Hi,
Have you looked at the specs of the ADS1115, can it measure negative voltages?
I double checked the common ground - there is none. The positive though seems to be common between the PV input and the battery charger.
Because you are effectively measuring the two differential voltages with respect to the positive supply.
When the charger drops out, the negative terminal of the PV will go to full open circuit volts which will be more than the system volts.
This means the ADS1115 inputs, or at lest one of them will go negative with respect to the ADS1115 gnd.
With a DMM, and the ADS1115 out of circuit, and the charger in the OFF condition, measure the open circuit volts of your PV, then measure the voltages where the ADS1115 is connected, ALL of the connections on the ADS with respect to gnd.
Thanks.. Tom... 