Arduino Voltage Divider Connection
The voltage divider reduces the battery voltage by a factor of \$\frac{R2}{R2+R3} = \frac{1000}{2500} = 0.40\$. This puts it in a range that allows the Arduino to measure it. 0 to 12.6 V (minus the diode drop) becomes 0 to 4.8 V, which can be measured by the ADC, which accepts 0 to 5 V.