile (seebox, "Ref 1," ). Battery characteristics change over time (self-discharge), with temperature, and with use and abuse. The battery type can experience varying degrees of these changes, and you have to account for these changes when choosing a battery-management scheme.

The complexity of the control schemes may impact your battery choice. For any product, the advantages of NiMH or even Li-ion batteries may not outweigh the costs of controlling them. Rayovac's Renewal alkaline batteries-which are suited more for low-power, handheld devices than for notebook computers-have very low self-discharge rates, so predicting remaining charge doesn't require as sophisticated a monitoring scheme as NiCd or NiMH batteries. An interesting feature of the Renewal battery is a mechanical interlock that allows a system to charge only the Renewal battery while running on another battery of a different chemistry.

Currently, you can implement either a full-blown smart battery or an intermediate level of battery management in three ways: You can use a fully integrated, retail smart battery, such as the DR15, 17, 30, 35, and 36 from Duracell. You can work with a manufacturer, such as Energizer Power Systems or Sanyo Energy, on a unique design based on a fully integrated smart battery. Or, you can mix and match products from companies that design battery-management and charge-control ICs, microcontrollers, and software.

Duracell's batteries epitomize the use of a fully integrated, retail smart battery. The company's smart batteries conform to one of five form factors with varying levels of capacity.

Products from Energizer Power Systems and Sanyo Energy epitomize the second approach. The companies have more or less a custom working relationship with OEM customers and offer design flexibility for the electronic design and hardware form factor. For example, when working with Energizer Power Systems, you can implement any proprietary bus system and battery-data set, or you can actually emulate elements of the Duracell/Intel Smart-Battery and SMBus specification.

Energizer Power Systems worked with National Semiconductor for the design of the actual silicon, which works with a variety of manufacturers' batteries and, thus, suits the final mix-and-match option. The chip set ($8 to $10 for the highest end) consists of the LMC6980 intelligent-battery development system and the LMC6984 intelligent-battery embedded controller ( Fig 1 shows a typical application circuit).

The LMC6980 contains the analog data-acquisition circuitry to monitor the battery's voltage, temperature, and dynamic current. The IC is unique because it contains 128 bytes of internal embedded EEPROM for storing numerous battery and charge-termination parameters. The LMC6984 contains all of the charge-control functions implemented by one of two versions of firmware. One is standard µC code. The other, which the company calls NeuFuz, is code the company derived using neural fuzzy-logic algorithms. The fuzzy-logic charge-control algorithms implement a charge time much faster than the typical two to three hours. Tests performed on NiCd batteries show charge times of around 20 to 30 minutes.

Other products that implement the mix-and-match approach are either complete battery-management ICs that you team with a selected battery, such as Benchmarq's bq2040 ($7 (1000)) and Microchip Technology's MTA-11200 ($3.75 (10,000)), or stand-alone gas-gauge and control ICs. Linear Technology's LT1325 contains a gas gauge and charge controller but requires the use of an external µC, typically the keyboard controller.

Note: You'll find essentially three types of individual ICs or chip sets: stand-alone charge ICs, stand-alone gas-gauge ICs, and battery-management ICs. The term battery management usually implies that the IC performs both charge control and monitoring of the battery. There are many inexpensive ICs available for charge control, but they don't determine battery capacity.

Some of the products are specialized ICs, and others are µCs specialized for battery-management functions. The MTA11200's design includes the company's 8-bit µC core and, based on a license agreement with Span Inc, uses purely digital methods to integrate battery charge and discharge current. Zilog Inc discusses a smart charger based solely on it Z8 µC in Ref 2.

Mix-and-match trade-offs

The bq2040 and bq2014 ($4.85 (10,000)) from Benchmarq Microelectronics highlight the trade-offs of various mix-and-match approaches. The 2040 is a gas-gauge IC that meets the Duracell/Intel SMBus interface and Smart-Battery Data specification. The 2014 is a proprietary stand-alone gas-gauge IC. Benchmarq teamed with SystemSoft to develop keyboard-controller software that translates the 2014's data to fit the Duracell/Intel spec. In this approach, the keyboard controller, rather than any IC in the battery pack, performs all the gas-gauge calculations. The bq2014 provides a more minimal data set than the bq2040 and has the minimum features necessary to do effective battery monitoring. Thus, the bq2014 is cheaper and more flexible, but doesn't provide as much information as the bq2040. Another difference that affects system implementation is that the bq2040 can't stop the charge but communicates its full status across the serial bus. The bq2014 can stop its own charging.

Narrowing your choices

When choosing a battery-management approach, you should consider numerous factors, including gas-gauge accuracy; charge control; 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. Other than for development systems, none of the products surveyed with set pricing were over $10 in 10,000-piece quantities.

The monitoring circuit's power consumption can be a big consideration, and you should determine how much operating and shutdown current any battery-management scheme requires. Most of the available ICs have shutdown and shelf-shutdown modes. In a part due this year, Microchip Technology will add a third "hibernate" power mode that draws 5 µA or less.

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Gas-gauge accuracy is a must

However, the most critical of the considerations are the accuracy of the monitoring circuit and implementation of the charge-control scheme.

For effective battery management, the self-monitoring of the battery—the gas-gauge function—must be highly accurate. Without accuracy, no level of control can improve the battery's performance. To achieve a high level of accuracy, the gas-gauge electronics may have to compensate for changing battery parameters and perform calibration.

A gas gauge measures some battery parameter that it uses to determine and report battery capacity (Ref 3). Some older, very inexpensive gas gauges simply measured voltage. This battery voltage is a highly inaccurate indication of a battery's capacity because it changes with temperature and battery load. Most of the more advanced gas-gauging products, including those discussed here, measure the current into and out of a battery to determine its capacity. Some manufacturers call this function coulomb counting.

Although measuring current is much more accurate, each manufacturer's implementation and accuracy claims differ. Because of the integration of the battery packs with the control electronics, Duracell claims its smart batteries have extremely high accuracy of around 1%, stemming from their accurate cell models and calculation algorithms. Benchmarq points to the precision ADC and reference in the bq2040 as the main accuracy-determining components. A V/F converter that uses residuals from one conversion for the next conversion actually performs this ADC function. The net result is zero quantization error. Microchip's MTA11200 gets within 3% accuracy or better by using good internal components and a calibration with external components. According to National Semiconductor, having the data-acquisition portion of the battery-management system residing in the battery pack results in much higher accuracy than solutions that put this function in a separate charger.

Note compensation

Part of a gauge's accuracy stems from applying compensation for varying conditions and calibration. For example, Microchip's MTA11200 adjusts the charge-efficiency calculation based on the present state of charge and the temperature. To improve accuracy, the IC does not apply the compensation factors to the state-of-charge calculation when the battery is discharging.

Also, a battery's capacity can vary over its lifetime, and, from time to time, many of these ICs need to self-calibrate and relearn 100% capacity. Benchmarq's bq2040 has an extra register that looks at discharge characteristics (Fig 2). If there have been no partial charges of the battery, the IC automatically updates capacity on a full-discharge cycle. When the battery reaches full charge, the IC resets capacity to full.

Capacity measurements should err on the conservative side and, after some number of partial charges, the system should inform the user that a full discharge is necessary.

Choose the right charge control

Charge-control schemes are also critical. All of the commonly accepted methods measure voltage, temperature, or their derivatives. For example, combinations of the dT/dt (change of temperature with time) and peak-voltage detect or dV/dt schemes (change of voltage with time) are recommended often for many batteries. For a time, negative delta-V, which detects the beginning of a negative voltage slope, was popular. According to some in the industry, the use of this technique appears to be waning in order to prevent overcharge of any battery type, including NiCds.

In addition to effectiveness of the charge control, you may want to choose a management scheme that allows you to change the charge-control regime easily. One of the advantages of National Semiconductor's approach is its flexibility: You can implement many charge techniques. To change the charge-termination scheme, you simply change the EEPROM look-up tables in the 6980. Microchip's MTA11200 allows you to choose charge-control regimes for three battery chemistries and numerical value to stop charging. Changing a small portion of the controlling µC's code changes the LTC1325 charge-control regime.

One final, important point about the charge-control scheme: It must be the one recommended by the battery manufacturer. Opinions and battery designs differ in the industry, and one NiMH manufacturer may recommend something slightly different than another. Don't second-guess battery manufacturers.

Another feature of a smart-battery or system is its ability to store and maintain information, such as the number of charge/discharge cycles and temperature exposure extremes. Microchip coins the MTA11200's data-logging function as the "flight recorder" of the battery. This feature, based on external EEPROM, lets you log a number of charge cycles and check if the battery has gone above or below certain limits.

The EEPROM data tables in National's LMC6980 holds values for load, data, and self-discharge correction. The EEPROM also stores three sets of phase termination and charge rates, min/max voltage and temperature limits, min/ max exposure temperatures, and the number of charge/discharge cycles.

Development systems

Development support is available for many of these battery-management schemes, including National's chip set, and Microchip Technology's MTA11200 ($499). Most include Windows-based software and some sort of demo board that includes the control and gas-gauge functions. These systems let you change various control parameters and test how the battery pack performs using those parameters. Benchmarq offers two versions of a development kit for its bq2040: a module ($25 each) that can fit on a pc board, or a larger pc board (the $149 EV2040) that you can hook to a battery pack.

FIGURE 3

Smart batteries have limits

Keep in mind what "smart" batteries can and can't do. They can report accurate state-of-charge information. They can implement a charge-control regime.

However, a smart battery or battery-management system can't make up for improper design. None of these products is completely fail-safe. For example, when using dT/dt methods to determine the end of charge, you can inadvertently fool the "smart" charger by changing charge rates. Abruptly slowing the charge rate for an almost-full battery may not trip the dT/dt mechanism properly, which results in overcharging. Also, power-supply noise can cause false terminations in peak-voltage-detect chargers.

System design considerations include first determining the appropriate level of battery management. Once you narrow down the choice, you need to determine the impact of various support components. For example, measuring battery current to gauge capacity requires a sense resistor. You have to look at the system's power requirements to choose the right value of this resistor. You want it small enough to not waste power but large enough to produce a decent signal that the system can measure.

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You can reach Technical Editor Anne Watson Swager at (610) 645-0544.

References

1. Small, Charles, "Batteries explode into new applications and ne chemistries," EDN, October 13, 1994, pg 63.
2. Cummings, Gary, Daniel Brotto, and James Goodhart, "Charge batteries safely in 15 minutes by detecting voltage inflection points," EDN, September 1, 1994, pg 89.
3. McClure, Malcolm, "Energy gauges add intelligence to rechargeable batteries," EDN, May 26, 1994, pg 125.
4. Kerridge, Brian, "Battery-management ICs," EDN, May 13, 1993, pg 100.