Problems using ADS1115 to measure battery voltage of Raspberry PI UPS Battery Pack
The ADS1115 maximum analog input pin voltage is its supply voltage + 0.3V.
Which means, an unpowered ADS1115 can take 0.3V on its input without exceeding the safe limits.
The ADC also has protection diodes from analog input pins to supply pins.
If you connect the analog input directly to a battery, current will flow via protection diodes to the 5V supply node, trying to power the all the circuitry up, and there is nothing to limit current.
It is possible that the overvoltages and overcurrents may have damaged your chips and circuits.
Perhaps a resistive divider of two 10k resistors would help, so the battery is not directly connected.