Re: tune is very inconsistent could big injectors be the problem?
 

tune AFR vs. battery voltage.

You will not be able to put enough drain on the car to take it down to 10 volts unless you unplug the alternator while it is running (WARNING DO NOT DO THIS - IT IS DANGEROUS)

More often than not, you will be able to explore the voltage range 14 - 12v
Extrapolate the shape of the curve from there.