Using arduino as a voltmeter, current drain with a voltage splitter?
Use a P channel mosfet to only enable your divider when you need it, (have a weak pullup to battery+ to switch it back off, and an N channel to pull its gate down to turn it on)

This is how its commonly done on other battery monitoring circuits, as its not like the battery voltage changes that much in say 10 seconds.

There are other approaches when you have control of the silicon in a chip, but for general user land, this is usually a good compromise.