Wiring for a 3rd amp

Time to a third amp to the system for some tower speakers. Currently I have a starting battery and a stereo bank with 2 group 27's with a Blue Seas relay in between the banks. The batteries are all in the back where the original was, but for weight distribution the stereo bank has one on each side. My issue is that I'm out of amp space on the port side so the new amp will go on the starboard side. Is it a problem to run this from the stereo battery on that side, or should I keep the amps all wired off the same battery? The batteries are paralleled with about a 4 ft run of wire between them and all grounds are on a common bus. The only problem I can see would be the real need for an onboard charger to bring everything up to the same level. Right now the 100amp alternator is doing just fine according to the DMM.

With the two G27's wired in parallel, they are in a sense, one big battery. So it doesnt matter if the new amp install terminates to the port or starboard battery. With 3 amps, it could be a good time to look into a charger, especially if you spend much time playing the system with the engine off. If you do spend a lot of time with the engine off, and the batteries are being cycled heavily, It may be a good idea to wire in a simple ON/OFF switch in between the two stereo batteries. This will minimize the load that it placed on the alternator when the engine is fired up after hours of play. Down side is, this would require you to terminate all the amps to the battery thats directly wired to the ACR, or loose that amp until battery A has recovered and you are ready to bring battery B back on line.

Whats the distance from the battery to where the amp is going as compared to where the other amps are located? Cant you just come off a distrobution block thats feeding the current amps and run to the new amp's location? If yur two amps are fed with their own cables, I would consider rewiring with w trunk line from the batteries and using distrobution block at the amp rack.

I'm wired with a switch between the 2 stereo batteries already, it stays on both until we leave the boat. The output of that switch goes to one side of the relay, the the charging wire from the alternator goes to the starting battery which is then wired to the relay. It tools a bit of sodering, but I had to keep everything in the back and still have acces to the electric hatch. The relay has a switch in the ground wire to keep it from looking for the stereo bank when it's off. So far I've run everything at the house at a normal volume for 7hrs to get down to 12.5 volts. I don't forsee the alternator having an issue, we don't sit much and I don't see many situations where all 3 amps would be up. Drives me crazy to listen others with no riders so WS 420 and an HSE Revolution were a must. If I wanted to listen to Rap or Country I'd have it on.

The amps are only about 2ft of wire from the batteries, just outside of the engine compartment. The current amps have about a foot of 1/0 wire to distribution blocks then through the fuses and on to the amps with 4ga. All the stuff sitting around from a project that never happened. For aesthetic purposes I'll probably do the new amp the same way.

By the way, thanks, I did think it would do any harm, just wasn't sure. The collective boat stereo knowledge around here is amazing.