12v/12v dc-dc auto transfer switch

Hi there - Looking for some input on my 12v power/charging setup. This topic might seems more geared towards the RV/Marine guys, but I'm hoping the solar people might have some more insight as it deals with strictly 12v power source switching.
It involves my Daily Driver 2002 Silverado Truck with dual 130 amp alternators, one battery bank (2-50Ah YellowTops) and isolated accessory/start battery. Batteries and alternators are currently isolated via Charles StartNow relay (which really only isolates batteries in non-charge state and does not isolate my alternators).
Ultimate goal is to be able to run stereo and accessories down to low voltages and have no problems starting vehicle. I am trying to setup two completely separate circuits (1- battery bank being charged with one alternator, regulated by future smart externally regulator, and 2- start battery/alternator powering all systems, including 150a max. stereo).
Upon turning off vehicle, the power source would be switched from starting battery/alternator to fully charged aux. battery bank. I love the idea of having a 100% fully charged battery bank being charged @ float at my disposal every time I turn off my truck.
All the transfer switches/relays I have come across are of the 120vac/12vdc type (marine-shore application). Is there such a thing as a 12vdc/12vdc transfer switch? I've thought about higher amperage relays, but this setup would involve dual inputs with common switchable output.
I like the idea of my truck running normally, and want to isolate the voltages that are sometimes associated with charging a depleted battery bank via external regulator from the normal operation of my truck.
Brands I've been researching are Xantrex, Sterling, Balmar, Charles. Can anyone suggest others?
Any and all input-comments welcome. -Nathan


  • blackswan555
    blackswan555 Solar Expert Posts: 246 ✭✭
    Re: 12v/12v dc-dc auto transfer switch

    Am I correct ?

    You want to run two totally separate charge/bat systems ? When the truck is running, The stereo runs off "main/truck" battery, when the engine is not running, the stereo runs off "aux" bats, ?

    Yes ? You need a large relay (Max stereo draw size) with NO & NC contacts, Feed the coil off the main alternator warning light, wire NC to aux bat, wire NO to truck bat, Com to stereo + As truck starts, alternator light provides a +, closes relay, stereo runs on truck bat, Stop charging, relay opens and defaults to aux bat, (You may get an audio click on change over, SSR would be better if poss)

    The more "normal" way to do it, used for most "AUX" bat circuits, Caravans, Boats, Campers, Winches, Stereo`s, is the same but only using one alternator, The relay switches the aux bat+ to the main bat/ alternator circuit, Charging aux bat only when running,
    Have a good one
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    Re: 12v/12v dc-dc auto transfer switch

    I have one of these charging switches for a camper I had and never installed if your interested. Forgot I even had it until MAMA made me work to clear out some of the "Junk". PM me if interested.
  • vcallaway
    vcallaway Solar Expert Posts: 157 ✭✭✭✭✭
    Re: 12v/12v dc-dc auto transfer switch

    Intelletec makes something like what you are asking.

    Could probably pick up the controller and a pair of relays at an RV salvage yard.
  • System2
    System2 Posts: 6,290 admin
    Re: 12v/12v dc-dc auto transfer switch

    Thanks Tim, it's amazing, got to thinking the same thing, but wasn't sure about the relay trigger. Just seems too simple, surprised such a device isn't readily available. Guess not to many people run two alternators.
    So the lamp wire off the alt. would be my best choice? (In the past I have used a relayed ignition trigger (key 'on/off')
    Haven't used the normally closed relays, are they identical to NO ones, but reversed?
    What can you tell me about SSR's?
    Any advice as to where to pick up some good NO/NC relays?
    What's your opinion on digital external alt. regulators? I've been told elsewhere that IC alternators are smart enough to recharge my aux. batteries and should be able to handle the load.
    I have read the external regulator manuals (Balmar, Xantrex, Sterling) and am leaning towards the Sterling, as it seems versatile ('A' or 'B' circuit alts.) and can be used in parallel with the existing internal regulator. (I have two Bosch 130a CS144 series alternators running).
    Perhaps the others can be used similarly as well. Do you think it useful to use
    one alt. as a 3 or 4 stage charger, or is it redundant in a daily driver vehicle? My goal is to maximize the ahrs from my agms, but until I get these relays setup, I won't know how well the alt. will charge the discharged bank.
    With my current setup (starter battery isolated while 'off'), the alt(s) seem to work fine (14.1-14.6), but after only 2 hrs with vehicle off (and no intended load), my voltmeter reads 12.1-12.3v. It has been -15C lately, but would you know how that correlates as an open circuit battery voltage? Tested for parasitic current draw last year, but maybe I should do it again.
    Thanks again for the input. - Nathan
  • System2
    System2 Posts: 6,290 admin
    Re: 12v/12v dc-dc auto transfer switch

    I run a Sterling Power Procharge-B on boat, between the starter battery and the radio battery so my radio battery is completely isolated on it's own from my start battery, but it
    still gets charged up by this waterproof battery charger every time I turn on my boat key.

    This thing runs thru 3 stage multistage charging while my boat is running on the water. I installed it, and my radio cranks.


    It was pretty easy to install, but I was nervous about how it worked so I called the guys at Sterling Power. They seemed nice and actually returned my call.
    I attached the manual I downloaded from their site.

    I'm going to add a 24v trolling motor bank and add a 12v to 24v model they have, but I havent got to that project yet. I'll post again when I do-