Running amp off a 2nd battery in my trunk?

Hi. I should start off by saying that I know very little about Car audio. I have a 2006 Ford Fusion. I bought my current 600w amp and pair of 12s in a box from my cousin. This was 9 years ago. For 7 years it was working great. I got it installed by a Car audio place. Worked nicely with my original car radio and when that went kaput, it worked fine with the pretty basic Pioneer deck I bought after that. An electrical guy in a shop installed that for me 4 years ago. I only use my system from the months of Nov. to Apr. as I need the trunk space for summer activities. However, my amp has always been screwed into the back of my backseat and the Power cable attached to the battery. One winter ago in Nov 2014 I noticed that when I connected the woofers to the amp again, I was getting really big bass but the lyrics and what not were all mumbled. I played around with it a bit and it would work for like 30 seconds and then same problem. I wondered if I had a bad amp. In Jan. 2015, my car battery started giving me problems. Couldn't start car sometimes. But hey that's winter right? Well that battery was only 11 months old. In Feb. 2015 I got my battery tested and the mech said it wasn't holding the right amount of power. I disconnected the Power cable of the amp from my positive terminal and continued with that battery for 3 more weeks. But continued having problems. So I replaced it with a much higher end battery that cost a hundred bucks more. And I did not connect the amp back to the battery for the rest of the winter or even spring and summer. Nov. 2015 I try my woofers again and connect them to the amp. And connect the amp power cable to my high end battery that is now 9 months old. Same problem of really big bass and mumbled lyrics and even some feedback. Now I'm thinking my amp is bad. Anyway, I take out the woofers until Jan. 2016 but leave the amp cable connected to battery. Jan. 2016 my car won't start. Battery problems. Get a boost and it's fine. Good for another 3 weeks. Then it does it again but boost didn't even start it. So I trickled charged it for an hour with a battery charger I use for my deep cycle boat battery. The manual says I can use it for car batteries. This time I disconnected amp cable from battery. It's fine again for the next 2 weeks. Feb. 2016 my car doesn't start. This top of the line battery is 1 year old. My car won't even try to crank or anything. This time I get it towed to the same mech. who sold and installed my battery. He says it's not holding the charge it should be. Says the alternator is fine. I get battery replaced under warranty for free. Mech. said it is possible I could have a bad amp that is drawing power on my battery even when my car is off. But he cannot say it definitively. So this got me thinking. Should I get a separate battery and keep it in my trunk to power my amp? I actually have found 2 old amps from 2 other people who had them stored in their garages after they had kids. They were working fine when they were last used 3 and 6 years ago. One is only 400w while the other I don't know because it doesn't say on the top of it but it is bigger. So should I take off the old amp from my back seat and drill in one of the others and connect the cables, and then run a new power cable from it to a new car battery in my trunk? And then charge that battery every few days with my charger that I use for my boat battery? I actually spend a total of 30 minutes a day in my car during the working week. Does this make any sense? I don't want to connect one of the other amps to my brand new battery I just got and have the hassle of what I've been going through again later on in the year. Would it have to be a car battery or could I use a general purpose battery? And I'm trying to keep the costs down since I only use my setup for 5 months a year and want to have kids soon so my trunk space would disappear. Thx for the suggestions.

Well, first of all, if your having dead battery issues, and you have tested your alternator (almost anywhere does this now, Autozone, etc) then you obviously have a grounding issue somewhere. If the audio your hearing is notably different than you remember before this whole mess and no settings have been changed on the amp, then it COULD be the amp, however you mention that vocals are an issue too. Most car amps for sub-woofers today are mono and usually set up to cut out any sounds above certain frequencies, like 85hz or higher, to prevent stressing the sub-woofer and distorted mid range sound from a speaker that's more tuned to running nothing but low end bass. If you have inadvertently switched your crossover to maybe a higher pass setting, or turned the frequency up that could explain the sound issues, but not really the grounding / power loss. Grounding could be happening anywhere in the vehicle, not just with the amp. My advice to you would be to pull the fuse on the power wire running to the amp and leave it out for a few days / week. See if you continue to suffer power loss with the battery. Buy a cheap voltage tester and check it periodically with DC volts setting to see if it's creeping lower by the day. If the battery continues to creep down then you may have another frayed wire somewhere grounding and draining your battery. You might also check with things like hood/glove box/trunk lights, accessories that aren't turning off, etc. If the power drain stops, then likely your amp is suffering some issues and needs either repaired or replaced. Not knocking the idea of running a 2nd battery, but a 600 watt amp shouldn't need another battery, and even if you install one without an automatic switch in between them you will drain them both if you don't address the actual problem. Let us know what you find. Rest easy, amps aren't that expensive these days. You can probably replace that one with a shiny new replacement for under 100 bucks. Less than the battery mod.