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Design fuel gauging for multicell Li-ion battery pack (Part 1)

Posted: 22 Feb 2008 ?? ?Print Version ?Bookmark and Share

Keywords:design fuel gauging? multicell Li-ion battery?

By Sihua Wen
Texas Instruments Inc.

Portable devices, such as laptop computers, cellphones and media players, have an increasing demand for power as the trend for functionality integration and technology convergence continues.

Li-ion battery, the main rechargeable power source for these devices, is having a difficult time keeping up with the power requirement of portable devices. While seeking generational advancement in new power sources, system designers also want to fully exploit the offerings of existing battery technologies. This puts a higher strain on the battery itself and sets an emphasis on adopting accurate fuel gauging methods to squeeze out the last drop of juice in the battery.

Many mobile applications, such as wireless account management, data processing and medical monitoring, depend on accurate remaining capacity information to avoid shutdown surprises resulting from depleted battery power. However, providing accurate remaining capacity information throughout battery lifetime and across temperatures and usage load profiles is often an underestimated challenge for many end users, and even some system designers.

This article describes how the fuel gauging technology, the Impedance Track from TI, tackles these challenges and presents a brief design example of a three-series, two-parallel battery pack solution.

Problems of existing fuel gauge
A misunderstanding about the Li-Ion battery is that the shrinking run-time of a battery in use is primarily due to the fading of the battery capacity. Contrary to this popular thinking, it is generally not the capacity loss but the increase of the battery impedance that causes the problem.

Figure 1 shows that after 100 cycles, the battery capacity drops by less than five percent, while the internal DC impedance of the battery, R(Z), increases by a factor or two! A direct effect of the higher impedance of an aged battery is a higher internal voltage drop in response to a load current. As a result, the battery reaches the minimum system operating voltage (or the termination voltage) much earlier than a fresh battery does.

Conventional fuel gauging technologies, mainly voltage-based and coulomb counting algorithms, have obvious limitations in gauging performance. The voltage-based scheme, widely adopted in handheld devices due to low cost and simplicity, suffers from battery impedance change over time.

A dynamic load condition and temperature change can render a gauging error of up to 50 percent. Meanwhile, the coulomb counting scheme takes the alternative approach by continuously integrating coulomb to compute the consumed charge and state-of-charge (SOC). With a pre-learned knowledge of full capacity, the remaining capacity can be obtained. A drawback of this approach is that self-discharge is difficult to model with accuracy. Then, without periodic full-cycle calibration, the gauging error accrues over time. None of these algorithms address impedance variation of the battery.

The designer then must terminate system operation prematurely by reserving more capacity to avoid the unexpected shutdown for aged cells. This leaves a significant amount of energy wasted for fresh batteries (Figure 2).

Battery impedance, chemical capacity
What makes the Impedance Track technology more accurate than existing solutions is its self-learning mechanism that accounts for the aging effects that cause the change of battery impedance and the no-load chemical full capacity (QMax).

Figure 1: Battery chemical capacity and impedance over 100 cycles.

The Impedance Track technology implements a dynamic modeling algorithm to learn the characteristics of a battery under the conditions of aging, temperature or usage history by tracking the impedance and capacity change during the actual usage of the battery. With this algorithm, no periodic full-cycle capacity calibration is required. Compensation for load and temperature is modeled accurately with the help of the knowledge of cell impedance.

Figure 2: Conservative design leaves larger amount of energy/capacity unused in new batteries.

Most importantly, the fuel gauging accuracy can be maintained during the whole lifetime of the battery, as a result of the dynamic learning of battery parameters. With accurate gauging from Impedance Track, the system design can be relieved from a conservative shutdown scheme. Now, battery capacity will not be wasted.

To view Part 2, click here.

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