Indeed, no external reference needed for the ADS1115, it's built in. 4.096V is fine for a 5V Arduino, or use 2.048 for a 3.3V Arduino (the maximum voltage that can be measured is the supply voltage, even if the range is higher).
A 1:10 divider would work very well (10k + 100k - don't make those larger or you run into problems with the rather low input impedance of the ADS1115). Then 36V would become 3.27V, offering a good resolution (almost 80% of full scale; about 26,000 ADC points for 1.4 mV resolution on the battery voltage), and allowing you to measure up to about 44V battery voltage which is important as at full charge your voltage will go well above nominal.
Note that due to the 1% resistor tolerance the measurement error is up to 2%, albeit highly constant over the range so if you have a more precise voltage meter you can use that to calibrate your sensor and add a correction factor. That way you should be able to get 0.1V or better accuracy.