Couple questions regarding how aux and trailer batteries play together

Discussion in 'Power Systems' started by reznunt, Aug 2, 2018.

  1. reznunt

    reznunt

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    Scenario: A parked vehicle has a dual battery system with it's aux battery feeding the trailer fuse block (via 7 pin socket), and the trailer has a battery of it's own, wired in parallel, and feeding the same fuse block:

    a) Does the usage of trailer accessories draw from the aux battery of the vehicle, the trailer battery, or both simultaneously? How do the two batteries and their voltages and the load interact?

    b) Would a solar panel/controller that's connected to the trailer battery charge only the trailer battery and detect when it is fully charged and shut off supply to prevent overcharging (thereby not charging the vehicle aux battery), or charge the trailer battery first and then float charge the vehicle aux battery, or try to charge both batteries simultaneously? In other words, how would it behave?

    Trying to plan out my setup and this has me a bit confused. Thanks in advance!
     
  2. aussie75series

    aussie75series

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    If the batteries + are linked through the fuse block or any other way then the solar and power drain thinks it is one big battery. Now people will say yes but ,which is true but at the end of the day the simple explanation is one big battery.
     
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  3. Suprasoup

    Suprasoup

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    A). Ohms Law. V=IR. The trailer load will draw from both batteries. The difference is that it will draw from the trailer battery more than the vehicle aux battery. This is due to how far the vehicle aux battery is from the load in comparison to the trailer battery. Ie longer distance = higher resistance. Higher resistance means lower current for same voltage.

    B). Same rules apply as above. The solar will try to charge both at the same time. Difference is the trailer battery will receive more of the current at the same voltage. Given enough time it’ll charge both. If you undersize the solar then neither battery will fully charge in a reasonable time frame.

    First step in sizing solar is determine your energy usage in a 24hr period. From there you can determine battery size and the.n determine how much solar you’ll need. Throw out some numbers and I can help you figure out how to properly size your trailer battery and solar panel size
     
  4. reznunt

    reznunt

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    Thanks for the response... I already have the usage calcs. I'm just trying to figure if I should install one or two panels, and on either the vehicle roof rack or the trailer itself. This all depends on the interactivity of the two batteries given the described setup, and how they would react to solar charging.

    Based on what you said and my estimated usage, I think two panels on the trailer would be ideal, and maybe one panel on the truck roof will suffice sans trailer.
     
  5. stevezero

    stevezero SILVER Star

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    On my teardrop, ive got a renogy 100 watt solar suitcase with its controller. It puts out about 7 amps in full sun. Ive got two group 29 deep cycle batteries (120ah each, rendering a total of 120ah usable). With my usual loading, I pull a max of 20 to 30 amps a day, and are usually recharged by lunchtime. The nice thing, the suitcase allows the camper to be in the shade, and the panel in the sun, and folds and zips up for travel.

    On my 80 series, I'm assembling my solar setup. 1 100 watt renogy rigid panel to go on my roof rack, feeding a 1/2/both/off selector switch (like on my teardrop). a group 24 or 27 starting batt, and a group 29 deep cycle. I still need to determine what controller im going to use in between the panel and battery selector.
     
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  6. reznunt

    reznunt

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    Thanks, I may have to look into the suitcase setup since I can't find any good spots on my trailer to install a panel. And being in the shade would be a huge plus.

    Do you have power feeding from your truck to your trailer via a 7-pin, or are your two electrical systems independent?
     
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  7. stevezero

    stevezero SILVER Star

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    Separate for now. Havent had the time and inclination at the same time to dig into it. My 7 pin does charge my breakaway battery for the brakes.

    Heres a pic of my battery box on my teardrop.

    Left to right, front to back. on front of box is a trolling motor connector. i have the matching end on my feed from my solar suitcase. left wall: Noco genius 20 two bank charger. Two group 29 flooded batteries. On back wall mounted to HPDE plastic. 1-2-both-off for solar feed from plug on box. West marine 1-2-both-off battery switch. 200 amp circuit breaker. All wiring for solar is 8ga. Noco wiring was stock for the unit (10 ga with fuses). All battery feeds are 1ga or 2 ga. Yellow cords are 12 gauge outdoor extension cords, 50ft each.

    Inside i have a 2 ga feed hitting a blue sea power point. 2 ga to 150 amp breaker, and 2 ga to my inverter. another leg is 4ga to a 50 or 100 amp bussman breaker, then 4ga to a blue sea 12 port fuse block. Grounds are matching in gauge. i have a 120v to 12v 5amp brick that feeds shore power to port on the fuse block.
    Inside shot was before i cleaned up the wiring

    Any time i have a shore power connection, the batteries are being charged.

    20171205_155700.jpg

    20171205_155628.jpg

    20170709_163112.jpg
     
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  8. george_tlc

    george_tlc SILVER Star

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    Looks nice, but I'd feel better with boots of the battery +ve terminals... especially given the metal box the whole system is installed in...

    cheers,
    george.
     
  9. Suprasoup

    Suprasoup

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    The 7 pin isn’t really designed to charge a depleted battery. The wire size is what 14-16ga? Voltage drop alone would make it impractical to do any real charging. If you are going to link the aux and trailer battery I would use larger sized wires that are fused and a quick disconnect like Anderson power poles. SB50s work well.

    If you go with suitcase style panels make sure you situate the charge controller at the battery and not at the panels. I’ve seen a few like Zamp that has the controller right at the panels.

    Buy a decent MPPT charge controller, skip PWM.

    Best bang for your buck on solar panels is house panels aka Gridtie panels. They are much cheaper per watt than 12v panels. They require a MPPT controller however.

    Best bang for your buck on batteries for a small trailer are usually 6v golf cart batteries. Buy 2 and wire in series for 12v. Do not buy 2 12v batteries and wire in parallel. Paralleling batteries are to be avoided.
    Flooded lead acid over AGM. Flooded last longer.
    I assume since you already have an aux battery that you have a battery isolator. For short jaunts like 1-2 days the vehicle will provide all the power you need. Just size your batteries accordingly. For RVs size for 3 days reserve. For full time trailers size the battery for 5 days reserve. Solars nice to have but for most people a second or larger battery with a good isolator (I like the blue sea ACR models) will be more cost effective.
    1EDC48D1-1695-44AF-A6DD-F07A524A34D7.jpeg

    My 4Runner has a 75w panel with a Morningstar SunSaver 10. Feeds 2-6v 232ah batteries wired in series. Blue sea ACR is the battery isolator. Runs a 100w pure sine wave inverter and a Dometic CF 80 refrigerator. The isolator handles probably 90% of the charging.
    809406FF-F56C-428E-9E8F-ACAD6EEE06FA.jpeg

    2-320w grid tie panels on my Zombie apolaypse Trailer. ZAT!
    688DAB1F-A528-4081-8C2E-3E3AD2857305.jpeg
    Zombie trailer runs 2-320w panels wired in series to a 30a MPPT controller. Batteries are 2 12v 200ah batteries wired in series for a 24v bank. Inverter is a 1000w pure sine wave inverter. Everything in the trailer is 110v. If your building a full time trailer I would ditch the 12v setup and wire the trailer for 110v. The savings in wiring make up the difference. Not to mention 110v accessories are way cheaper than their 12v equivalent. I go completely off grid for weeks at a time with this setup. 100% charging from the solar. If I needed to charge my 4runners batteries I have a simple 10a 12v battery charger that runs off my power inverter.

    Many ways to skin the solar cat. Depends on what you wanna do. Let me know if you got any questions.
     
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