You can use 0.1R resistor, in serial with batery. And measure voltage drop on it. I=U/R
Because you use smal value resistor( to reduce dissipation) you need to amplify resistor voltage. You can use just simple Non-Inverting Amplifier. Or something like MAX4080. Look datasheet for details. I used it, and it very simple, and it give 0-5V output, wich is perfect to use with ADC.