Detect battery/cell's charge
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) Potential divider for ADC. (b) Calibration circuit.

You only need a potential divider such as Figure 1a to bring the battery voltage down into the range of the ADC. Just make sure that at maximum voltage that you don't exceed the ADC input voltage.

You can calibrate and test as shown in Figure 1b.