Hi!
I'm working on a door logging machine that displays time on a a display.
I'm using a DS1307 with a backup battery.
I want the Arduino to constantly measure the voltage left on the little backup battery, and write on the screen when the voltage is critically low.
I know how I measure voltage, but will this draw a noticeable amount of current from the battery?
I want the little button cell to last as long as possible.
That is not easy.. You need high impedance input to the ADC, at least 10Meg in order not to load the battery too much. Or, measure from time to time only - that may not load the battery.. (the RTC takes 0.8uA out of the battery)
You don't do this, you use a CR2032 lithium cell of equivalent and replace it
every 5 years or so. Just measuring the voltage is likely to drain the battery
dramatically faster than the RTC drains it.