INA219 measure voltage of battery with RPi
See the schematic below, taken from the datasheet. Reading the datasheet explains that VIN- is the input from which the bus voltage is measured. You should connect this VIN- to your battery's positive terminal if you only want to read voltage. If you want both current and voltage, connect as shown below with "Supply" being your battery positive terminal and "Load" being the power input on your "Device powered from battery".

Edit: to be clear, the measured voltage is VIN- minus the INA219's GND. You will need to connect the INA's GND to the battery's negative terminal, and likely to the RPi's GND as well.