Dual battery systems

Discussion in '80-Series Tech' started by ParadiseCruiser, Jan 25, 2004.

  1. ParadiseCruiser


    Likes Received:
    Mar 27, 2003
    In another thread on batteries, the topic was drifting in the direction of dual battery systems in general - a subject I would like to present to the group for some considered input.

    I begin with a quote fron "Rich"...
    >> To maximize the life of dual batteries it is highly recommended to do one of the following: 1) use exactly identical batteries including age, or 2) ensure that your charging system individually senses the voltage of each battery and charges each battery seperatley, and also ensure that the batteries are never in parallel other than in emergency situations such as winching, engine starting when one battery is insufficient, or jump starting your buddy. Otherwise one battery can end up being continuously overcharged and the other continuously undercharged, which is bad for both."

    I generally agree with Rich's comments, though I am not 100% certain I agree with "Otherwise one battery can end up being continuously overcharged and the other continuously undercharged" if the charging circuit and both batteries are working properly.

    In any case, in my experience this is not the way most of us do it - nor the way that many OEM installations are designed. In other words, this may be the ideal system. BUT, as a practical matter, is it really necessary?

    I have two petrol powered trucks with dual 12v batteries. The first (a Ford) uses an isolator (SureFire, NwWranger, HellRoaring, etc.) - and the second (an 80) a solenoid and relay system, similar to the circuits widely cited here (Slee, Scolaro, etc.). Both have used different batteries (type, kind, age, etc). and both work exceptionally well.

    Isolator. I just recently pulled a 7-year, deep cycle aux battery from this truck after 9 years of service. This 7.5L powered truck has the whole bit: lights, winch, radios, refrigerator, etc., and uses a standard, high current starting battery that has consistently yielded more than 5-years of service.

    Solenoid. This system has only seen 2 years of service, with somewhat less of a load on the aux battery (truck is still being built up). Batteries were dissimilar originally, now I am experimenting with like-type batteries. The system works well either way with no apparent problems.

    My neighbor has a newer Ford 3/4-ton diesel pickup with dual 12v batteries (truck uses a 12v starting circuit) that are always in parallel, running or not. This is an OEM installation.

    Truck with an isolator. Engine off: Batteries are isolated. Eng running, charging circuit is in parallel, discharging circuit batteries are isolated.

    Truck with solenoid. Engine off: Batteries are isolated. Eng running, both charging and discharging circuits are in parallel.

    I can think of significant ways in which these three systems differ, both short comings, and advantages. I believe we would all prefer to have the best, most reliable system for our 80s, but which would you (do you) use and, most importantly, why.

    Thanks, Ron -
  2. squeezer


    Likes Received:
    Oct 13, 2003
    As a data point:

    I have a 78 Ford with a factory dual battery system, uses a solenoid and fuse link...

    This truck has been in the family since new and is on its 2nd set of batteries. The OEM's lasted untill right around 1993'ish and the 2nd set (Optima's) are just starting to show some age.

    I thought is was a fluke with the OEM's but the red tops have lasted way longer than expected as well.

    I can only guess that always running a matched pair and having a relatively mild 70 amp alternator are the key to the long life in this application.
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