Battery Charging Current Determination
What I have always seen/done is a small-valued resistor (in the range of 0.01-0.05 Ohms; they're called current sense resistors) in between the battery's negative terminal and the system ground. Measuring the voltage at the battery's negative terminal (with respect to system ground) will give you the current flowing into the battery, and you can adjust the applied voltage at the positive terminal to control it.