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Kd7vea's avatar
Kd7vea
Explorer
Apr 14, 2020

What solar panels, and what charge controller do I need?

I’m hoping you can help me with this, or at lest point me in the right direction. I have a motorhome that I’m going to add solar panels to. I have 12 100ah gel cell batteries. These are alpha cell 195 gxl batteries, I got them for free, so that is why I am using them. I know that you want to put a Slower charge on gel cell batteries, so the question is do I want to go with a 40 amp charge controller and 600-900 watts of solar, but will I not be able to utilize all of that charging/wattage coming from the solar panels? The controller I have been planning on is the Tracer 4215BN, but I’m just starting to wonder if that is more than I will need.
  • pianotuna's avatar
    pianotuna
    Nomad III
    Apr 14, 2020
    Kd7vea wrote:
    pianotuna Trust me, I'm not worried about having too much solar, I'm worried about having plenty of solar, but the batteries not being able to take all of the charge that the panels are capable of producing. My thought is why produce 10kw of solar energy if the charge controller can only use 1kw to charge the batteries. I would love to run a lithium battery bank, but I got 12 good Gel cells for free, so I figured I would try them out, but I know that the Gell Batteries have to take a slower charge. Also, we purchased our motorhome for $3000, and have sunk another $4000 into getting it put back together inside, so I dont want to waste money where it wouldnt benifit us. As normal, everything we have done to get this thing ready to use has cost 2 times what we initially planned. Welcome to the joys of owning an RV!


    Simple solution to solar power waste. Turn on the inverter, turn on the water heater. Do this after the battery bank reaches 85% state of charge.

    If that draws too much power, then switch the fridge to the 120 volt electric setting, and save on propane.

    Even with my tiny system and large battery bank (at one point, 875 amp-hours @ 12 volts), on a sunny day in June I'm sometimes able to do this.

    I do all of my cooking on an induction hob (NuWave is the best imo). I run my microwave when ever I wish.
  • pianotuna's avatar
    pianotuna
    Nomad III
    Apr 14, 2020
    KD7eva,

    Since there are multiple batteries, this is what is balanced and best for twin twelve volt batteries.



    As it often doesn't cost a dime more to do this, I think it is worth the trouble.

    If you wish to understand the "why" surf here:

    correctly interconnecting multiple twelve volt batteries

    See #3 wiring for more than two 12 volt jars.

    Others may say it doesn't matter--but unless there is a compelling reason to not optimize charging and discharging why not do it the best possible way?
  • KD4UPL's avatar
    KD4UPL
    Explorer
    Apr 15, 2020
    You need to look up the manufacturers specs for your batteries and see what the maximum charge current is. Until that happens we're all just guessing.
  • time2roll's avatar
    time2roll
    Nomad
    Apr 15, 2020
    You have 1200 Ah of gel batteries?

    900 watts is a bit much but should be fine. 600 watts is probably enough mounted near flat on the roof. You could keep the 3rd panel lose to attach in winter and tilt toward the low sun. Consider a 60 amp controller if you need 900 watts.
  • drmopar's avatar
    drmopar
    Explorer
    Apr 15, 2020
    Gel Cell Batteries will only take 13.9 Volts MAX!....and charge with low amps. I sell Batteries at my Marina/RV Park. Authorized Dealer for most major brands, since 1982. We cut open a Gel Cell after 6 months at 14.4 volts. The Gell was like tough rubber. Toast. With that amount of Solar, you may have a fire issue with Gells.

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