This example shows how to design protection for an 18-V Li-ion battery pack using 4.2-V cells with the following requirements:

• The system operates from 15 V to 21.5 V.
• The battery allows 4-A continuous current.
• The battery protects with 8-A discharge current > 500 ms.
• The battery has short circuit protection in < 2 ms.
• The system is for operation in an office environment: 10°C to 30°C.
• The cell normal charge voltage is 4.2 ±0.05 V to 0.05 C.
• The cell cutoff voltage is 2.75 V.
• The charge temperature is 0°C to 45°C.
• A cell configuration is selected to provide 5 Ah over the system range of operation.
• The cell assembly is capable of > 30-A short circuit current.
• Cell balancing is desired with a current of 10% of termination current.
• Low current drain is desired when the pack is removed from the system.
• Load removal for fault recovery is required. Recovery by connecting the charger is acceptable.

To start the design:

1. Start the schematic:
   • An 18-V pack using 3.6-V nominal cells requires a 5-series configuration. A single BQ77915 device is needed.
   • Follow the 5-series reference schematic in this document. Follow the recommended design parameters in Design Requirements.
   • Because a single device is needed, CTRC and CTRD are connected directly to GND.
   • The power FET used in this type of application usually has an absolute maximum of 20-V Vgs. For an 18-V pack design, transient voltage during an OCD may exceed 20 V, so the diode across the 1-MΩ R_CHG2 is used. R_CHG helps to slow the charge FET from turning on.
   • Because a charger connection for UV recovery is acceptable, the condition in Using Load Detect for UV Fault Recovery is not a concern. A 1-MΩ R_{GS_CHG} can be used for the schematic.
   • The optional sense input filter is selected for the circuit.
   • Because low current storage is desired, the PRES pin is brought out of the pack for control by the system. The standard recommended R_HIB value is used.
   • Because cell balancing is required:
     • Connect the CBI pin to VSS.
     • Determine the resistance for the R_IN filter resistors. Since the charge taper current will be 0.05 × 5 A or 250 mA, 10% is 25 mA. With a 4.1-V cell, 25 mA would require 164-Ω resistance. This resistance includes the internal R_BAL resistance and two R_IN resistors. 75-Ω resistors are selected for R_IN.
2. Decide the value of the sense resistor, R_SNS.
   • When selecting the value of R_SNS, verify the voltage drop across SRP and SRN is within the available current protection threshold range.
   • In this example, only one protection threshold is specified. The minimum available OCD threshold is the –10-mV OCD1 threshold, but this would result in an odd value for R_SNS and the tolerance of the threshold is 30%. Using the –60-mV threshold of the BQ77915 configuration, a 10-mΩ sense resistor would give a 6-A nominal OCD threshold. With the 20% tolerance, 4 A can pass without OCD and 8 A will always cross the threshold.
   • A 30-A SCD with a 10-mΩ sense resistor would be a nominal 300-mV threshold. Tolerance must be considered and the protection threshold can be lower than the battery capability. The 120-mV threshold of the BQ77915 configuration with a 10-mΩ R_SNS will give a 12-A nominal short circuit threshold.
   • Select R_SNS = 10 mΩ for this example.
3. Determine the remaining BQ77915 protection configuration:
   • Charging the cells at a lower than maximum voltage allows a margin on setting the OV threshold. The system could allow a 4.15-V OV, while the cells might allow a 4.3-V OV. Since the charge voltage will be 4.1 V/cell, this is the desired VFC point of the BQ77915 device. The 4200-mV OV threshold and 100 mV VOV – VFC of the BQ77915 device are suitable.
   • OV hysteresis and delay values are not specified requirements. A 1-s delay will be selected. Some hysteresis is desired to prevent cycling if the battery were to reach OV. 200 mV is acceptable.
   • The system will stop operation at a nominal 3 V per cell, while the cells could operate to 2.75 V. Some margin below the 3 V should be allowed, because cell voltages possibly vary at low states of charge. A 2750-mV threshold option is available, but the existing BQ77915 configuration has the 2900 threshold.
   • UV hysteresis and delay are not specified requirements. A 1-s delay is selected. Generally, a larger UV hysteresis will avoid system cycling from automatic recovery; however, in this design load, removal is required and charger connection is expected for UV recovery. The value could vary, but 400 mV is selected.
   • Open-wire protection is selected at the 100-nA level.
   • t_OCD1 or t_OCD2 could be programmed to 350 ms to protect in less than 500 ms, or the default BQ77915 180 ms is used. However, the 350 ms can be selected with R_OCD. Use 604 kΩ 1% for R_OCD.
   • The 2-ms SCD response time allows either SCD delay selection.
   • Overcurrent charge protection is not specified in the requirements. The BQ77915 60-mV setting will allow a 1C charge.
   • For temperature protections, the 0°C to 45°C charge temperature thresholds match the range for the cells. Use the lower range for discharge.
   • The VCBTH – VCBTHL determines the voltage spread during constant current charge when balancing is allowed. 100 mV allows some spread without balancing.
   • See the summary in Table 10-2.
4. Review the available release in the Device Comparison Table to determine if it is a suitable option. In this example, the BQ7791500 configuration is suitable. If it is not suitable for your design, contact a TI representative for further assistance and for information on BQ77915 PRODUCT PREVIEW devices.