I know that when using the rechargeable 9.6 NiCd battery in the transmitter, the real time battery voltage is conveniently displayed. The transmitter must obviously be placing some appropriate load on the battery for this reading to be accurate.
Can the transmitter also be used, essentially, as a voltage meter with the 7.2 volt batteries meant for the robot. Is the load placed on the battery sufficient for an accurate reading with this 7.2 volt battery as well?
I believe the voltage measurement is done with a simple voltage divider feeding into one of the analog inputs of the transmitter’s microprocessor. In fact, I believe it is wired exactly like one of the joystick channels.
So, I think that as long as the battery has enough power to run the microprocessor and LCD on the transmitter, it should provide a reasonably accurate reading.
Of course, this is not reading the voltage “under load”, so once the battery is placed into a robot with a high current-demand, the voltage will dip lower.
This is all conjecture from poking around the innards of a transmitter, so don’t count on this being perfectly correct.
It’s not at as good as a proper battery tester that also applies a load, but we have found through empiricism that if the 7.2V battery reads more than 8.0 volts on the transmitter display, it’s doing well. A freshly-charged battery usually reads 8.2-8.4 volts. One that reads 7.8 or lower is not going to last long in the robot. Like I said – not a deeply scientific analysis but sometimes rules of thumb work well enough.
To sum up others’ replies:
The voltmeter’s load is enough to accurately measure no-load (or close to it) voltage; but says little or nothing about how the battery will behave under a significant load. You have to keep track of that aspect of battery health separately from measuring the battery’s non-load voltage.