Suitable for battery-powered portable system circuit design

　　Although voltage measurement alone has been used in many portable products to estimate the remaining charge of a battery, this method can have an error of up to 50%. The relationship between battery voltage and charge varies with discharge rate, temperature, and battery age. For example, a high discharge rate will result in a greater voltage drop than a low discharge rate with the same power loss. When batteries are discharged at different temperatures, we notice some similar characteristics. As demand for long-runtime products continues to grow, system designers need a more precise solution. Using a battery gauge IC to measure the charge flowing into or out of the battery will provide a better estimate of battery charge over a wide range of application power levels .

　　Battery pack circuit design

　　Figure 1 depicts the application circuit in the battery pack. Depending on the fuel gauge IC used, the battery pack will have at least three to four external terminals available. The VCC and BAT pins will be connected to the battery voltage for IC power and battery voltage measurement. A low value sense resistor is installed at the battery ground terminal so that the voltage across the sense resistor can be monitored by the high impedance SRP and SRN inputs of the battery fuel gauge. The current flowing through the sensing resistor helps us determine how much the battery has been charged or discharged. When selecting the sense resistor value, the designer must consider that the voltage across it should not exceed 100 mV. A resistor value that is too small may introduce errors 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 the respective endpoints of the sense resistor; that is, Kelvin connection measurements. The HDQ/SDA and SCL pins are open-drain devices, and both require an external pull-up resistor. This resistor should be located on the host side or main application side to allow the battery fuel gauge sleep function to be activated after the battery pack is disconnected from the portable device. The recommended pull-up resistor value is 10 kΩ.

　　　Figure 1 Typical application circuit

　　Battery pack verification circuit design

　　Rechargeable batteries in portable devices must be replaced before the end of the device's life. This opens up a huge market for manufacturers offering cheaper replacement batteries that may not have the safety and protection circuitry required by the original equipment manufacturer. 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 a calculated cyclic redundancy check (CRC) IC (TI's bq26150). This CRC is based on this authentication and a CRC polynomial secretly defined in the IC. The host also calculates the CRC and compares the various values ​​to determine whether authentication was successfully obtained. If not, the host will decide whether to perform another verification or disallow system power from that battery.

　　Figure 2 Circuit with BQ27000 and verification IC

　　Once the battery is verified, the bq26150 receives a command to ensure that all communication over the data line is carried between the host and the battery fuel gauge. From this point of view, the host can continue to utilize the function of the battery fuel gauge. The entire verification process must be repeated both when disconnecting and reconnecting to the battery.

　　Two-cell battery application circuit design

　　Figure 3 shows a typical application circuit supporting two Li-ion batteries with the bq26500. An adjustable voltage regulator is added to the system to enable multi-cell support. The BAT pin of the battery fuel gauge is connected to the positive terminal of the bottom battery to enable regulated voltage measurement of the battery pack. Figure 3 Two-cell application with bq26500 The host needs to interpret the battery pack regulation voltage measured by the battery fuel gauge to determine the end-of-discharge threshold and end-of-charge time. We can take advantage of information such as "state of charge remaining" as reported by the battery gauge.

　　Transfer files for PCB proofing

　　The bq2650x and bq27x00 provide battery manufacturers with a simple alternative to battery charge reporting. Simply by reading data from the register of the battery fuel gauge, the host can obtain the remaining power value and then display this result to the end user. With a battery fuel gauge, end users can use as much of a battery's charge as possible with very little power remaining, because the charge estimate will be more accurate than simply measuring battery voltage. Battery fuel gauges can be used in a variety of configurations and have proven features that allow use in many two-cell powered applications.