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Solar charge controller, what for?

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  • Originally posted by PNjunction View Post
    Heh, many of us thought about external cooling schemes that always fail, are impractical, or just too expensive (both power and financially) to do the cooling.

    Also consider you may be witnessing "edge of cloud" lensing affects, which tends to have the same big but temporary jumps in current. That's why most CC's have a "nominal" and a peak rating for current.
    +1. I'd also suggest the situation of improving output can be better addressed by improving the power coefff. of panel temp. rather than lowering the panel temp. Get to the root. Reducing panel temps. is good for performance and probably equipment life, increased temps. not being good for electronic stuff, but it may be money better spent to eliminate the need for cooling by improvements in tech. that make additional cooling unnecessary.

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    • I think the main effect I'm seeing is probably due to panel temp than edge of cloud. If a cloud goes over when panels are hoy it only happens a little if at all. The days I observed the big difference were cold and if the sun was on them for a bit the amps decreased the longer the sun was on them.

      Speaking of clouds, Yesterday was heavy overcast. No switching to HWS, Normal usage. Batteries were at 13.24V AGAIN this morning.

      This is going well.

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      • Everything seems to be normal. If you really want to mess about, grab a handheld IR thermometer and start pointing it close to and across the panels searching for hot spots, particularly those cells on the opposite side of any junction boxes that may contain panel dividing / bypass diodes. Point it at your interconnects. It might reveal a trend if not any particular problem.

        Reward yourself with a Tooheys, but only AFTER you come down off the roof.

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        • Nearly one year on, I'm wondering how this turned out for you BungaWalbyn? I'm looking to add some solar to my LiFePO4 bank and had the same thought process as you - why use a charge controller when LiFePO4 is effectively in Bulk stage until full. Like you, I already have multiple redundancies with my BMS and other monitoring equipment. Why not just pass the solar directly through a relay which is switched on and off by the BMS/monitoring system?

          I recognize that MPPT can capture some lost power, but I was only planning to have one 100W panel for my small application. A second panel would be much more cost effective than MPPT, especially if I don't even get a PWM anymore.

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          • Originally posted by nixsee View Post
            I'm looking to add some solar to my LiFePO4 bank and had the same thought process as you - why use a charge controller when LiFePO4 is effectively in Bulk stage until full.
            Good way to destroy your batteries, even Pb batteries.

            A solar panel is a current source, not voltage. Both Pb and LFP use the exact same charging algorythim. They both use Constant Current and Constant Voltage. aka CCCV. A solar panel alone can act and does provide the Constant Current portion of the charge, but cannot perform the Constant Voltage Phase. You cannot fully charge either a PB or LFP battery without a Constant Voltage phase.

            Example assume a 12 volt LFP or Pb battery. You charge both with a CC until the battery voltage reaches 14.4 volts. When you reach 14.4 volts either battery is roughly 80% charged up. At that point you hold the voltage at 14.4 volts until the charge current tapers to 3% of C. So if it is a 100 AH battery you hold 14.4 volts until the current tapers down to 3 amps and terminate the charge. At that point you are at 100% SOC.

            If you do not have a controller to hold 14.4 volts, the voltage will keep rising because you are still in Constant Current mode. A 12 volt Solar panel does not become a Constant Voltage source until you reach 20 to 22 volts. By th etime you reach that point your battery was destroyed long before you reached 20 volts. If you were using a Grid Tie panel the voltage is even much higher.

            Now what you could do is use a BMS to disconnect the panel when the voltage reaches 14.4 volts, but you are only 80% charged up which is not a problem for LFP, but a big problem for a Pb battery.

            Bottom line there are two possible outcomes. Either a destroyed battery or partially charged battery. And for 12 and 24 volt systems, no BMS is needed to start with. Just buy a good MPPT controller with that money and get 30% more power and use much less expensive rid tied panels. A GT panel cost 1/2 to 1/5 of a battery panel.
            MSEE, PE

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            • extrafu
              extrafu commented
              Editing a comment
              One more brilliant and useful answer. Thanks!

          • Originally posted by nixsee View Post
            Nearly one year on, I'm wondering how this turned out for you BungaWalbyn? I'm looking to add some solar to my LiFePO4 bank and had the same thought process as you - why use a charge controller when LiFePO4 is effectively in Bulk stage until full.
            Because you will overvolt and damage them. Or you will have to cut them off before they are close to fully charged.
            Like you, I already have multiple redundancies with my BMS and other monitoring equipment. Why not just pass the solar directly through a relay which is switched on and off by the BMS/monitoring system?
            NEVER use your BMS to terminate charge. BMSes are last ditch protections to prevent destruction of the battery pack and/or fire. They are NOT designed to manage charging.


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            • Thanks for the responses, but I probably should have provided more details of my setup.

              My application is a campervan, and I have been successfully charging off of my alternator thus far (LiFePO4 is paralled with my starting battery so as to not kill the alternator when I cut the LiFePO4 off)

              I would like the solar so that I can charge when I am stationary rather than idling. I am looking to get some flexible panels to keep the weight and visible/aerodynamic profile down (likely going to get two of these: https://www.amazon.com/HQST-Monocrys...dp/B017OMTAV6/) so grid tie/higher voltage wont work.

              ​I already cut off charging when the cells reach 3.5/14.0, and I am happy with this level with whatever pros/cons it might have, so CV shouldn't an issue. Furthermore, I'm likely to get 8-10A max out of those two panels, and my bank is 210Ah, so they will already be charging at ~0.03C, again meaning that I won't need a CV period for the current to taper.

              This sounds like almost the exact same situation as BungaWalbyn, and people eventually agreed that it was likely a reasonable setup given that he essentially built his own charge controller. I just wanted to know how it has worked out for him​.

              So back to my original question: why would I spend $100+ on an MPPT controller for a 30% boost on one panel when I can spend $190 for a 100% boost with a second panel. It seems to me that MPPT only makes economical sense once you start getting above 5-600W of panels. I don't have the budget, need or even physical space, for this. And PWM simply seems to feed through current at whatever voltage the battery is at until CV stage hits and limits current, but that will never happen with my setup where my multiple existing, redundant voltage monitoring alarms/BMS devices act as a charge controller. Hence, no need for a charge controller?

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              • Hi all. It's been a while. The system is working well. I actually simplified the controls. Took out the BMS and cut out contactor, after thinking about SunKing's advice. So it's all running on the voltage sensor and heavy relay. I've had to adjust voltage parameters a few times as the year progressed. Still a little troubled by voltage drop in the control panel, but can adjust for it.
                On solar under constant​ use the full charge thing is somewhat irrelevant. I keep the max voltage under charge below 13.8. The voltage starts rising more quickly above this so I make the assumption I'm getting up on the charge curve

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                • nixsee
                  nixsee commented
                  Editing a comment
                  This is exactly what I was looking to hear - confirms everything that I was thinking about my system. I'm going to keep my BMS since I use it for other charging sources/loads, but will just use a simple SPDT relay to toggle the solar between my LiFePO4 (being cut off around 13.8-14.0V) and a hot water dump load. I'll only have 200W of solar, so a $5 40A automotive style relay should be more than sufficient. Thanks!
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