what do you mean by the "real deal" ?
if the code relies on, say; int battVolt = analogRead(sensorReadingBatteryVoltage);
then simulating the decrease of battVolt by code is all you can really do.
if you want to model how it really decreases (like a graphed curve), then you should refer to the datasheet for how it behaves - failing which, you just have to actually do it for real - on site and live run.