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Dual Battery Wiring Question

Discussion in 'Technical Chat' started by Slamuel, Jul 7, 2021.

  1. Jul 7, 2021 at 7:34 AM
    #1
    Slamuel

    Slamuel [OP] Well-Known Member

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    Well I didnt pay any attention in my circuits class so here I am.

    I have seen alot about dual battery setups for camping. Typically isolators or relays are used, but what prevents me from just using a switch? I will wire them in parallel and use a switch in the positive side to disconnect the starting battery while I'm using the camping battery, then reconnect the two while I am running to recharge it.

    Why is this not often done, and is this a good idea?
     
    Last edited: Jul 7, 2021
  2. Jul 7, 2021 at 7:36 AM
    #2
    0xDEADBEEF

    0xDEADBEEF River Trash Aficionado

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    Thats how they do it on boats sometimes.

    https://www.amazon.com/Perko-8501DP-Marine-Battery-Selector/dp/B00144B6AE


    Definitely don't want them in series though.
     
  3. Jul 7, 2021 at 7:40 AM
    #3
    ToyoTaco25

    ToyoTaco25 Well-Known Member

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    Why wire them in series? What do you need that’s 24v?
     
  4. Jul 7, 2021 at 7:41 AM
    #4
    Slamuel

    Slamuel [OP] Well-Known Member

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    Yeah I meant parallel im sorry, I'll fix the OP too.
    I have that exact selector on my boat too. I was thinking even more primitive like a single on/off disconnect. I would leave it grounded to the frame and disconnect it to run a little fan through the night and maybe a phone charger or something.

    Sorry if any of this sounds dumb, I really did grease my way through my one and only circuits class lol. big regret.
     
  5. Jul 7, 2021 at 7:42 AM
    #5
    0xDEADBEEF

    0xDEADBEEF River Trash Aficionado

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    Always time to learn more!

    If you're not running a big load, I'd just get a good single battery or maybe a portable pack and call it good. They've got more capacity than people realize.
     
  6. Jul 7, 2021 at 7:43 AM
    #6
    Slamuel

    Slamuel [OP] Well-Known Member

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    I fixed the op lol, I always write it backwards. I dont wanna turn the truck into a mortar lol so we're gonna keep it 12v :D
     
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  7. Jul 7, 2021 at 7:45 AM
    #7
    Slamuel

    Slamuel [OP] Well-Known Member

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    Would you probably opt for a high end deep cycle and run off of that? I just want to try to keep the truck bed camping area decently cool overnight is all honestly. Small fan is all I'll do, maybe a second one for the wife. She's the heat baby....
    FLORIDA MAN PREVAILS
     
  8. Jul 7, 2021 at 7:47 AM
    #8
    Slamuel

    Slamuel [OP] Well-Known Member

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    I actually just bought a battery for my boat anyways so I might hijack that for these purposes...
    Does that sound rational? it'll keep it charged too since I'll always have a 4-6 hour drive home after usage.

    I'm driving to a MX track far away occasionally and planning on bed camping overnight.
     
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  9. Jul 7, 2021 at 7:52 AM
    #9
    0xDEADBEEF

    0xDEADBEEF River Trash Aficionado

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    So for example, this little guy pulls 2 amps:
    https://www.etrailer.com/question-299194.html

    So if you run that overnight, thats like 16-20 Ah. Seems like a lot of the group 24f batteries have 50+ Ah of capacity for a regular battery. It might be worth going dual purporse or deep cycle if you're running it multiple nights.
     
  10. Jul 7, 2021 at 8:04 AM
    #10
    Slamuel

    Slamuel [OP] Well-Known Member

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    Okay so I definitely need to learn some more about draw and amp-hours and such I see. In the mean time, it doesnt seem bogus to do the more simple "switch" right? I'm not going to harm the alternator or anything charging a battery like that? I just don't want to get down the road and have a face palm moment that I should know better about.
     
  11. Jul 7, 2021 at 8:43 AM
    #11
    0xDEADBEEF

    0xDEADBEEF River Trash Aficionado

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    My inclination is that lead acid batteries do okay with this, but it'll be more strain on the alternator if they're both drained. AGM batteries may have issues with uneven charging in parallel though, I'd have to go look.

    So as usual, it depends. :laughing:

    That said, if your house wiring is entirely separate and you just want to charge off the alternator when it needs it, this is designed for that:
    https://www.bluesea.com/support/articles/1366/Automatic_Charging_Relay_[ACR]_Explained
     
  12. Jul 7, 2021 at 8:56 AM
    #12
    Slamuel

    Slamuel [OP] Well-Known Member

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    I was thinking about it and I don't want to overload the alternator. FWIW this is actually for an 05 ram with alot of miles. I'll probably upgrade the alternator wiring when I do this too. Luckily, it is right there. Changing alternator is about as easy as the battery honestly.
    I honestly could probably just charge the battery at home and then take it with me. it'll never be more than one night honestly. That could be a cheaper and simpler than any of this but I will eventually want to be able to go on a fun trip or something for more than just an overnight track trip. Even a multi day trip. I think I'll call BSS and extract some info from them lol.
     
  13. Jul 7, 2021 at 9:01 AM
    #13
    Sandman614

    Sandman614 Ex-Snarky TWSS elf, Travis #hotsavannahdotcom

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    Because it's not overland cool... Gotta spend money on the high end stuff.

    But really for the added features such as automation, dual voltage sensing, and not having one battery significantly lower than the other(unless it's the house battery after the isolator disconnected the starting battery)
     
  14. Jul 7, 2021 at 5:40 PM
    #14
    ascendr

    ascendr Active Member

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    You could split the positive wire to the second battery with a continuous duty N.O. starter solenoid. Then you can use a dash switch wired to ignition to engage the solenoid when you want to charge said battery.
     
  15. Jul 7, 2021 at 10:17 PM
    #15
    6 gearT444E

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    The alternator is not there to charge your battery from a dead state after discharging, rather it's there to top off after using some amps from starting and sitting for a couple days of losing a charge. It will recharge extremely slow from a discharged state. The switch OP talks about is feasible but will require.

    1 - remembering to isolate and sync the batteries
    2 - not discharge to the point of no return - you can damage a lead acid battery if connecting in parallel with an AGM as they require different charging voltages.
     
  16. Jul 8, 2021 at 5:19 AM
    #16
    Sandman614

    Sandman614 Ex-Snarky TWSS elf, Travis #hotsavannahdotcom

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  17. Jul 8, 2021 at 5:44 AM
    #17
    0xDEADBEEF

    0xDEADBEEF River Trash Aficionado

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    Good point . It really depends on the vehicle. A good rule of thumb for alternators is you can safely pull half its rated output continuously. if you've got a bunch of other electronic shit, then you may not have enough left to charge a battery. From a quick google, it seems like toyota likes to scrape by with the weakest alternator they can, so maybe we should do more digging on that.

    I hadn't considered the possibility of having 1 as an agm and 1 as a lead acid. Thats asking for trouble imo. If thats the route that we're going down, then a DC-DC charger would be best:
    https://www.renogy.com/12v-dc-to-dc-on-board-battery-charger/
     
  18. Jul 14, 2021 at 10:58 AM
    #18
    Slamuel

    Slamuel [OP] Well-Known Member

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    Yeah I'm out of my depth here. I don't really know much about batteries as I've said so I might prefer to just charge it from home.
    I really fancy that DC-DC charger you linked as well. That seems like the golden ticket to me for mobile setups.
     

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