TRACKING BATTERY CAPACITY
In most handheld devices, the battery-management circuits implement a rough form of fuel gauging based on voltage monitoring. This approach adds no additional components to the system and doesn't increase power dissipation. Voltage monitoring is commonly used to detect low-battery conditions. Yet this technique is very inaccurate because the battery voltage corresponding to a selected level of capacity varies with battery discharge rate, temperature, and aging.
The difficulty of picking a voltage point at which to issue a low-battery warning is illustrated by a graph of battery voltage versus discharge rate and temperature (Fig. 2). In this graph, the voltage corresponding to a fixed amount of remaining battery capacity varies widely over the discharge rate and temperature ranges shown. Consequently, when a fixed-voltage point is selected for low-battery warning, the actual capacity for that voltage point will vary a great deal over temperature and discharge rate (Fig. 3).
To account for the variations in capacity for a fixed battery voltage, large capacity guardbands are required when voltage monitoring is used to issue a low-battery warning. Unfortunately, that means low-battery warnings will be issued too soon or too late in some cases.
With the advent of cell phones offering color displays and Internet access, power dissipation has risen dramatically and reduced battery runtime significantly. In addition, the consequences of inaccurate fuel gauging are higher now that data can be lost when the battery runs out. As a result, there's greater demand for more-accurate fuel gauging in handhelds.
To obtain this higher accuracy, designers will need to adopt coulomb counting. When combined with an accurate fuel-gauging algorithm, coulomb counting permits measurement of battery capacity to within 1% of actual capacity. That's because this technique can account for capacity variations due to changes in discharge current levels and temperature. Although fuel gauge ICs have been in use for multicell Li-ion applications such as notebook PCs, these devices have not met the cost and size constraints of the handheld equipment.
However, semiconductor vendors have responded to the new requirements for capacity measurement by developing fuel-gauge ICs, also referred to as gas-gauge ICs, aimed at the single-cell applications. (Most handhelds use just a single Li-ion cell, not two.) These fuel-gauge ICs will compete with each other on the basis of their accuracy, ease of implementation, power dissipation, cost, and package size. Size is particularly important because the fuel gauge and its associated components often must fit in the battery pack along with the cell's protection circuit.
One recently introduced single-cell Li-ion battery gas gauge IC is Texas Instruments' bqJunior, also known as the bq2700x. Packaged in an eight-lead TSSOP, this fully integrated gas gauge measures the battery's charge and discharge currents (via an external sense resistor), battery voltage, and temperature using an on-chip analog-to-digital converter (ADC) and voltage-to-frequency converter. Dynamically balanced, the chip can measure and correct for its own offset. Like other fuel gauges, it requires a calibration step when installed in the battery pack.
The bqJunior's measurements are fed to an on-chip processor, which executes a coulomb-counting algorithm to calculate the remaining battery capacity and system runtime. The host simply reads the gas gauge's data through an HDQ interface. That data includes available power, average current, temperature, voltage, and time to empty and full charge. This device will be priced at $2.55 in quantities of 1000.
Other vendors have also indicated their plans to introduce fuel-gauge ICs for single-cell applications in the near future. One such company is Intersil. Unlike TI's approach, the Intersil fuel gauge will rely on the host microprocessor for capacity and runtime calculations. The company will employ an ADC to sample battery data, which will then be passed over an I2C bus to the host. According to Intersil, this approach can reduce power consumption by an order of magnitude versus having a microcontroller on-board, while also permitting use of a smaller die and package.
Nevertheless, another vendor plans to develop an on-chip-microcontroller-based solution that will integrate a fuel gauge with the battery protection circuit and the battery tag. (The latter function identifies the type of battery in the pack.) This device, which is slated for launch at the end of the second quarter, will take advantage of a very low-power microcontroller that consumes less than 100 nA in standby.
References:
1. "Power Management Challenges in Hand-held Products: A Holistic Perspective" by Ed Bordeaux, Staff Applications Engineer, Intersil; Power2002 presentation.
2. "New Handheld Devices Need Accurate Battery Management" by J. Norman Allen, Vice President Strategy, Microchip Technology; Power2002 presentation.
TRACKING BATTERY CAPACITY
In most handheld devices, the battery-management circuits implement a rough form of fuel gauging based on voltage monitoring. This approach adds no additional components to the system and doesn't increase power dissipation. Voltage monitoring is commonly used to detect low-battery conditions. Yet this technique is very inaccurate because the battery voltage corresponding to a selected level of capacity varies with battery discharge rate, temperature, and aging.
The difficulty of picking a voltage point at which to issue a low-battery warning is illustrated by a graph of battery voltage versus discharge rate and temperature (Fig. 2). In this graph, the voltage corresponding to a fixed amount of remaining battery capacity varies widely over the discharge rate and temperature ranges shown. Consequently, when a fixed-voltage point is selected for low-battery warning, the actual capacity for that voltage point will vary a great deal over temperature and discharge rate (Fig. 3).
To account for the variations in capacity for a fixed battery voltage, large capacity guardbands are required when voltage monitoring is used to issue a low-battery warning. Unfortunately, that means low-battery warnings will be issued too soon or too late in some cases.
With the advent of cell phones offering color displays and Internet access, power dissipation has risen dramatically and reduced battery runtime significantly. In addition, the consequences of inaccurate fuel gauging are higher now that data can be lost when the battery runs out. As a result, there's greater demand for more-accurate fuel gauging in handhelds.
To obtain this higher accuracy, designers will need to adopt coulomb counting. When combined with an accurate fuel-gauging algorithm, coulomb counting permits measurement of battery capacity to within 1% of actual capacity. That's because this technique can account for capacity variations due to changes in discharge current levels and temperature. Although fuel gauge ICs have been in use for multicell Li-ion applications such as notebook PCs, these devices have not met the cost and size constraints of the handheld equipment.
However, semiconductor vendors have responded to the new requirements for capacity measurement by developing fuel-gauge ICs, also referred to as gas-gauge ICs, aimed at the single-cell applications. (Most handhelds use just a single Li-ion cell, not two.) These fuel-gauge ICs will compete with each other on the basis of their accuracy, ease of implementation, power dissipation, cost, and package size. Size is particularly important because the fuel gauge and its associated components often must fit in the battery pack along with the cell's protection circuit.
One recently introduced single-cell Li-ion battery gas gauge IC is Texas Instruments' bqJunior, also known as the bq2700x. Packaged in an eight-lead TSSOP, this fully integrated gas gauge measures the battery's charge and discharge currents (via an external sense resistor), battery voltage, and temperature using an on-chip analog-to-digital converter (ADC) and voltage-to-frequency converter. Dynamically balanced, the chip can measure and correct for its own offset. Like other fuel gauges, it requires a calibration step when installed in the battery pack.
The bqJunior's measurements are fed to an on-chip processor, which executes a coulomb-counting algorithm to calculate the remaining battery capacity and system runtime. The host simply reads the gas gauge's data through an HDQ interface. That data includes available power, average current, temperature, voltage, and time to empty and full charge. This device will be priced at $2.55 in quantities of 1000.
Other vendors have also indicated their plans to introduce fuel-gauge ICs for single-cell applications in the near future. One such company is Intersil. Unlike TI's approach, the Intersil fuel gauge will rely on the host microprocessor for capacity and runtime calculations. The company will employ an ADC to sample battery data, which will then be passed over an I2C bus to the host. According to Intersil, this approach can reduce power consumption by an order of magnitude versus having a microcontroller on-board, while also permitting use of a smaller die and package.
Nevertheless, another vendor plans to develop an on-chip-microcontroller-based solution that will integrate a fuel gauge with the battery protection circuit and the battery tag. (The latter function identifies the type of battery in the pack.) This device, which is slated for launch at the end of the second quarter, will take advantage of a very low-power microcontroller that consumes less than 100 nA in standby.
References:
1. "Power Management Challenges in Hand-held Products: A Holistic Perspective" by Ed Bordeaux, Staff Applications Engineer, Intersil; Power2002 presentation.
2. "New Handheld Devices Need Accurate Battery Management" by J. Norman Allen, Vice President Strategy, Microchip Technology; Power2002 presentation.