I'm going to be adding a 1000watt (rms) amp & 15" sub (audiopipe TXX). I just upgraded my alternator to an AD-244 with excellent results for my ~800rms system (2x cheap lanzar 10's), but it's about to be a 1800watt system...

So I want to add at least one deep cycle battery in the rear (either gel cell or more likely a custom bracket in the tire well underneath).

Now, does it make sense to isolate the batteries with diodes?

I think the biggest diode isolator I found online is 160amp. Is this ok? I have 2/0 from the alternator and I know I'll be needing a bigger fuse for it...

 

Those diode isolators suck....they drop too much voltage through the diode(s). I was trying to solve the same issue and found a device called POWER-GATE. Installed it in my boat and now I have them in my boat and my car. Brilliant little device and performs as advertised.

 

I've seen some solid state non-diode devices advertised but I'm skeptical of how reliable they will be with the high current draw of a big stereo. I have to further explore this option.

The diode isolators I'm looking at claim low voltage drop, but I've done enough EE to know there will still be a loss of voltage...

On the other hand, does a slight voltage drop really matter? I suppose all of the lights will be a little less bright... As long as the output voltage is over 12.8 or so, the battery will still charge just as well.

My amp probably pushes the subs harder at 14.5v than 13.1v, but it's probably faster to fry a MOSFET at the higher voltage (I have one such amp on my desk in pieces right now... =).

 

I wouldn't worry about going thru the trouble. IF your amp is a true 1800 watts, then you may want to get a deep cycle battery. But, I think an Optima blue top will be just fine, stock location, no modification needed. IF that doesn't work, perhaps a power cap will do what you need. But I think you'll be fine with the blue top battery

 

1800 watts isn't necessary for a second battery. You're wasting your money.

 

These guys use MOSFET arrays and advertise less than 100 micro-ohms resistance. My voltmeter confirmed their specs and yes, there will always be some resistance but whatever these guys are doing just works. That said, their devices aren't cheap but I find quality products never are.