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Choose the right battery fuel-gauge for smartphones, tablets

23 Jul 2012  | Ming Yu, David Maxwell

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As they gain more and more functionality, smartphones and tablets also become more indispensable. At the same time, these devices are expected to become both thinner and cheaper. This requires balancing the competing objectives of both smaller batteries and increased usable capacity. To solve this dilemma a highly accurate battery fuel-gauge is needed. In this article we discuss the trends in smartphone and tablet design. We also identify how accurate fuel-gauging can increase customer satisfaction, reduce battery costs, and extract the maximum run-time.

Smartphone applications such as video players or gaming systems require application processors (APs) based on relatively power-hungry two- or even four-core AP chipsets. The smartphone display screen is also getting bigger, from 4.3 inch to 4.7 inch and upwards, which also require more power.

The traditional battery pack in smartphones has been removable. As the devices form factor continues to become thinner, many are shifting to non-removable battery packs, a trend first started by tablets. Embedding battery packs within the device allows for more creative placement options to save space. With space at a premium the system designer needs to use the smallest cells possible to achieve the desired specifications of run-time. Not only are cells with higher energy density used, but also all available cell energy is needed. If an inaccurate fuel-gauge underestimates the available energy, it can lead to early termination. This means that energy is still available, but the end user only sees is a shorter run-time.

One solution is to use an over-sized battery to compensate for the unused capacity while allowing a looser gauging estimation, but this wastes space. A more cost-effective solution is to use a more accurate fuel-gauge to efficiently utilize all of the available energy while still avoiding a surprise shutdown.

To save pc board space and reduce cost, additional features are integrated into the baseband processor, application processor, and power management IC (PMIC). On PMICs, in particular, there is an attempt to integrate battery fuel-gauge functions. However, compared to the complexity of advanced discrete fuel-gauges, the battery fuel-gauge found in PMICs basically is a simple "coulomb counter" that requires the host processor to provide the gauging algorithm. Furthermore, the heat and interference generated by the PMIC switch-mode charger and converters can impact the integrated voltage and current measurement. This greatly reduces the gauging accuracy of this all-in-one approach.

Most tablet designs place the fuel-gauge within the battery pack, and most tablet batteries cannot be removed by the user. This trend of embedded batteries has spread to smartphone designs as phones get thinner. More designs now use pack-side gauges to reduce development and manufacturing cost.

There are some misconceptions about battery fuel-gauges due to the traditional thinking of coulomb counters. A true battery fuel-gauge must do more than just simply count the Coulombs or report the battery voltage. So what exactly is a battery fuel-gauge?

A battery fuel-gauge is an intelligent device that can provide information such as state-of-charge (SOC), available charge (in mAh or mWh), battery run time (in minutes), and battery state-of-health (SOH). One point of confusion is that there are many types of "fuel-gauges" that are no more than a battery monitor. A battery monitor simply reports battery voltage, charge and discharge current, and battery temperature. It usually incorporates a coulomb counting analog-to-digital converter (ADC) to track charge. A battery monitor can't accurately report the actual battery remaining capacity without a sophisticated gauging algorithm. A battery monitor just reports measurements and requires the host processor to apply compensations for temperature, load rate, and other factors that can affect the usable capacity. In this approach, cost of software development can be higher while the accuracy will be lower when compared to a fully-featured fuel-gauge. Performing gauging algorithm by the host processor will also increase power consumption and still lack the features of most standalone fuel-gauges.

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