Voltage backflow into Analog inputs

I'm using an analog input of an Arduino UNO to monitor battery voltage for a robot. The battery is a 2-cell Li-Ion stack producing about 7.2 V.

Rather than use a voltage divider on the total battery stack to get the monitored voltage within the allowable analog input voltage range, I decided to simply monitor the voltage of the bottom cell in the stack. Unfortunately, this means that when the battery is disconnected from the robot and Arduino via the main power switch, the 3.6V from the bottom cell is still present at the Arduino input pin.

The 3.6V at the analog input pin is clearly back-flowing into the Arduino to some degree, as the power LED illuminates, and the loaded program appears to run. Is this a feature, or a bug, or what? Although I haven't seen any evidence to date that this back-flow is damaging my Arduino, it is at least very worrisome.

Any thoughts?

TIA,

Frank

When the Uno's MCU is turned off, the inputs basically act as they're connected through back to back diodes to GND. This is normal, however note that voltage should not be applied to inputs while power is disconnected.

Try something like this (untested). You'll need to account for the voltage drop across the diode.

Use either a voltage divider or a protection resistor (to the analog input) of 4k7 or 10k.

A series resistor would help ... I guess 10K would limit the "back flow" current to around 0.33mA, which would definitely protect the UNO's input. OK if 0.33mA drain on the battery is acceptable. |500x163