If your car is still starting without any kind of charge or jump, then it's not the battery.EVERY Alt is grounded to the block of the motor unless you've got a plastic mounting system. When alts are tested before they leave my local shop they don't have an extra ground wire, just the bolt to hold it in place.
Remove the capacitor especially if you're running more than one amp. Once they discharge then the amps are starving for power. Where are you reading dropping to below 11v? Dmm on the battery, dash voltage meter or trusting the capacitor's digital readout?
Big 3 consists of battery positive to alt post (fused), battery ground to frame, frame to engine block. Then make sure your grounds on the amps are connected to the frame as well and that's it.