volts up and down 150 V6 Blackmax

I have a 1988 MERCURY V6 150 Blackmax on a 18ft mustang boat will do 100kms..

Question.......when my boat is running at idile it sits on 12.5volt when i raise the RPM it can get as high as 17volts. If i turn the stereo on it will lower the volts back down to 12 to 13 the louder the stereo the lower the volts get. This problem only started after I installed a 1200watt amp with 4 x 310watt 3ways and 1 x 1000watt sub, stereo amplifier main power is connected to master kill switch . I run twin batteries which are both new marine batteries all hooked up to a Narva Battery Master Rotary Isolator Cut Off Kill Switch 4 Positions (bat 1, bat 2, both bat, off) pictured below.

I'm guessing its the rectifier do you think this is my problem or could it be more???, Thanks Andrew

if you have a rectifier only charging system and you installed maintenance free(sealed) batteries this sounds about right..you will eventually lose the rectifier....there are many entries in here and other forums on the problem and there are many theories on the difference in the batteries..but the one i go by is there is no internal discharge on the newer style batteries so the voltage just keeps going up...if you have a rectifier only system then you need the old type barreries where you add water etc...an additional load such as your music system will over come the problem..

if you have a regulator system then it is broke but i think you have the rectifier only one...

if you have the owners manuel you will find that it calls for a much smaller battery in amp hours etc than you have...

dont feel bad about it...almost every boater i know,including me,buys good(the best he can afford) batteries for the boat...thats a mistake on the old motors with no regulator...

Your motor has a 16amp charging system and thats 16amp at WOT, the lower the RPM the less charging. So if your idling around your only putting out 2-3 amps and your stereo package is drawing more that system can supply. Also make sure the battery switch is a make before break or switching it with motor running can spike and take out rectifier.

Thanks guys, I just never had volt fluctuations before the amp was hooked up it would stay at a constant 12volts no matter what the RPM? I always switch the motor off anytime I turn the kill switch.