Battery bank wiring

Discussion in 'Electrical Stuff' started by Jmac572, Nov 4, 2021.

  1. Jmac572

    Jmac572 Active Member

    Dec 20, 2016
    Plymouth, MA
    2004 Sea Ray 390 Sundancer "Freedom"
    Twin Mercury 8.1
    Hi everyone... On a new-to-me boat I have four batteries connected in parallel on two banks. The prior owner has a group 27 and a group 31 on each bank (below). The manufactured dates are 3/21 on the 27's (Duracell) and 7/20 on the 31's (Odyssey), the PO never did work himself, the marina always did it so this was very likely them. I always thought that batteries on the same bank should be the same type/age, should I move these batteries so the Odyssey's are on one bank and the Duracell's on the other or is this a design from Sea Ray? It's a 2004 390 Sundancer w/ 8.1's. (and before anyone mentions it, the batteries/engine bay is MUCH cleaner and de-scaled since this photo was taken during survey)

  2. Korkie

    Korkie Active Member GOLD Sponsor

    Feb 2, 2016
    Leonardtown, MD Potomac River / Chesapeake Bay
    2006 340 Sundancer, Garmin 7610, xHD 24" Radar, Shadow Drive AP, VV703
    Merc 496 Mags
    Bravo III Drives
    I have always kept the same batteries throughout our boats. Mainly just by replacing them with same thing or better.
    Our 340 has 2 banks, one with 2 batteries and the other with 3., All of them are group 31 AGMs from Batteries Plus. Theory I have been told is the stronger battery will damage the smaller one. So safe answer to me was keep them the same.
  3. ttmott

    ttmott PhD in OCD TECHNICAL Contributor GOLD Sponsor

    Apr 3, 2012
    Space Coast Florida
    2006 52 Sedan Bridge
    Cummins QSM11
    That arrangement is a problem. The Gp31's will never get fully charged. You are correct to arrange them with the two 31's in one bank and the two 27's in the other. I would completely disconnect all of the batteries and charge each independently to float before reconnecting. After full charge remove the charger, bleed off the surface charge, and then check their resting voltage; If the batteries in a bank are within 0.1 volts of each other and resting between 12.6 and 12.7 volts without the charger operating you are good to go.
    Last edited: Nov 7, 2021
    Chris-380 likes this.
  4. Chris-380

    Chris-380 Well-Known Member

    Jan 20, 2019
    Pontoon Beach IL
    2000 380 Sundancer
    7.4L Horizon w/V drives
    Yes, as an electrician for over 30 years, batteries should be the same type, size and age. I would do as @ttmott suggested
  5. BlueYonder

    BlueYonder Active Member

    Jan 12, 2015
    Chesapeake Bay
    QSC 8.3 600
    Make sure you install covers over all positive terminals in order to comply with Coast Guard regulations. Covers on all terminals is even better.
    Chris-380 likes this.

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