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Small Cabin Forum / Off-Grid Living / Solar Charge Controller ?
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gcrank1
Member
# Posted: 10 Apr 2021 22:40
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Ive read a LOT online but would like real-world experience comments if my thought on this is correct:
An mppt scc will always perform 'at least as well' as a pwm.

Steve_S
Member
# Posted: 11 Apr 2021 04:48
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MPPT will equal or do better than PWM.

ICC
Member
# Posted: 11 Apr 2021 09:11 - Edited by: ICC
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The only thing PWM does better than MPPT is a cheaper price, but at the cost of less performance.

rpe
Member
# Posted: 11 Apr 2021 09:14
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We inherited a 220W panel my son wants to use to charge the 12V car battery at his sugar shack. Voltage output of the panel is up over 24V. I'll follow this thread in the hopes of learning what charge controller gadget he needs for that rig.

spoofer
Member
# Posted: 11 Apr 2021 09:25 - Edited by: spoofer
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He needs a mppt charge controller that can handle, say 250 watts. It prevents the battery from overcharging etc. A deep cycle marine style battery is a better "cheap" choice.

ICC
Member
# Posted: 11 Apr 2021 10:18 - Edited by: ICC
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Maybe a (hopefully) simple explanation of how the PWM and MPPT chargers differ will make it easier to understand why MPPT offers better performance?

First, the charge current going into a battery must be at a voltage higher than the battery's at rest voltage, but not too much higher or the electrolyte will be overheated and evaporate.

Using the same (pretend) PV panel and battery for purposes of illustration: 24 volt panel output at 10 amps with a 12 volt battery. The battery will need 14 volts coming into it for charging. (actually more like 14.4, but this is a simplified example).

It also helps to know that Volts x Amps = Watts and that Watts are the amount of energy.

The PWM charger takes 14 of the incoming volts and uses them at 10 amps to charge the battery. 10 amps x 14 volts = 140 watts going into battery. The unused volts and their amps are wasted as heat. 100 watts wasted.

The MPPT charger first looks at the battery voltage; 12, and decides it needs to output 14 volts to charge. The MPPT charger then converts the 24 volts to 14 volts. (24 volts x 10 amps = 240 watts) So the 240 watts of energy is changed to 14 volts.... (Watts/ Volts = Amps)....(240 watts divided by 14 = 17.14 amps). So the Volts are reduced and the Amps increase. Yes, there are internal conversion losses but for now we will ignore that. The MPPT charger outputs 17 Amps instead of 10, a big gain of energy going into the battery.

FYI, an MPPT charger is not 100% efficient. The efficiency rate also varies with the difference between the voltage from the PV panels and the voltage output to batteries. To complicate things as the voltage difference increases the efficiency rate falls. So a PV array that supplies 120 volts to an MPPT charger used on a 12 volt battery bank will be a little less efficient thatn an array wired to supply 50 volts to the MPPT charger. BUT! there are times when the higher voltage is better, is needed, to reduce losses in the long wires when an PV array is distant from the controller. Such is the life of a PV system designer.

Other variables also have to be considered.

ICC
Member
# Posted: 11 Apr 2021 10:28 - Edited by: ICC
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If you have an MPPT controller you can connect PV paanels in series and harvest power when the solar is insufficient to make enough power from a single panel that is "correctly" voltage matched to the system.

For example when using a PWM controller it is best, with a 12 volt battery, to use what are called 12 volt panels, which are panels that have a Vmp of something like 17 to 18 volts. When the sun is obscured these may not produce a high enough voltage to actually charge a 12 volt battery through a PWM controller. This is exacerbated when the panel temperature increases as voltage falls as panels heat up. An MPPT controller will allow two of those panels to be placed in series. The voltage will likely be high enough even with heavy clouds.

We have actually made power by moonlight just to see if it could be done. We waited until we had a full moon and it was after 11 PM at night. We reconfigured the wiring of some panels to place many more in series. We got almost 1 amp by moonlight. Of course, when the sun would come up the overvoltage would have shut the controller down. But it was fun.

paulz
Member
# Posted: 11 Apr 2021 10:37 - Edited by: paulz
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Thanks ICC, good explanation. I already knew the answer to the first question.

I'm still using my old PWM (Harbor Freight 150w setup) to keep the batteries for several vehicles in my shop topped up, on a cord with alligator clips I can move from one to another, and I don't bother to unhook the panels when moving. Seems to work well for that, where ultimate o output isn't really important.

ICC
Member
# Posted: 11 Apr 2021 11:38
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I would not go to the expense of an MPPT controller for anything like that. I am thinking of everything else, everything that is a little larger. IMO, if you have more than one small battery MPPT is best. Or if the battery is used for something as simple as a single night light, then MPPT is not needed.

gcrank1
Member
# Posted: 11 Apr 2021 14:45
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It kinda looks like about 200w of array might be the tipping point but you gotta have at least 24v input to the cc, and more is better?(within the input limits of the cc)
I see an Epever 20a mppt Tracer model on Amazon that looks 'attractive' for the features included and price.
Rpe, what are the specs on the panel, they should be listed on a mfg plate on the back.

ICC
Member
# Posted: 11 Apr 2021 15:34 - Edited by: ICC
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Quoting: gcrank1
.....but you gotta have at least 24v input to the cc, and more is better?


re input voltage: If the battery is 12-volt lead-acid type it needs about 14.2 to 14.4 to do a charge as quickly as possible and without damaging the battery..

So any volts higher than that much is going to be wasted with a PWM controller. Looking at it that way I see an MPPT still being a winner over the PWM. Not as big a winner if the incoming panel voltage was higher, but still a winner.

More is better only because then if the solar drops the incoming because of clouds or time of day, then the MPPT can still convert to something that still can charge. The only thing MPPT cannot do is increase the incoming voltage. That could be designed for but is pretty much not needed.

And then one must factor in their own economics. I don't like buying the cheapest of anything unless I know I only need or want it to work a short time. I do like a good deal, but the quality I percieve is important to me. I guess that is why I do not shop at Harbor Freight. I still use a few corded Makita and Porter-Cable tools which are 35+ years old. The Outback MPPT charge controller I bought in 2002 is still working, though it is working for someone else now.

Brettny
Member
# Posted: 11 Apr 2021 18:52
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The only thing I'm going to use PWM on is my outhouse light.
Today I'm at our property and I didnt see the sun once, infact is drizzled most of the day. With my 230w 32v pannels i was seeing 13.1v @ 5a at one point. At that point the pannels where inputing 29v to the CC. With PWM anything over 14v (or so) would be wasted.

I have a 40a EPever BN series, the one that looks like a big heat sink. You can input 150v and output 12v. You gain alot of flexibility with a MPPT CC. At one point I even had my pannels in series.

gcrank1
Member
# Posted: 11 Apr 2021 21:50 - Edited by: gcrank1
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My 10w(?) Coleman and 15w SunForce panels get the SF 7a cheapy that came in the SF 'kit', Im pretty sure it is just a simple switching control, not even pwm. The panels each put out about 17-18v in bright sun. Just for a trial I put each onto my depleted old LA 18ah jump pack when at 50ish % dod at mid-day WITHOUT the cc, in a few hours prime sun it charged up. I figured with the very low amps it wouldnt cook it, they are really just battery 'maintainers' not chargers anyway.
Guess I should say, For Info Only, Dont Try This

But for my 3 banks of 2 x 102w in series arrays (for nominal 24v into the mppt cc) it sure seems sticking with mppt will maximize my limited available sun. Ive been mixing the arrays up depending upon how much battery bank I have so as to keep in the est. C.1 - C.15 charge rate for LA AGM.
I agree that for Rpe's son the car battery is a poor choice, waayyy low amp-hours available. But, he has it....and for limited use of some LED lights it can work. When it dies (not 'if' but when) get at least a marine deep cycle 100ish ah, typically a Group 27. Note that only 50% of those listed amp-hours are net usable each cycle or you murder the battery. There are better batteries for solar storage, the marine grp27 is the basement model and price.
Fwiw, iirc, a 'car' battery with something like 850CCA only has like 65ah/32ah net and nowhere near the number of cycles of even the cheap marine battery.
An old style RV 50w incandescent light bulb would use that 32ah's up by calc in about 8 hrs., ime more like 5 hr's, You definitely want LED bulbs!

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