You need to monitor the current, not a voltage. Put a small-value resistor, like 0.1 ohms, in series with the power. Measure the voltage across the resistor. Since you don't actually need to measure the current, just detect a threshold, you can use a comparator such as LM311. If you did want to measure the current, for example to detect end of charging, use a current sense amplifier such as an INA199 to gain-up the small voltage across the sense resistor to a level your Arduino ADC can easily measure.