At -40 degrees, the DS2781 capacity drops to 0 when discharging because the DS2781 has detected an empty point (battery voltage lower than empty voltage, and discharge current satisfies that of an empty voltage). After detecting an empty condition, the DS2781 clears the ACR register. Therefore, even if the temperature rises and the actual battery capacity is not zero, the detected battery capacity is still zero due to the zero value in the ACR register.
When using the DS2781 at low temperatures, the empty voltage of the battery can be set to a lower point so that DS2781 is less likely to detect an empty voltage at low temperatures by mistake, and will not reset the ACR to zero. Note that the empty voltage should be set according to the battery specifications. If set too low, the DS2781 may not be able to detect when the battery is empty.