Small MPPT Charge Controller?

[Amy@altE]

Maybe I'm using the wrong terminology. The panels are 37 Vmp. They are 45 Voc and are 72 cell panels. It's a short run from the panels to the controller and the wires are sized correctly. I ended up going with a Prostar PS-MPPT-25 charge controller, so the input from the panels in parallel is well within it's limits. Is something else significant to warrant climbing on the roof and rewiring things?

With 72 cell panels and a short run, you are fine in parallel for 24V. Sorry for the confusion, too many voltages without enough details.

[jflorey2]

Hmmm. $200 for another panel and $300 for a 25A MPPT charge controller, so, $500 to maximize the life of $340 worth of batteries.

Pretty much. If it saves you two battery replacements over ten years it's paid for itself.

[NorthRick]

37Voc is a 60 cell panel. That is nominal 20V, designed for grid tied solar. 44Voc 72 cell panels are for charging 24V batteries. (22Voc is 36 cell 12V nominal panels) It will be fine in cool weather, but in hot weather, the voltage drops, and may not be high enough to effectively charge the battery. Trust me on this one. If you are switching to MPPT, switch the wiring to series. You also get a bonus of lower voltage drop and lower current in to the charge controller.

Maybe I'm using the wrong terminology. The panels are 37 Vmp. They are 45 Voc and are 72 cell panels. It's a short run from the panels to the controller and the wires are sized correctly. I ended up going with a Prostar PS-MPPT-25 charge controller, so the input from the panels in parallel is well within it's limits. Is something else significant to warrant climbing on the roof and rewiring things?

[Amy@altE]

Actually the panels are 37v and are currently in parallel, which I believe is fine for a 24v battery bank.

37Voc is a 60 cell panel. That is nominal 20V, designed for grid tied solar. 44Voc 72 cell panels are for charging 24V batteries. (22Voc is 36 cell 12V nominal panels) It will be fine in cool weather, but in hot weather, the voltage drops, and may not be high enough to effectively charge the battery. Trust me on this one. If you are switching to MPPT, switch the wiring to series. You also get a bonus of lower voltage drop and lower current in to the charge controller.

[Sunking]

Rick you biggest mistake is using PWM controller with 2 195 watt panels wired in parallel. What you have done is turned your potential 390 watts panels into 220 watts. That is what you get using PWM controllers You shot yourself in the foot from the start. At best you can only generate 9 amps of charge current which is not near enough for a 205 AH battery. Ideally you want 20 to 25 amps.

Simple fix, buy a good 15 to 20 amp MPPT charge controller and something amazing happens. You actually get about 390 watts from 390 watts of panels. Who would have thought that was possible? PWM controllers are antiquated 8-Track tapes. That is why they are so cheap.

Small MPPT Charge Controller?

That was the title of your thread right? Now go get a good 15 to 20 amp mppt controller and wire the panels in series and correct your mistake. What more do you want?

OK get a Morningstar Sun Saver MPPT, and sell the Sun Saver PWM to a sucker who does not know the difference and will loose 30 to 50% of the power from the panel using PWM.

If it were me I would have just dug a shallow trench, and ran an AC circuit. Lot less expensive and works.

[PNPmacnab]

It is hard to believe there aren't industry devices ready to serve this market. Solar panels are the cheapest thing in the system and you want to double or triple them and run nearly without a battery at all using linear current boosters. That would make the system very cost effective. If the sun isn't shining, you don't need a fan and at lower light levels they just run slower. Alas, the world is battery centric and stuck with stupid control systems. Find yourself a sharp 12 year old to get involved with this experiment.

[NorthRick]

So what is your point? You asked for unreasonable electric prices, outages and problems. What are you complaining about? Anything you take off-grid is going to cost you 5 to 10 times more than just buying it from the POCO and have many days without power. You are getting exactly what you asked for.

Couple that with an undersized system to start with, and you have to start over and do it right. Your system has to be designed for WORSE CASE, half the year is not going to cut it and destroy batteries. Battery replacement cost alone is going to be many times more than just buying power. That is what off-grid solar is all about.

My point is, that although the existing system is not set up to get the maximum life out of the batteries, it ain't going to kill them by the end of the week. It's a hobby greenhouse that is in use only 5 months out of the year (they float with no load the other 7). It appears to be undersized for just 2 months out of the 5 that it's in use. I am looking to make some minor tweaks to get more life out of the batteries I have, not spend nearly double their worth to maybe get them to go one extra season.

This is for fun and education, not some misguided attempt to stick it to the POCO. I am still messing around with how best to garden up here and may change the greenhouse set up substantially at some point, I don't know, hence I don't want to spend money on a whole new system right now.

Heck, if I really wanted to just waste money, I'd take up something like golf.

[NorthRick]

Be sure when you switch to the MPPT that you rewire the panels to be in series, not parallel. You have nominal 20V panels trying to charge a 24V battery bank. When it is hot out, the voltage drops from the panels, making it even harder to get a good charge. Rewiring them to be in series, nominal 40V will fix that. The MPPT charge controller will drop the voltage to the correct level while raising the current on the output.

Actually the panels are 37v and are currently in parallel, which I believe is fine for a 24v battery bank.

[Sunking]

Hmmm. $200 for another panel and $300 for a 25A MPPT charge controller, so, $500 to maximize the life of $340 worth of batteries.

So what is your point? You asked for unreasonable electric prices, outages and problems. What are you complaining about? Anything you take off-grid is going to cost you 5 to 10 times more than just buying it from the POCO and have many days without power. You are getting exactly what you asked for.

Couple that with an undersized system to start with, and you have to start over and do it right. Your system has to be designed for WORSE CASE, half the year is not going to cut it and destroy batteries. Battery replacement cost alone is going to be many times more than just buying power. That is what off-grid solar is all about.

[Amy@altE]

Be sure when you switch to the MPPT that you rewire the panels to be in series, not parallel. You have nominal 20V panels trying to charge a 24V battery bank. When it is hot out, the voltage drops from the panels, making it even harder to get a good charge. Rewiring them to be in series, nominal 40V will fix that. The MPPT charge controller will drop the voltage to the correct level while raising the current on the output.

And yes, Morningstar is a great brand.

[NorthRick]

Hmmm. $200 for another panel and $300 for a 25A MPPT charge controller, so, $500 to maximize the life of $340 worth of batteries. I am pretty sure for the first half of the season the batteries do get fully charged most days.

I have three main loads: a circulation fan on a timer for moving air around the greenhouse, an exhaust fan on a thermostat to keep the inside temperature from going too high, and the parasitic loads of keeping the system on.

The circulation fan is 45 watts and last year I had it set to come on a 3am and run until 9am. Six hours, or 270 watthours, mostly just before the sun really starts hitting the panels.

The exhaust fan is 105 watts and comes on when the greenhouse temp exceeds 90F. This only happens on a sunny day and typically not until about 2pm or 3. Most of the time when I get home from work around 5pm or 6, I will open the greenhouse doors, which will drop the temperature and the fan shuts off. So call it 105 watts for 3 hours or 315 whrs - only on sunny days. On a cloudy day this fan never kicks on. Also, if we are home on a weekend I will open the greenhouse doors once the outside temperature warms up and, again, this fan does not come on.

The other loads are the parasitic draw of the 300W inverter (10W), the fan timer at 3W, and a small radio, say 10W for an hour a day. So, 322 whrs daily.

Assuming the batteries were full at 9pm, by 9am the next morning 431 whrs has been pulled out (1/2 the parasitic load, 161, plus the 270 whrs from the circulation fan). Now if it is a sunny day and say I am averaging 40% of the panel rating or 156w for the 5 hour period from 9am to 2pm that is 780 whrs available to replace the 431. The battery bank should be pretty close to fully charged at this point.

Now, at 2pm the circulation fan kicks on and starts pulling 105W. Our solar noon is 2pm and lets say the panels are now putting out 60% or 234W. That should be enough to run the fan and continue to top off the battery bank. At 5pm, I show up, open up the doors, the fan goes off and now the load drops to 23W (I turned the radio on). That is 6% of the panel rating and the panels will receive direct sunlight until around 8pm. The sun will be out for several more hours but it will start coming from the northwest after about 8.

Bottom line is, I believe the battery bank does get fully charged by the end of each day from late April when I set things up until about early August. What happens in August is that we typically get many more cloudy days. The output from the panels does not keep up with the load and that is why I was thinking that the additional efficiency of a MPPT over a PWM would make the difference.

Maybe I would be better off getting a 24v battery charger capable of equalizing the batteries and firing up the Honda 2000 generator after a string of cloudy days in August/September to get the bank fully charged and equalized? By the end of September it is getting too cold to keep the greenhouse going and I shut everything down for the season.

[Sunking]

I know it is lower than ideal and I figured it would only have a minor impact on battery life as long as they get fully charged regularly. Am I way off on that?

OK there are two conditions that need to met to obtain maximum battery cycle life.

1. Minimum charge currents must met in order to prevent Stratification where the acid settles to the bottom of the battery jar with the water floating on top. To do that FLA batteries need at least a C/12 charge current where C is the battery amp hours capacity, and 12 or X is hours. What that says is you charge at a current that will charge the battery X hours. You said the batteries are 205 ah so C/12 is 205 ah / 12 h = 17 amps. Even a MPPT charge controller with 390 watts does not meet the minimum requirement. Ideally you want C/10 to C/8 of 20 to 25 amps.

2. Lead acid batteries are unique in that they must remain fully charged at all times to maximise battery life. Any time lass than 100% and the clock is running. That means you should fully charge a battery ASAP after a discharge cycle.

Keep in mind even though a charge indicates the battery is charged does not mean it is fully charged. Example 29 volts. You have to hold 29 volts until charge current STOPS. At that point the battery is fully saturated, aka Absobed. Most solar systems are not capable of doing that because it takes 4 to 12 hours. EQ charges take up to 24 hours.

[NorthRick]

Its a start, but not a total solution. You failed to do the first step, determine how many Watt Hours you need in a full day, and determine worse case sun hours.

While that is technically right, it implies I paid no attention to what my loads would be. I did estimate my loads but the whole thing is a bit of an experiment. Not just the solar but how to effectively manage the greenhouse. That said, the actual loads, as I managed the greenhouse last summer, are higher than what I thought they would be (that never happens right?).

But, I would like to set that discussion aside for the moment and go back to the charging amps from the panels. As I said, I know it is lower than ideal and I figured it would only have a minor impact on battery life as long as they get fully charged regularly. Am I way off on that? The existing controller has an equalize mode that the manual says it goes into every 28 days for 3 hrs. It brings the charge voltage up to 29.8V. I have seen it go into this mode and verified with a volt meter that it does bring the battery voltage up to 29.8V.

[Sunking]

Its a start, but not a total solution. You failed to do the first step, determine how many Watt Hours you need in a full day, and determine worse case sun hours. What I can tell you for sure is 400 watts of panels is not enough to support a 24 volt 200 AH battery, and that a 24 volt 200 AH battery is only good for 1 Kwh per day. You need at least 500 watts of panels with a 20 amp MPPT Controller.

Problem is no one knows if that will work or not. Will only work if you use 1 Kwh/day or less. You need to go back and start over before you spend a dime.

[SunEagle]

And 15amp is on the low side of properly charging that 205Ah battery system but based on 195w and Vmp = 37v you are only getting ~ 5amps (195w / 37v = 5.3a) for each panel which is a little over 10A total. That is not enough for your battery system."

Would that not be closer to 15A into the batteries? 390w/24v = 16.25amps minus loses. Regardless, I understand that is on the low side for charging amps, but that battery bank spends 7 months a year in float. It does sound like the Epever may be the way to go though.

Yes a 15A MPPT would be better then your current PWM. The problem is that even though your batteries stay in float mode you still need to kick them in the butt every once and a while. To do that you need more charging amps or you will end up sulfating the plates and turn a 205Ah battery into a 190Ah battery.

FLA batteries do not like to just sit there. They need to be used but not abused. Part of that requires hitting them hard or at least charging them at a higher amp rate then C/13 which is what 15 amps get you.