[AlecS]

The voltage the battery outputs is proportional to the amperage being drawn, and the resistance of the circuit. This is also known as ohm's law.

V=IR

Although batteries are not always perfect voltage sources, you can use ohm's law to determine the voltage drop in your robot fairly accurately. If you were drawing 300 amps, and the total circuit resistance was .020 ohms, you would lose 6 volts. This does not fluctuate over time, it is directly related to the current being drawn. (ignoring temperature and other factors). Hopefully this clears things up.