Cap is killing my battery!

if you have a dmm, please use it and check to see if your alternator is healthy....meaning that the voltage reading should be at the very least 14v when your engine is running. please also use this same dmm to find out if you have a battery power drain....anything more than 75mA is higher than normal. you mentioned that you have a second battery in the trunk....did you put in a battery isolator? it is highly recommended that you use one if your car is a daily driver so that you don't overload your alternator or cause any circuit loops. I agree with RufNUSD and auex. caps do not turn "on" or "off", they just simply store energy to help avoid major fluctations in power flow to the amp. those caps that do have a digital display but lack a remote turn-on lead do have a voltage sensing mechanism to turn off the display after a while. btw, how old is your battery? another thing to keep in mind is that the life of a battery shortens considerably if it has been drained previously. their ability hold energy is weaker and eventually will have trouble holding a charge for very long. good luck

The average output of an alternator should be at least 13.8 volts.