PeterPan321:
No. Reading voltages on A/D inputs whose sources are the same (or different) from the power supply source is a vary common thing. But the battery voltage applied still MUST be brought within the accepted range of 5V max. Hence, the voltage divider I suggested. Even if there could be a short, the high resistances in the voltage divider I explained would limit the current to a tiny amount. That current would be the difference in voltage divided by at least 10,000 ohms. But you could use a pair of 100K resistors instead if you feel you need the extra safety or isolation.
I forgot to mention that I use 6 Volt Lead Acid battery and a step-down-DC-to-DC-converter to give me 5 Volts... So if I understood correctly I get the 6 Volt of the battery (before the step-down-DC-to-DC-converter) and I supply the Voltage divider you said...?