first of all your altenator stops charging the battery after it senses the right voltage.
the only way for that not to occour is if you had your system running at max volume the minute after you start your car and continuously never lowering the volume. hence altenator would never see the expected voltage to shut down charging mode and decrease current. this is also why you system sounds the loudest and cleanest right after you start the car. because there is two power sources feeding accessories. but who the heck would do that not me, hopefully no one. unless they were trying to show off or something. now before i explain what you can do before you upgrade alt. i will give you a little tip, neons dont take a lot of power. to prove that i once hooked up an entire underbody single transformer neon kit to a nine volt battery. which may or may not have been new. but it stayed on for well over an hour. now thats nine volts and the batt is small now image a 12 volt battery normal state and that same battery at aprox 13.5 when car is running. well my point is that if street glow puts more than 50 neons in there show car and never even think of a second battery or anything(car not running) than you should not have to worry about it unless you have a short than it will not matter how many batteries or amps your alt has.
now back to original post. traditional battery diode isolaters suck. especially for the sound person why 2 reasons
1 unneccessary noise introduced into the power lead.
2 a drop in voltage in and outputs of the isolator.
now you have to have an isolator simple cause your batteries will compete against each other at resting state (no power being used) and the weaker batt will begin to brake down.
same thing take two nine volts and put them together they get hot and maybe one starts to leak battery acid or the acid just decais inside and the battery or batteries both die. pretty much the same thing.
now its really simple and not expensive
buy a pac bg12 and a pac 200 amp relay(non diode isolator)
than wire your trigger through the voltage sensor/warning module (bg12) than wire up the relay the two main large terminals are for the power lead and the two small terminals are for the relay trip.
that unit alows you to monitor and set voltage to turn off the back battery ( which all things should be connected through after fuses) so that if you sit in a parking lot there is three led's on the box which in my car have be desoldered and relocated above my voltage guage. any way there is a green-good. yellow- warning and red- dead. well depending on the set voltage red or even yellow would de activate the relay so that your main battery would not be affected. and thats sounds like what your looking for. help this really helps instead of the dead posts already posted. no offence but there just not informational.
Jon
Installer/Help Technician
---coral springs florida---
mecp certification is not always needed. I have it and it has not helped me out at all. my experience out shines it.