I am wondering how many batteries I am going to need in my boat. It is going to have a 800w stereo in it with 2 amps, with twin 525's. It has 2 optima batteries and I am wondering if I should opt for a 3rd for backup?

i would put a third in just for the stereo so you know your motors will always start....

Is your boat a twin or single engine? My boat has a single engine,when it had the stock alternator and stereo was on,lights were on plus a bigger than stock fuel pump,msd ignition etc, plus all the normal electrical load, it would show a steady discharge while running. I ended up putting a bigger alternator(130 amp vs stock) to help keep batteries charged.( I use 2 optima batteries too). Twin engine guys don't have that problem. For peace of mind you could add 1 or 2 more batteries and have some kind of isolator that allows them to charge while running but not discharge from accessories,Smitty

I'd just try isolating to one batt when jammin and make sure you switch between just sarting on one and just the other to make sure both are "good" batts. Unless you want to dance for hours without charging,,then you do need a "stereo battery".When testing it's limits bring some jumper cables.

I went with 4 Optimas. I am going to have 2000 watts total.

The Optimas have a farily low Amp-Hr rating compared to a conventional lead acid battery (about 50 vs 80 or more), so while they do provide enough CCA to start your motors, you will discharge them pretty rapidly with house loads. Best bet would be to install more, or put a bank of one or two deep-cycle lead acid batteries on isolators to use as your house bank. Theyn you'll never have to worry!

P.S. I have two Optimas in my boat, and I think they are wonderful for starting batteries.

Go with 3. Be sure to set it up professionally. Don't just "link in a third". Research the proper set-up...its got quirks...but you'll end up with years of trouble free service.

Yeah... I like the idea of a deep cycle for designated "house" use.

I actually have two switches and trays for four batteries, but I only use three. Two crankers and one good deep cycle. Whenever I dock, I switch off the cranker and use only the deep cycle. I switch back to both usually while I am running. When I get to the next stop the deep cycle has recharged.

Your "800 watt" stereo probably only draws a lot when you are running and really jamming to get over the boat noise.
When you are docked and playing tunes, my guess is that most of the time you are not running the stereo wide open. If this is true... one would do it.
If you are the stereo king of the lake... go with two house batts.

Like Christian says... do the research and build a solid trouble free system. Don't just add one in.

Don't everyone croak, but I have six (6) large deep cycle Interstates in my 38 Cig and also 130 amp alternators. We spend 5-6 nights a month onboard and have a large stereo system, DVD player w/17" LCD screen, 12VDC fridge and lot's of LED lighting and some nav electronics as well. I charge batteries as three banks of two off my three shorepower charger outputs. I had extremely heavy duty battery cables custom made and I check and monitor the batteries regularly.

Never any problems with DC power at all. If absolute top speed is your mantra, you probably could not stand all the weight. But for us it's perfect. Never a worry in the world about anything 12 volts at all.

If I wanted to spend a lot of time away from shore and overnight often, that's what I would do. Plenty of good batteries.

I am wondering if mine is set up properly, I have 2 batteries and when I switch them back and fourth nothing seems to change. Where is a good resource to look for wiring diagrams??