Hooking Solar to Dual Batteries?

ScottGScottG Moderator Posts: 4,388
Of late I have been revisiting the idea of carrying a spare battery. I'll detail my thoughts on that in a future post, but for now I have a question for you solar gurus... 

The basic set-up is diagrammed in the schematic below. To maximize the versatility of this system, I would run both batteries through a four-position switch that would allow connection to either battery individually, or to both at the same time in what would be a parallel configuration. In this scenario, the pigtail to the spare battery would be no longer than a few feet.

If the solar is only connected directly to the main battery, is this set-up adequate to charge both batteries when in dual battery mode? Stated differently, is there a compelling reason why I should ensure the solar is connected directly to both batteries? In terms of electrical continuity I can't see a difference, but I suspect solar may have some other considerations that are beyond my current understanding. 

Thanks in advance for any opinions or info.


Comments

  • CharlieRNCharlieRN Member Posts: 170
    Your diagram shows the solar panel connected directly to the battery however, the output from the panel(s) to the battery needs to be via a controller, not directly. If the batteries are connected in parallel as the diagram indicates, I believe that the controller will see it simply as one large battery and charge both. That said, it also looks like your 4-way switch can isolate batteries. If this is the case, then the controller would not be able to charge both batteries in some switch positions. I'm sure others with way more experience will weigh in on this with more helpful information.
    2021 [email protected] 320 S Boondock / 2008 XC-90 V8 Sport - Phillies Territory
  • ScottGScottG Moderator Posts: 4,388
    edited March 18
    I do have a controller--I just left it off the diagram for the sake of simplicity and laziness. Sorry for the confusion!  :-)

    And you are right about the 4-way switch--if only the spare were connected to the camper, then the charge from the solar would not reach the spare in this arrangement. 

    Ideally, I'd like to set the solar up on a similar switching arrangement, but that may be getting a little too elaborate for the number of times I expect to need to use the spare.
  • Denny16Denny16 Member Posts: 3,013
    And in lieu of the 4-way switch, all you need is a Marine A/B house battery switch, which has a charge source connection that by-passes the switch A/B position to keep both batteries on the charge circuit (solar controller). Yiu can get one with an off/A/B /Both positions also.  This is what I would use, and they are waterproof so could be installed in the front Tub with the batteries.
    cheers
    2018 TaB400 Custom Outback, Jeep Gladiator truck, Northern California Coast.
  • jkjennjkjenn Member Posts: 6,314
    A little different but similar, Scott, I have used my GZ Lithoum battery to charge my [email protected] batteries via a smart charger. I already had a bus bar for my connections, so I added an the smart charger quick connect to the bus bars. Similarly, I added A Denson plug ls for my solar connection. When not using the Yeti, they were just left unplugged.there was no need for a switch.

    2021 [email protected] 320 Boondock "Mattie Ross" | 2021 [email protected] Nights: 80 | Total nights in a [email protected] 291 | 2018 Jeep Grand Cherokee Trailhawk | [email protected] owner since 2014

  • rfuss928rfuss928 Member Posts: 685
    The switch Denny16 describes is on the right in this photo.  In my setup the solar input switches with the battery selected.




  • DougHDougH Member Posts: 1,071
    edited March 20
    @ScottG Looks good to me.  I have two batteries in back connected to the inverter, solar charger, battery monitor.  Then thick cables going through a switch and up to the battery in the tub that's connected to the shore power converter and 12V bus.  When batteries are bridged the shore power or solar charger can charge all three. Or I can isolate the solar system from the stock electronics. I think I had a reason for doing it at the time... maybe returning the [email protected] to stock slightly quicker for resale, or some such nonsense.

    2021 Jeep Gladiator, 2017 [email protected] S Max, Austin TX

  • ScottGScottG Moderator Posts: 4,388
    Thanks, all. I have the switch in hand and it is is indeed like that described by @Denny16 and @rfuss. My question (perhaps not clearly stated) was less about the switch and more about the hook-up of the solar--specifically whether hooking the solar to the main battery would adequately charge the spare battery wired several feet away.

    After a little more poking I think I have determined the answer is maybe. It seems the correct way to attach a charge (or load) to parallel batteries is to attach the leads on different batteries (see image below). Theoretically, this better balances the charge/load over both batteries.

     
    Not that doing it the other way won't still work. @DougH, it sounds like that is how your set up is wired when your three batteries are bridged--the battery in the tub is some distance from the interior batteries where the solar is actually connected. Do you find that--practically speaking--all three batteries still charge just fine?

    That is really the root of the question. There are a lot of things that are theoretically best practice, but in reality, the operational differences are negligible.  
  • DougHDougH Member Posts: 1,071
    edited March 20
    @ScottG Yes, the more distant battery charges fine. I tested it both ways, covering up the solar and letting the stock shore power converter charge the front one, and after running through 8' of cable the rear set... and then with solar enabled the other way. Both ways always within 0.1V of each other when at rest. That let me know that the wiring was thick enough between the batteries.  I also tested separating the batteries and charging one set or another, then eventually bridging them to see how soon they'd equalize. I was concerned if I hooked up a 13.8V battery to a 11.9V battery that there would be a sudden rush of current that would blow the inline fuse between the two. Nope. Worked fine, and they reported the same voltage when both charged and reseparated the following day. This was with 125Ah AGMs. I was mildly surprised it worked so well... but I guess it takes more effort for Mr. and Mrs. Electron to charge a battery than to travel down a 4AWG cable.  But I do nonetheless use leads on different batteries for the rear pair, because I'd read that it's the better practice, even if it doesn't seem to make much difference on my 8' apart test case.
    2021 Jeep Gladiator, 2017 [email protected] S Max, Austin TX

  • ScottGScottG Moderator Posts: 4,388
    Thanks, @DougH. I was also wondering about the effects of hooking two batteries at different states of charge together for charging purposes, but you seem to have real-world success in that are as well.

    Though my knowledge of this topic is limited (but growing), I suspect the 4AWG was a factor for exactly the reasons you stated. I may consider making my pigtails a little heavier than planned!
  • DougHDougH Member Posts: 1,071
    @ScottG I was a bit nervous using 4AWG instead of 0000AWG or equally ridiculous thick stuff that would have added considerable weight to the camper. Consulted several AWG charts on amperage and distance for 12V, and for pulling 50A like running a couple major appliances, 4AWG allegedly would be insufficient. But I only charge at 10-20A with my solar controller, and don't see over 250W of solar from the roof, and I'd read that even when plugged into a 30A shore power circuit our stock WFCO converters never charge the batteries at anything close to 30A either. But I wouldn't feel safe rerunning the experiment with 10AWG or thinner, especially for eight feet distance. I suspect using the really thick stuff is more important for the battery to inverter very short connections, where there may be sudden 100A surges.
    2021 Jeep Gladiator, 2017 [email protected] S Max, Austin TX

  • rfuss928rfuss928 Member Posts: 685
    If the batteries are connected in parallel to the charge controller, the selector switch will not do anything.  The parallel connection will share both the charging current and the load.  The solar output needs to be connected to the convertor side of the selector switch to keep the batteries separated.  It is basically impossible to charge both batteries with a single source while drawing current from only one of the batteries.


  • ScottGScottG Moderator Posts: 4,388
    @rfuss928, my intent (albeit not well reflected in my diagrams) is that the charge current would only pass through to the spare battery when the two batteries were connected in parallel through the 4-way switch.

    However, as I think it through, the simplest and most versatile option may be to just put separate, dedicated MC4 solar pigtails on both batteries. Since my current solar set-up is highly modular anyway, that would allow me to hook up either battery individually (regardless of which one was running the camper) or--if the batteries were joined--properly charge them both as shown in my second diagram.

    I'm probably making it sound more complicated than it is. Ultimately it's a pretty low tech solution mostly controlled by simply plugging and unplugging and switching various connections as needed. Cumbersome, perhaps, but probably adequate for what I envision as more of a highly versatile back-up system that will probably get little actual use.

    I appreciate all the input!   
  • Denny16Denny16 Member Posts: 3,013
    Doug, nothing in the TaB draws 50 amps of DC power, it is fused for 30 amps, so 4AWG is fine.
    cheers
    2018 TaB400 Custom Outback, Jeep Gladiator truck, Northern California Coast.
  • Jax0913Jax0913 Member Posts: 25
    Late to the party on this... but love the idea. I have the same thing. 200 watt solar coming into a charge controller connected to the factory installed group 24 battery in my 400. I would love to add a separate battery and also use solar to charge that if necessary. @ScottG did you give up on the switch idea? 
    2018 [email protected] 400
    2020 Toyota Tacoma Double Cab
  • Denny16Denny16 Member Posts: 3,013
    edited April 30
    The factory nüCamp installed battery on a 2018 TaB 400 is a single 12 VDC group 31D, 224 amp hr AGM battery, and the later 2018 (after Feb 2018 builds) and 2019 onwards have two group 24 AGM 200+ amp, 6-VDC AGM batteries, wired in serie. Which do you have?

    We have a 2018 TaB400 with factory solar, and I was going to add a second battery to the single group 31D 200-Amp AGM battery.  But after camping with it, and one with three days off grid boondocking, I discovered the solar will keep the battery charged, even in the fall partial shade, getting mostly afternoon sun.  The battery never went below 35% discharge, so a second battery is not necessary for most camping situations.
    cheers
    2018 TaB400 Custom Outback, Jeep Gladiator truck, Northern California Coast.
  • ScottGScottG Moderator Posts: 4,388
    Jax0913 said:
    Late to the party on this... but love the idea. I have the same thing. 200 watt solar coming into a charge controller connected to the factory installed group 24 battery in my 400. I would love to add a separate battery and also use solar to charge that if necessary. @ScottG did you give up on the switch idea? 
    @Jax0913, nope. I've got the switch but beyond that I haven't given this project any more attention. I had to get my glycol change completed, and next on the agenda are my wheel bearings. I'm looking forward to making improvements again rather than just doing overdue maintenance!  :-)
  • Jax0913Jax0913 Member Posts: 25
    @ScottG I hear you! If I go down this rabbit hole, I will certainly give you a tag to follow the process.
    2018 [email protected] 400
    2020 Toyota Tacoma Double Cab
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