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Voltage Drop between Main battery and Additional battery


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I have a 2007 Ford Fusion with a stock alternator, probably stock battery (could be anything between 120-150 amp), and a Crescendo Logic F-31HP AGM Powercell battery in the trunk. The batteries are connected in parallel via 0 gauge wires (one positive and one negative) under the car. I installed the battery about a year ago and have had zero power/electrical problems since.

Within the last week my car has been having a hard time starting and in fact the last time I had to have someone jump me. I have a voltmeter up by the driver's seat that is connected to the back crescendo battery and it read 9.5 volts. When the car is running with the radio off, the voltage stays between 3.7 - 4.0 volts. HOWEVER, when I go to the trunk to maunally check the voltage of that crescendo battery, it reads almost 0.3 volts more. So because of the 0.3 volt drop, when I mentioned before that the battery was 9.5, it was actually 9.8, which is still bad. But, the problem is when I then check the battery in front, it reads 0.17 more than the back battery. When I first installed the back battery, the voltages were always within 0.01 volts of each other. Now the voltage drop is concerning me greatly.

So my theory is that when the car is off and I don't drive it for a few days the front battery is trying to charge the back battery up to its voltage, which in affect is discharging the front or maybe even both batteries. But, it is just a theory and I really don't know, that is why I am asking you guys what you think is happening and what I should do.

PS, I bought the car used like 3 years ago and have never changed the front battery and I am thinking it has never been changed, so it might be at the end of its life cycle

PSS, I know that it is bad when you add a 2nd battery and don't uprade the alternator or add one; so that might be a factor as well.

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Wish I could help you more, but to start I know it's been advised to not mix AGM's and wet cell batteries. What I'm thinking happened is, similar to your theory, that over time the weak wet cell has been drawing power and weakening your AGM...and the stock alt probably didn't help. Have you checked your fuses? Any corrosion on grounds/any connections or loose grounds/connections?

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All connections are good and tight. No cuts or problems in the wires connecting alt or batts together. I very tiny small amount of that battery corrosion dust stuff on negative terminal of the crescendo batt in back.

It is gotten even worse in the last day. I am pretty sure the front stock batt is dead. I am taking it out now, doing a load test on just it, which i am pretty sure it will fail and then go get a new replacement battery.

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Put your agm on a good amount charger to see if it will come back. Upgrade front batt to agm. Big 3 if you haven't already.

CURRENT SYSTEM:

  • Alternator:
    1. Stock alt on mids/highs
    2. Isolated DC Power 270 SP - Charging @ 16.8v
  • Batteries: (2) XS D1400s
  • Power Cable: Double Runs of 1/0 KNU Kolossus Fleks
  • Headunit: 80-PRS
  • Sub Amp: DC 5.0k
  • Subwoofers: 2 RE MT 15s /PSI dual .7ohm recones
  • Subwoofer Enclosure: 9.1cubes @ 32hz - brutal.
  • Mids & Highs Amp: CT125.4
  • Active Components:
    • Mids RE XXX 6.5c
    • Highs - 3 RE XXX tweeters per side(A pillar)
  • Noise Control: Damplifier Pro all over the cab.

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