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  1. #1
    Join Date
    Apr 2019
    Posts
    1

    Default Dual batteries, but which loads for each?

    I’ve been reading about installing a second battery in various boats, but there appears to be a difference of opinion on what loads should be connected to each battery. Obviously this depends on the type of vessel in question.

    In a motorboat with an inboard engine for example, many advocate only the starter motor be connected to the starter battery, and all other loads be connected to the ‘house’ battery. Others stipulate that loads such as engine instrumentation, fuel pump, ECM, windscreen wipers, cabin heaters as an example be connected to the starter battery as these are loads specific to the engine running.

    Any thoughts?

  2. #2
    Join Date
    Jul 2003
    Location
    West Australia
    Posts
    11,490

    Default Re: Dual batteries, but which loads for each?

    The concept of dual batteries is to permit discharge of a second battery which is quarantined form the essential battery which starts the engine. So if sleeping over night using the service battery you can have no fear that running your tV or fridge, cabin heater, etc will leave you with a flat battery and unable to start the engine for charging. Hence these services which may be run with engine stopped should be run from the isolated battery.
    Now it would seem sensible to keep only engine start on the engine battery. However it usually includes gauges and warning systems associated with the engine.
    Now when it comes to anchor winch, windscreen wipers fuel pump etc which are only used with engine running perhaps it does not matter so much. The current especially of an anchor winch taking a lot of current might best be taken straight from the engine battery to avoid this current going through the charging connection to the service battery. VSR or battery switch or diode. (less volt drop less stress on the device). All this presupposes that you understand the concept of having both batteries in parallel with engine running for charging and isolated when engine is not running.
    In the end you decide regarding your boat use habits and the need for load isolation philosophy. ol'will
    Last edited by William_H; 26-04-19 at 03:07.

  3. #3
    Join Date
    Sep 2001
    Location
    Home - Sothampton, Boat - Gosport
    Posts
    9,693

    Default Re: Dual batteries, but which loads for each?

    It makes sense to have engine services such as gauges and wipers run through the starter battery, via an "ignition" switch just like a car, but I'd put cabin heating on the domestic side. You may think you'll never use it without the engine, but one cold, damp evening...

    I'd also wire things up so you can use the domestic battery to start the engine in an emergency. I use an off-1-both-2 switch, with 1 going to the engine battery, 2 to the domestic one and the common to the starter. A voltage sensitive relay will allow charging of both batteries. (I use a £2 headlamp relay, but I'm a tightwad, and VSRs were a lot more expensive when I set my system up)
    Steve
    Never believe anything until it has been officially denied.

  4. #4
    Join Date
    Jun 2006
    Location
    Algarve
    Posts
    10,844

    Default Re: Dual batteries, but which loads for each?

    The more domestic equipment you have, the larger the domestic battery you need and hence the need for different charging regimes for starter and domestic batteries. That leads to having a smart charger that treats the batteries differently.
    One tip is to fit a tiny AGM battery for the starter and so to make space for more domestic battery capacity.

  5. #5
    Join Date
    Mar 2010
    Location
    Ammersee, Bavaria
    Posts
    4,551

    Default Re: Dual batteries, but which loads for each?

    The point of a starter battery is to get the engine started. Once it is running, everything on board, basically every possible electrical load, is provided by the alternator - at least it should be. I have one battery for the engine (starter and engine panel) and everything else runs off house batteries .... The starter battery is back to 100% very soon after starting the engine, then the alternator recharges the house batteries and runs everything else at the same time. Isolate the house batteries from the starter battery, and the alternator should be wired to charge both. The most important thing is it must not be possible to discharge the starter battery once the engine is switched off.
    The best argument against democracy is a five-minute conversation with the average voter.

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