It doesn't mater how loud the "POP" is, the FPS will still be the same. You're not thinking about how an AEG works. The FPS is determined by the spring and the battery only helps pull that spring. For every shot, the spring has to be fully retract for the shot to happen. If not (like a "weak battery"), then you will not get a shot and your piston will be fully back and compressing the spring. Hope that helps.
Originally Posted by Percuvius
If you want to see a 9.6V with a "weak" charge left fire just watch my VFC M4 SOPMOD full loadout video in the media section. Only a COMPLETELY DEAD battery will not fire at all. It could barely retract the spring and gave just a little pop when pulling the trigger. After I recharged it, it was firing full throttle again.