Battery power detection chip

Battery Capacity Detection Principle

A battery fuel gauge is an IC that automatically monitors the battery charge and reports back to a processor that makes system power management decisions. At a minimum, a good battery fuel gauge requires some method of measuring the battery voltage, battery pack temperature and current, a microprocessor, and a proven battery fuel-gauging algorithm. The bq2650x and bq27x00 are complete battery fuel gauges that have an analog-to-digital converter (ADC) for voltage and temperature measurement and an ADC for current and charge sensing. These battery fuel gauges also have an internal microprocessor that runs TI's battery fuel-gauging algorithms. These algorithms compensate for self-discharge, aging, temperature and discharge rate of lithium-ion (Li-ion) cells. The microprocessor frees the host system processor from having to perform endless calculations.

The battery fuel gauge provides information such as "power remaining state" and the bq27x00 system also provides "remaining run time" information. The host can query this information at any time and decide whether to inform the end user about the battery information through LEDs or screen display messages. Using the battery fuel gauge is very simple because the system processor only needs an I2C or an HDQ communication driver.

Battery Pack Circuit Description
Figure 1 describes the application circuit in a battery pack. Depending on the battery fuel gauge IC used, the battery pack will have at least three or four external terminals available.
Figure 1 Typical Application Circuit

The VCC and BAT pins are connected to the battery voltage for IC power and battery voltage measurement. A low-value sense resistor is placed at the ground end 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 fuel gauge. The current flowing through the sense resistor helps us determine how much the battery has been charged or discharged. When choosing the sense resistor value, the designer must consider that the voltage across it should not exceed 100 mV. Too small a resistor value may introduce errors under low current conditions. The board layout must ensure that the connections from SRP and SRN to the sense resistor are as close as possible to the respective ends of the sense resistor; that is, Kelvin connection measurement.

Both the HDQ/SDA and SCL pins are open-drain devices and 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.

Battery Pack Validation

Rechargeable batteries in portable devices must be replaced before the device reaches the end of its life. This has created a large market for manufacturers to offer cheaper replacement batteries that may not have the safety and protection circuitry required by the OEM.

Therefore, in addition to the battery gas gauge function, the battery pack may include an authentication feature (see Figure 2). The host will authenticate the battery pack, which contains an IC that calculates a cyclic redundancy check (CRC), such as TI’s bq26150. This CRC is based on this authentication and a CRC polynomial that is secretly defined in the IC. The host also calculates the CRC and compares the values ​​to determine if the authentication was successful. If not, the host will decide whether to perform another authentication or not allow the battery to power the system. Once the battery is authenticated, the bq26150 will receive a command to ensure that all communication over the data line is transmitted between the host and the battery gas gauge.

At this point, the host can continue to utilize the battery fuel gauge functionality.
The entire authentication process must be repeated when disconnecting and reconnecting to the battery.