retrolefty:
Well to be fair you also might be making it more complicated then it really is. In context with a nimh peak charger there is no reason for a series resistor to be used to detect when 'peak' voltage has been reached, in fact I'm not sure you could even detect it that way, just measurement directly across the battery terminals will work. A series resistor's voltage drop tells you about the value of current flowing into the battery rather then it's gradually increasing terminal voltage as it accepts a constant current charge from the charger. Now there might have to be a voltage divider wired across the battery terminals to keep the measurement value within say an arduino analogRead() range and scaled back up in software to track when peak voltage has been reached, but that is not effected by the current flow through the battery.Lefty
Perhaps you could explain how you are supplying 7.2V to charge a 7.2V battery and with only the battery in parallel with the 7.2V supply you would expect to read anything different than 7.2V? That makes no sense whatsoever.
Yes, you are measuring current. With Ohm's law, you have three components: voltage, current, and resistance. You need to know two of them to solve for the last unknown. So you use a known resistance, measure the voltage drop (which give you current) and calculate that the voltage across the battery by subtracting that voltage drop from the 7.2V using Thevenin's thereom. This is considered a half bridge circuit, where a wheatstone bridge would be a full bridge.
THAT makes it sound complicated. The OP doesn't really have to understand how all that works to apply it.
But to think that you would expect to ever measure anything different from a 7.2V supply in parallel with one component (the battery) makes no sense at all. And THAT was what the OP was (rightly) confused about.
BTW, I am an electrical engineer in the aerospace and defense business. I would hope that I could understand something as simple as a battery charger. Just saying...