Extending Battery Life:Today's battery-monitoring ICs must combine a high level of accuracy with a host of intelligent power-down modes to help extend run time. The battery manufacturers have done a pretty good job increasing the energy densities within their products. But there's only so much they can do. Each new chemistry seems to push the envelope a little further in terms of increasing the energy without increasing the volume or weight of the battery. But those steps are incremental, with nothing revolutionary coming in the immediate future. This puts the onus squarely on the battery-management IC designers to help eke out a few more minutes of run time and/or a faster recharge time, combined with more accurate gas gauging to meet users' expectations. In terms of chemistry, the premier technology today for portable products is lithium ion (LiIon). Some industry followers group lithium polymer together with LiIon, but there are some distinctions. This includes the cell's packaging, the removal of the exterior can, and the use of a laminated packaging material. The interior of LiIon tends to be liquid-like, hence the need for the can, or container, to hold the liquid. Polymer tends to be more of a gel. "The key point is the lack of free liquids," says Eric Dix, general manager of rechargeable batteries at Ultralife Batteries. "From a safety standpoint, if you have a battery that ruptures around an electronic device, like a laptop computer, cell phone, or PDA, these are very volatile liquids. So inherently, the lack of free solvents is a big upside for safety." Ultralife is one of the leaders in LiPolymer cells. In fact, that's the only battery chemistry the company produces, albeit in many different varieties. They claim to be shipping volume quantities of polymer batteries in commercial applications. "We've invested more money on LiPolymer technology than anyone in the western hemisphere," boasts Dix. "We believe we have the safest and highest performing solution available." LiPolymer has the same charge and discharge characteristics as LiIon, so the battery management is handled in a similar fashion. As a result, the chargers and charger circuitry that was developed for LiIon can be employed by systems taking advantage of LiPolymer. Older chemistries like nickel cadmium (NiCd) and nickel metal hydride (NiMH) are still fairly prevalent, although NiCd use is diminishing in many applications because of environmental concerns. They're prevalent in power tools because of their power capabilities, as well as their ability to operate over a wide temperature range. Little change, small improvements All the chemistries mentioned continue to receive incremental improvements. The most promise today is held for the lithium variants because they're relatively new. "If you look at the basic active materials used in today's LiIon cells, they haven't changed much since they became popular in the early 1990s, but the energy density has doubled," says Jason Howard, a staff scientist at Motorola's Energy Systems Group. "The basic chemistry hasn't changed. The biggest thing they've been able to do is improve the cell engineering, pack more material into the cans. Some of the materials have improved, such as using better metal-oxide compounds, but it's still a basic LiIon chemistry." The Motorola Energy Systems Group tends to serve the cellular, computer, and vertical markets, often building custom solutions. "There's probably more customization dealing with the charger side because that tends to interface directly with the mechanical constraints of various customer components," says Howard. "But we have the ability to do full customization if that's required." STMicroelectronics offers a generic charger, the L6902, that works with any popular chemistry. It drives a constant current into the pack, up to the battery's set voltage level, then it supplies a constant voltage. Designers can tailor the programming of the current to whatever the battery requires, up to 1 A, from 1.235 to 34 V. It employs an internal switching regulator to generate the constant current or voltage. "When we're designing a chip like this one, we ask our customers a lot of questions, then we design a part that will encompass as many features as the major customers need," explains Ed Friedman, product marketing manager at ST. "We try to include the majority of features that most customers need. But if you include all the features that everybody wants, the part is likely to become prohibitively expensive to those that don't need some of the features." Charge and recharge On the recharge side of the equation, the application often determines the recharge time. For some applications, a quick recharge is more important than it is for others. The tradeoff for a fast recharge is that the number of times a battery can be cycled is reduced. Most vendors recommend charging at the C rate. For LiIon, a full recharge occurs in about 2.5 to 3 hours, although the battery is charged to about 90% of its capacity in the first hour. Each of the chemistries requires a different charging solution. To ensure that the battery and battery-management components operate cohesively, it's imperative that the two product makers work together. "We work closely with the battery manufacturers in two areas," says Peter Fundaro, TI's marketing manager for battery-management products. "One is with regard to the algorithms that we incorporate into our gas-gauge IC. It's gotten to the point where you can only do so much with analog measurement accuracy. You have to take it one step further and model the batteries closely in the microcontroller code to increase the accuracy down to the 1% or 2% level. Anybody can interpret an analog signal very accurately, but where you get that additional accuracy is if you can model the battery's discharge current very closely in the gas gauge." TI writes all the code for its family of gas-gauge ICs. Not only must you account for batteries from different manufacturers, but also different battery characteristics from the same manufacturer. This is done in the ROM code of the microcontroller. TI recently released a battery monitor for LiIon and LiPolymer applications, specifically those that require just one cell. The bq2023 measures critical parameters in a battery pack, such as charge, discharge, self-discharge, and temperature, and employs an auto-calibrating VFC (voltage-to-frequency converter) for continuous charge/discharge integration for maximum accuracy. The part includes 224 bytes of flash memory and a 64-bit ID ROM register. This lets system designers add an accurate fuel gauge while replacing serial EEPROMs or ID chips. Guaging the fuel The primary purpose of the fuel gauge is to help extend the system's run time by accurately telling the system how much capacity remains in the battery. Instead of cutting the system off at 10% remaining capacity, for exampleÑmade from some inaccurate low-voltage readingÑyou can cut the voltage off, or warn the user they have to charge the battery at 1% or 2%. This effectively turns that 8% or 9% of battery capacity into actual run time for the user. In general, gas gauges are designed for specific chemistries. Understanding how the batteries operate under different conditions comes from working closely with the battery manufacturers. Note that having an accurate gas gauge only solves half the problem. Just because you're feeding accurate information to the host system doesn't mean that the system is taking advantage of that information. The information that's generally passed to the host controller from the gas gauge (which typically resides within the battery pack) refers to the voltage, temperature, current, and remaining capacity. Obviously, the gauges are necessary in battery-powered systems, which tend to focus on small size and light weight. Simply put, the gauge is taking up premium space that could otherwise be used for energy to power the system. Gauges are often tailored for the differing space requirements of specific applications. For example, the space in a cell phone comes at a higher premium than the space in a laptop computer. And this is one way IC vendors can leverage their more advanced technologies. Laptop computers sometimes rely on the battery-management flexibility as a product differentiator. In such a system, the gas-gauge information is usually passed through the keyboard controller, which communicates with the charger IC, the host controller, and possibly the protection circuitry. One significant trend that's becoming apparent, particularly in handheld devices, is the incorporation of a battery cell directly on the system's main board, thereby eliminating the traditional battery pack. Says Fundaro, "That says a lot about today's battery technology. If you incorporate the battery right onto the main board, you better be sure that the battery can last at least as long as your projected life of the system, because replacing the battery isn't really feasible." This puts more pressure on the battery management circuit to ensure that the battery is managed properly in both run time and charging. Overcharge/discharge dangers It's fair to say that a battery's life can be severely affected by overcharge and overdischarge. In simple terms, overcharge means that the charger applies more than enough amp-hours (Ah). "When you overcharge the battery, it heats up," offers Shahiar Esmaili, director of engineering at Saft America. "For example, in a NiMH cell, overcharging produces oxygen. When combined with the heated cell, the alloy of the battery can become corroded. In a NiCd cell, there's also an unfavorable chemical reaction." Saft offers a NiMH D-cell battery that achieves a capacity of 8500 mAh and an energy density of 66Wh/kg (see the figure). The VH series battery is suited for such high-energy applications as lighting products, home appliances, and power tools. One way to guard against high temperatures in a battery is to use a resettable fuse, which is typically employed as secondary protection behind the primary circuit. For example, if the cell temperature goes above a predefined setting, the circuit can be placed in an open condition until the temperature settles back to a reasonable operating level. If it's a one-time fuse, one unruly situation forces the user to return the phone for servicing. According to Littelfuse, one of the makers of resettable fuses, lots of options are available, and some going as low as 15 cents in high volumes. The fuses can also handle overcurrent or overtemperature conditions. The buck stops at the system house In the end-users' eyes, the battery management falls on the shoulders of the system developer. They wouldn't look to the battery supplier or battery-management IC vendors to prolong the life of the system. Consequently, most system developers spend a fair amount of development effort pushing their battery subsystems to the limit. For system manufacturer Melard Technologies, the burden was especially heavy because its systems are used by field technicians who have no access to a recharge, yet must get a full day's work from the system. "We developed system- and battery-management software that gracefully powers down the unit to different levels depending on the setting," says Chris Reitz, chief operating officer at Melard. "In a wireless system, it allows your radio to be up and listening for incoming messages or communications from the network, even in a powered-down state." The battery pack in the latest Melard system, called the Sidearm, is a custom intelligent pack that holds four standard LiIon cells. The 3.4-Ah pack supplies about 12 hours of life. The Sidearm runs the Windows CE operating system, and is based on Intel's StrongArm microprocessor. According to the company, the system delivers twice the performance with one-third the power consumption, at 45% less voltage, when compared with the competition. "We grew up in the field service market," explains Reitz. "When you'd supply product, the engineers had to stay in the field for a full 8- or 10-hour shift. So we had to build a device that would stay powered for that long without needing a recharge. As the feature set increases, with things like faster processors and wireless radios, the power drain expands pretty quickly." The systems with the wireless radios present an interesting challenge, in that each radio type has its own characteristics that must be handled differently. The different radios operate with differing sleep modes.
Source: Portable Design June, 2001
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