dual battery question

Discussion in 'GM Electrical Tech' started by eightbol21, Jan 20, 2012.

  1. eightbol21

    eightbol21 Rockstar

    my wife and i have been talking about getting a small camping trailer and my truck has a spot to put a second battery in it so i started thinking it would make sense so we could run some electronics if we get the power converter so here is my question. i read somewhere else on here you can get an isolater to hook to batts up and have them charge on the alternator but is the second battery supplying any power to anything or just sitting there adding weight? would you have to run accessories off the battery itself or can they run through the power accessories like everything else does. im sure somebody out there knows what i mean. thanks in advance for the help:great:
  2. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    If you add a second battery in parallel without an isolator you must use two identical batteries and as one is drained by things connected to it the other will replenish it. Effectively your voltage remains the same but your battery capacity doubles. Keep in mind that it also means that if you connect a power inverter to the second battery you CAN drain both it and your primary battery dry. The wiring diagram looks something like this for a pair of batteries connected in parallel:

    If you add a second battery in WITH an isolator you may elect to use different batteries and as one is drained by things connected to it the other will NOT replenish it. Your voltage remains the same AND your capacity of each battery also remains the same. If you connect a power inverter to the second battery you CANNOT drain both it and your primary battery dry. The wiring diagram looks something like this for a pair of batteries connected using a battery isolator:

    Note that if you add a second battery using an isolator then nothing will magically be connected to it. The only things that can draw power from it are things that you connect to it. If you connect nothing to it, then it will just be added weight in the truck that you could potentially use to jump the primary battery in the event that it was dead.

    On the flip side, if you add an identical second battery in parallel without using an isolator then anything/everything that can draw from the primary battery effectively draws from the pair ... but you run the risk of draining the pair dead if you are not careful about your use.

  3. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    I'm unsure why the first diagram was cut off. Here's one that shouldn't be:

    Here's what my dual battery setup looks like. The blue wire runs from the auxiliary battery to an amplifier in the cab, but it could just as easily go to a fuse block I installed, an inverter, or anything else of my choosing. That wire is the means by which power from the second battery is utilized elsewhere in the vehicle. Notice that my battery terminals are of a type that allow me to make multiple connections if I choose to do so.

    I only marked positive wires and didn't bother with grounds, as they should be obvious. I should also state that all of my grounds are directly to the frame [including the amplifier's] and I'm using 1/0 AWG cables for all alternator & battery connections -- both positive and negative. 4ga cable is used for my amplifier as that's what was required. Be sure to choose the proper gauge wire for the amount of current (amps) you intend to carry on the wire. Use of wire that is too small is a no-no.
    Last edited: Jan 20, 2012
  4. jake's silverado

    jake's silverado Epic Member 5+ Years 500 Posts

    Nice job Surreal! Real Nice! Clean install... Kudos!:great:
    Nice clear write up too...
  5. KDL2618

    KDL2618 New Member

    Definitely, as SurrealOne recommends, plan on including a battery isolator in your plan. You can be assured that when you accidently drain the aux batt you will still have a way to start the vehicle. The 3-post diode isolator is a great no-hassle solution. Another solution you may find in your research is a more traditional solenoid which connects the positives of the two batteries only when the ignition is on. But that solution requires splicing into a switched ignition wire - somewhere. I like the third option which uses a product of www.colehersee.com which is their "Smart Battery Isolator" (I have some experience with their model 48530). It connects the aux battery to the vehicle's main battery when it sees that the main battery is at or above 13.2 volts (i.e. engine spinning the alternator) and has been so for two minutes. This way, the alternator has a chance to somewhat top off the main vehicle's battery before feeling the charging load of the aux battery. The "Smart Battery Isolator" contact drops out (disconnecting the aux battery from the main battery) after the voltage of the connected batteries drops below 12.7. Try going to the cole hersee site and navigate to battery isolators and download pamphlet #D-617. BUT there's always a caveat. . . my experience is with an older system in a chevy pickup. Can someone comment on the reliability of this smart solenoid solution if installed in a vehicle which has SARVC like I just discovered on my 2006 GMC Sierra which I've had for a week? I think it would work OK - my concern is, if after starting the vehicle the SARCV discovers that the battery has a high state of charge, the system may not elevate the system voltage above 13.2.....tell you what, I'll wire it up and post the results later. For that reason, SurrealOne's solution may be the most attractive.
  6. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    Something to consider: All battery isolators produce some amount of voltage drop. Diode-based isolators cause the most voltage drop. Solenoids used as isolators cause less voltage drop than diode-based isolators. MOSFET-based isolators cause the least voltage drop. I consider diode-based isolators to have unacceptable amounts of voltage drop, as they can drop as much as 0.6 volts...

    In terms of price, solenoids are the cheapest of the bunch and may suit your budget the best, but they are also the most prone to failure. Diode-based isolators are the second cheapest and fail less often than solenoids. MOSFET-based isolators are the most expensive of the bunch and also the most reliable. I consider the failure rate of all of them acceptable, by the way...

    I personally chose a MOSFET-based isolator because I wanted the reliability, low voltage drop, it was within my budget, and I was looking for a 300 (continuous) amp isolator and could not find a suitable-sized solenoid to handle a load that high ... but I found a MOSTFET-based dual rectifier (i.e. dual isolation -- such that I can jump either battery from the other if I want) that -could- handle it.

    If my needs had been closer to a 200amp requirement and my budget been more constrained I'd likely have gone for a solenoid as an isolator...
    Last edited: Jan 20, 2012
  7. zigger215

    zigger215 Member 2 Years 500 Posts

    +1. Excellent
  8. wayned

    wayned Rockstar 100 Posts

    [​IMG]You need to have the battery as close as you can to your power inverter.
    less then 3 feet you can use #4 wire.
    A 200 A pac solenoid $50,two 100A fuses $20 (amazon.com)
    Your second battery will only be connected to your starter battery when the key is on.
    You do not hook anything to the alternator with a solenoid and there is no power loss.
    Last edited: Jan 27, 2012
  9. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    That's definitely workable with a covered bed. And by the way, solenoids DO cause voltage drop. (Google the following and see for yourself: solenoid voltage drop)
  10. eightbol21

    eightbol21 Rockstar

    Wow is all i can say now. Thank you for the info definatly gonna be planning this out and possibly have a couple red tops in by summertime

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