Small MPPT Charge Controller?

[NorthRick]

I have a small solar system for our greenhouse, primarily to power circulation and exhaust fans. It has two 195 watt panels with a Vmp of 37v. The battery bank is flooded lead acid - 205ah at 24v. Currently I have a fairly basic SunSaver PWM charge controller (SS-20L-24V).

The issue is that the actual fan loads are higher than what I had estimated when setting up the system. This comes into play latter in our growing season when we typically get more cloudy weather. Several times I have had to disconnect the load for a day or more to allow the battery bank to get back up to full charge.

My thought for a fix is to replace the PWM charge controller with a MPPT. I believe with my current setup the PWM is leaving about 25% of the available power on the roof. I am looking at getting a SunSaver MPPT (SS-MPPT-15L) to replace the PWM controller. It is about $210 online. I've seen cheaper ones but not sure about them.

Two questions:

1) Does switching from a PWM to MPPT seem like the best fix for my situation?

2) Are there other small MPPT charge controllers I should consider?

[sensij]

EpEver's Tracer series offers a good entry level mppt at a lower price point.

[SunEagle]

Usually if you have more than 200 watts of panels and a PWM CC you are losing about 33% of the wattage. Going to an MPPT should use more of the panel wattage but with ~ 400 watts a 15amp CC might be cutting it close even for a 24Volt battery system.

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.

[sensij]

Usually if you have more than 200 watts of panels and a PWM CC you are losing about 33% of the wattage. Going to an MPPT should use more of the panel wattage but with ~ 400 watts a 15amp CC might be cutting it close even for a 24Volt battery system.

400 W of panels will not produce more than 15 A of charge current on a 24 V system. Even if the panels were producing STC wattage (they probably won't for any meaningful length of time), the charge controller efficiency would eat up enough to keep the output to 15 A. That said, the Epever 20 A CC with remote monitor is $120-$130, and going smaller doesn't save much unless you go all the way down to 10 A, which is too small for this system.

[SunEagle]

400 W of panels will not produce more than 15 A of charge current on a 24 V system. Even if the panels were producing STC wattage (they probably won't for any meaningful length of time), the charge controller efficiency would eat up enough to keep the output to 15 A. That said, the Epever 20 A CC with remote monitor is $120-$130, and going smaller doesn't save much unless you go all the way down to 10 A, which is too small for this system.

I guess when you figure in the inefficiency of the system 400w will work with a 15A MPPT CC and a 24v battery system.

I just didn't want the OP to try to save a few bucks and limit the CC if they find out 400watts is not enough to run their system. Not worth painting yourself into a corner if you don't have to.

[NorthRick]

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.

[sensij]

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.

The 10 A is his estimate of the charge you might be getting from the PWM controller.

[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.

[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.

[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]

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]

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.

[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.

[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.

[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.