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  • Running Load ONLY While the Sun is Shining?

    I"ll try to keep this as simple as possible.

    I plan to configure a normal setup (two 100w solar panels, MPPT charger/controller, battery, 12v fuse panel, and various loads). The panels will be outputting 12v to the charger, it charges the battery, loads connected as usual. FWIW, I've been looking at a Renogy kit something like this: https://www.ebay.com/itm/283229534405

    Here's the catch: I want part of my load to ONLY run while the sun is shining, and it's a very small load at that (a few amps max: two 12v->12v battery trickle chargers, and a small 12v circulating fan). Most of the controllers have a load function that would separately handle this small load, but the problem is that it seems that near everyone of them either has the load lugs designed only to run when the sun is not shining, or continuously.

    Simply put, I don't want those loads to drain the batteries, which while small, could be lethal. I forgot to mention, this is going on an enclosed trailer, and I need to keep a couple of motorcycle batteries topped up while in storage.

    I can't seem to find anyone who has a clear and obvious (or elegant) solution, so here's my running idea right now...

    Where the panels connect to the charger (panels in series outputting 12v), I connect the coil side of a solid state relay across the panel wires (the relay is rated for 12v-36v input), and then run a load line from the fuse panel through the relay load side to feed my devices. If the controllers behave as I think they should, the relay should only close and make the load live while the panels are getting sun, and then open to kill the load at night.

    Does this sound at all logical or doable? Or am I missing something altogether?

    I'm not an electrical novice, but I don't yet have a controller to tinker with, and I don't want to buy one yet (especially if there is an affordable option that does what I want out-of-the-box).

    Any help or thoughts would be MOST appreciated.

    Zang
    Last edited by zangort; 01-06-2020, 02:01 PM.

  • #2
    Alternately if that doesn't work as hoped you can use a relay on the load terminals which are only powered when the sun isn't shining. In that case you would use a normally closed relay that would open at night (presumably when sun is not shining). It would not provide a fail safe operation but that is not much different risk than SSR which often fail closed.
    9 kW solar. Driving EVs since 2012

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    • #3
      I did consider using a normally closed relay on the load terminals, but then it means the relay is (ironically) drawing power when the sun isn't shining, which sort of defeats the purpose of minimizing drain while the sun is down.

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      • #4
        By the way, this is the kind of relay i was looking at: https://www.amazon.com/dp/B07PPF85ZN/

        I thought the fact that the 'coil' input had a range of 3-32VDC would be ideal to use with a solar array that will put out between 12 and ~23 Voc.

        As another thought, I considered the fact that clouds could reduce the output, but i thought that, if the relay stays energized with as little as 3VDC, then it really shouldn't disconnect until the solar panel is near-as-it-matters in the dark. That would - I hope - help keep the loads from constantly going on and off on cloudy days.

        Again this is guesswork and it may fall to experimentation, but I want to make sure I've considered all my avenues before I go buy a controller and then find out there was a simpler/easier way to do it after all.

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        • #5
          A couple of things, an MPPT controller allows the panels to operate at their max power voltage (Vmp) and current (Imp), so in normal operation, two "12V" panels in series into an MPPT controller would be running at about 36V (twice the Vmp of a single panel) when charging, and could go over 40V (twice the open circuit panel voltage) when the charging cycle is finished, Your relay would need to be able to handle that. The other thing is that the panels will produce voltage early in the morning and late in the evening where they aren't able to produce meaningful power (current along with the voltage), so you might not be accomplishing what you want. I think basing this on charging current would be the way to go.
          Last edited by sdold; 01-06-2020, 04:36 PM.

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          • #6
            I have items running off my load terminal through a relay and I have preset cut out and cut in Voltages, it is a Plasmatronics PL40, PWM.

            Looking at Victron MPPT contollers they would do what you want direct off the load terminal, they have an auto setting as well as some preset options.

            You could use one grid connect panel with that controller and possibly come out better off than the renogy kit.

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            • #7
              Originally posted by zangort View Post
              I"ll try to keep this as simple as possible.

              I plan to configure a normal setup (two 100w solar panels, MPPT charger/controller, battery, 12v fuse panel, and various loads). The panels will be outputting 12v to the charger, it charges the battery, loads connected as usual. FWIW, I've been looking at a Renogy kit something like this: https://www.ebay.com/itm/283229534405

              Here's the catch: I want part of my load to ONLY run while the sun is shining, and it's a very small load at that (a few amps max: two 12v->12v battery trickle chargers, and a small 12v circulating fan). Most of the controllers have a load function that would separately handle this small load, but the problem is that it seems that near everyone of them either has the load lugs designed only to run when the sun is not shining, or continuously.

              Simply put, I don't want those loads to drain the batteries, which while small, could be lethal. I forgot to mention, this is going on an enclosed trailer, and I need to keep a couple of motorcycle batteries topped up while in storage.

              I can't seem to find anyone who has a clear and obvious (or elegant) solution, so here's my running idea right now...

              Where the panels connect to the charger (panels in series outputting 12v), I connect the coil side of a solid state relay across the panel wires (the relay is rated for 12v-36v input), and then run a load line from the fuse panel through the relay load side to feed my devices. If the controllers behave as I think they should, the relay should only close and make the load live while the panels are getting sun, and then open to kill the load at night.

              Does this sound at all logical or doable? Or am I missing something altogether?

              I'm not an electrical novice, but I don't yet have a controller to tinker with, and I don't want to buy one yet (especially if there is an affordable option that does what I want out-of-the-box).

              Any help or thoughts would be MOST appreciated.

              Zang
              Just some information. That Renogy charge controller only has a max of 25V input rating and those panels have a Voc rating of 21.4V so you can't wire them in series to the CC. They need to be wired in parallel. They also have an Imp of 5.62 amps so you will get a maximum of about 11 charging amps for your batteries.

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              • #8
                So, several good responses, I'll try to hit each in order...

                1. Good point on the input needing to be in parallel instead of series; with that in mind and wired in parallel, I think the relay could still handle the incoming.

                2. I considered the fact that early and late day power was going to be poor, but I figured that a voltage-based on-off was good enough. Running the loads an hour or two with poor incoming power is still better than running them all night long with none. I've seen current sensing relays, but for most all of them I looked at, the simple mechanical ones appeared to be based on AC induced crrents (no good on DC wiring), and the DC-based ones had vampire drains from running the circuitry.

                3. Someone else suggested privately that - because the motorcycles are standard 12V AGM batteries and the main bank is also going to be (2) 12v deep cycle in parallel - that I could actually run wires to the motorcycles and tie each of them to the bank 'in parallel' and just let the solar charger keep everything happy. They also noted that, if I was paranoid, I could install a couple of ACRs between each of the motorcycles and the bank.

                Thinking about, that would eliminate the need for DC-DC trickle chargers, and it does seem 'allowable' by all other wiring standards (generally, batteries of different capacities can be charged in parallel). Thoughts? (If you're wondering, I was originally looking at using a couple of Tekonsha float chargers.)

                4. I'll have to look closer at the Victron controllers. If they check the boxes, I'll reply here to update.

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                • #9
                  So a quick loopback...

                  Unless I've missed something in the manuals, all the Victron controllers (MPPT and PWM) are pretty much like everyone else... the load outputs can be configured to either turn off when the battery voltage drops too low (which, from my perspective, means it's too late), or on a timer which is always restricted to turning ON at sunset and OFF at sunrise, the opposite of what I need. Even it were a simple clock timer (e.g., 12 hrs on, 12 hrs off) I suppose that would be better than nothing, but that's not an option either.

                  For those worried about my capacity requirements, other than running some LED lights here and ther and a winch (for about 2 minutes per day at best), the main bank doesn't actually have a lot to do. I'll eventually add an inverter to allow me to charge some cordless batteries and operate some small tools, but again nothing major.

                  Thanks again!

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                  • #10
                    The OP mentioned that he wants an MPPT controller, and an MPPT with 100V limit is one of the options on the page he linked to in the pull-down menu. Zangort, is that what you had in mind? A PWM controller would be cheaper and give you about 11A of charge current, while an MPPT controller would be more expensive, but give you around 16A charge current with the same panels.
                    Last edited by sdold; 01-06-2020, 08:46 PM. Reason: Corrected "PWM" to "MPPT"

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                    • #11
                      This is available on the victron, may be suitable??

                      1.4 Battery Life: intelligent battery managementWhen a solar charge controller is not able to recharge the battery to its full capacity within one day, the result is often that the battery will continually be cycled between a ‘partially charged’ state and the ‘end of discharge’ state. This mode of operation (no regular full recharge) will destroy a lead-acid battery within weeks or months.The Battery Life algorithm will monitor the state of charge of the battery and, if needed, day by day slightly increase the load disconnect level (i.e. disconnect the load earlier) until the harvested solar energy is sufficient to recharge the battery to nearly the full 100%. From that point onwards the load disconnect level will be modulated so that a nearly 100% recharge is achieved about once every week.



                      The Plasmatronics PL range is programmable to what ever cut in/out voltage you want but is PWM, made in Australia but expensive so I dont think and option.

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                      • #12
                        sdold - Yes, I planned on doing the 40A MPPT controller option. As I say, 11 amps, 16 amps, it's all good to me right now... which I realize as I type this would make most peoples' heads explode on this forum, but seriously my demands are large burst over short times, and I'm not at the point where I need to squeeze every amp out of the system. I know the MPPT is a better long-term investment, though.

                        My big concern is keeping those motorcycle batteries topped up, because I can't tell you how many of those batteries I've replaced over the years in various bikes. Those little AGMs just don't hold a charge if they're not consistently topped up. At home I keep then on a Tender Jr. and they 'll crank right over even if not ran for 6 months, but when they're disconnected they'll go completely flat in just a few months. In addition, I need to make sure the trailer's electronic brakes battery stays topped up too... I've had the trailer sitting at times where it hasn't been hooked up to a vehicle for 8+ months!

                        Anyways...

                        Bala - The problem with that whole system is that the timing of when the load runs is dictated by the controller, not by the sun. What it's REALLY saying is that the load will still only go hot at sunset, but if it's too draining it will simply pull back on when the timer turns it off over time (e.g., if the timer was set to turn off after 4 hours, the unit might pull it back to 3 or 2 hours as needed) to ensure the battery doesn't get over-drained. This is still running the load at night, and not really helpful as I want the load to run during the day.

                        It does seem somehow there's some irony hidden in all of this that a 'solar charger' can't/won't control the use of solar power during the day.

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                        • #13
                          Have you thought about having a separate panel or panels for the motocycle batteries? Panel would connect to a PWM CC and charge the motorcycle battery directly. If the motorcycle batteries are the same size you could charge them off one CC.

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                          • #14
                            hammick - It was a thought, but I'm really trying to avoid having more panels than I need. The 200 watts I plan to put up should be plenty for my needs, so adding more panels 'purely on principle is more than I'd like to hassle with.

                            And of course, despite these distractions, I'm no closer to figuring out a 'proper' way to disconnect loads at night when the sun goes down. Right now I might just experiment with the relay design, and see if I can get that to work, since it appears the controllers out there (at least the small/affordable ones) won't provide any such capability.

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                            • #15
                              If you use a Voltage Sensing Relay ( VSR ) ,as used in cars for dual battery systems it will effectively only come on when there is enough sun to get the batteries up past the cut in V.

                              If it clouds over it will cut out. Thats is effectively how I have my controller set up.

                              VSR will come in different V set points depending on brand but above 13V to cut in and 12.7 / 12,8? cut out. I am going to do just that to run a fan in a shipping container.

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