TI Power Switch Design Tips 5 - Complete battery pack design for single or two battery powered portable applications

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introduction

Although voltage measurements have been used alone in many portable products to estimate the remaining battery capacity, this approach can have up to 50% error. The relationship between battery voltage and battery will vary with discharge rate, temperature, and battery age. For example, a high discharge rate results in a larger pressure drop than a low discharge rate of the same charge loss. When the battery is discharged at different temperatures, we will notice some similar features.

As the demand for long-running products continues to grow, system designers need a more accurate solution. In a wide range of application power levels, using a battery fuel gauge IC to measure the charge flowing into or out of the battery will result in a better battery power estimation method.

Battery fuel gauge principle

A battery fuel gauge is an IC that automatically monitors battery power and reports monitoring to a processor that makes system power management decisions. A good battery fuel gauge requires at least some method of measuring battery voltage, battery pack temperature and current, a microprocessor, and a proven battery fuel gauge calculation.

Both the bq2650x and bq27x00 are complete battery fuel gauges with an analog-to-digital converter (ADC) for voltage and temperature measurements and a current and charge-sensing ADC. These battery fuel gauges also have an internal microprocessor that runs TI's battery fuel gauge calculations. These algorithms compensate for the self-discharge, aging, temperature, and discharge rates of Li-ion batteries. The microprocessor allows the host system processor to eliminate the need for endless calculations.

The battery fuel gauge provides information such as “remaining battery status” and the bq27x00 system also provides “remaining runtime” information. The host can ask for this information at any time, and the host decides whether to notify the end user about the battery via LEDs or via on-screen messages. Since the system processor only requires one 12C or one HDQ communication driver, using a battery fuel gauge is very simple.

Battery pack circuit description

Figure 1 depicts the application circuit in the battery pack. The battery pack will have at least three to four available external terminals, depending on the battery fuel gauge IC used.

Figure 1 Typical application circuit

The VCC and BAT pins are connected to the battery voltage for measurement of IC power and battery voltage. A low-resistance sense resistor is placed on the ground of the battery so that the voltage across the sense resistor can be monitored by the high-impedance SRP and SRN inputs of the battery gauge. The current flowing through the sense resistor helps us determine the amount of charge or discharge that has been discharged. When choosing the value of the sense resistor, the designer must consider that the voltage across it should not exceed 100 mV. Too small a resistance value may cause an error under low current conditions. The board layout must ensure that the SRP and SRN connections to the sense resistor are as close as possible to each end of the sense resistor; that is, Kelvin connection measurements.

The HDQ/SDA and SCL pins are open-drain devices, both of which require an external pull-up resistor. This resistor should be located on the host side or on the main application side so that the sleep function of the battery fuel gauge can be activated after the battery pack is disconnected from the portable device. The recommended pull-up resistor value is 10 kΩ.

Battery pack verification

Rechargeable batteries for portable devices must be replaced before the end of their life. This opens up a huge market for manufacturers that offer cheaper alternative batteries that may not have the safety and protection circuitry required by original equipment manufacturers.

Therefore, in addition to the battery fuel gauge function, the battery pack may also include verification features (see Figure 2). The host will verify the battery pack containing the Calculated Cyclic Redundancy Check (CRC) IC (TI's bq26150). This CRC is based on this authentication and the CRC polynomial that is secretly defined in the IC. The host also calculates the CRC and compares the various values ​​to determine if the verification was successful. If not, the host will decide whether to perform another verification or not to power the battery's system.

Once the battery is verified, the bq26150 will receive a command to ensure that all communication over the data line is transferred between the host and the battery fuel gauge.

Figure 2 Circuit with bq27000 and verification IC

In this regard, the host can continue to take advantage of the battery gauge. The entire verification process must be repeated when the battery is disconnected and reconnected to the battery.

Two battery applications

Figure 3 shows a typical application circuit with a bq26500 supporting a two-cell Li-Ion battery. An adjustable voltage regulator is added to the system for multi-cell support. The BAT pin of the battery fuel gauge is connected to the positive terminal of the bottom battery to achieve regulated voltage measurement of the battery pack.

Figure 3 Two-cell battery application with bq26500

The host needs to interpret the battery pack regulation voltage measured by the battery fuel gauge to determine the discharge end threshold and charge termination time. We can use information such as "remaining battery status" as reported by the battery fuel gauge.

in conclusion

The bq2650x and bq27x00 offer battery manufacturers a simple alternative to battery power reporting. Simply by reading the data from the battery gauge's registers, the host can get the remaining charge value and then display the result to the end user. With a battery fuel gauge, the end user can use as much of the battery as possible with minimal remaining power, as the battery estimate will be more accurate than the data obtained by measuring only the battery voltage. Battery fuel gauges can be used in a variety of configurations, have verification features, and are used in many two-cell battery-powered applications.