aux batt in, new probs

Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=117473
Printed Date: May 12, 2025 at 11:24 AM

Topic: aux batt in, new probs

Posted By: coolen
Subject: aux batt in, new probs
Date Posted: November 04, 2009 at 7:16 PM

Well I installed my second battery in my 95 Nissan around 2-3 months ago with no issues. However all of the sudden, the truck has been flickering warning lamps and exhibits other issues as well. When I was under the hood today I heard a small "boiling" sound around my battery (starting). Come to realize, that when the starting battery is connected without the truck running, it drops to 12.06v, but when disconnected it sits at 12.3. I had the battery tested tonight and it tested fine. However, when I disconnect it, the "boiling" hiss goes away.

My isolator is a 120amp Princess auto model, and it has 4 terminals. Batt 1, ALt, Exciter, and Batt 2. I figured it was pretty much straight forward when I installed it. The charge wire from my alt goes the the "ALT", my starting battery goes to "BATT 1", the exciter is empty, and my auxillary battery is connected to "Batt 2". I tested for power on the pos term cable on my start battery when it was disconnected, and there was none. And there is a constant 13.5 on my aux battery.

Am I missing something here?

Thanks

Replies:

Posted By: coolen
Date Posted: November 04, 2009 at 7:46 PM

here is the manual for the isolator, maybe i grouped it wrong or something:

https://www.princessauto.com/components/com_virtuemart/shop_manuals/8235699_manual.pdf

Thanks

Posted By: haemphyst
Date Posted: November 06, 2009 at 9:57 AM

Ahhh.... this warms the c-ockles of my heart, and vindicates my obsession to DETER the use of isolators!

You are ruining the front battery! You are likely getting a charge sense from only the back battery, and WAY overcharging the front one... The isolator isn't telling the alternator to stop outputting when both batteries are charged, only when the back one is done. Since the audio system is powered off the back battery, it never gets a full charge, (the system is pulling more than the alternator can put into it) thus never telling the alternator to turn off or slow down. Replace both batteries now, and wire them in parallel, WITHOUT the isolator.

To do it correctly and safely, both batteries MUST be the same type and age, this is why I am telling you to replace them both.

This is also an indication that your alternator is too small for your system. Do the Big Three, and upgrade the ULTIMATE POWER SOURCE IN THE CAR - your alternator. The batteries don't magically make power, my friend! They are there primarily to start the car and secondarily to filter the electrical power produced by the alternator, NOT run all your accessories!

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."