The MAX17043 is a low-cost, host-side fuel-gauge system for lithium-ion (Li+) batteries, manufactured by Maxim Integrated.
It monitors a single lithium cell and - using a sophisticated Li+ battery-modeling scheme, called ModelGauge™ - can estimate the battery's relative state-of-charge (SOC).
It offers an i2c interface to communicate with a microcontroller.
An Arduino Library is available.
Using that library you can get the battery's voltage and SoC; the IC has also an alert function that triggers an alert when the battery's SoC goes below a threshold value.
DSSCircuits sells a breakout board for this IC, as its package is very hard to be soldered by hand.
It's an i2c device, so connection to Arduino is straightforward: