MPPT vs standard PWM?

[Dave3011]

OK just before the professionals shoot this argument down in a spectacular fireball let me make a valid point: MONEY IS AN ISSUE HERE!

I was just wondering under what circumstances a MPPT charge controller actually improves energy effeciency?

I'll give you my scenario:

2x300 watt panels connected in parallel. Nameplate specifications on each of the panels are: Open circuit voltage is 44.8 volts, Power voltage is 36.6volts. Power current is 8.2 amps, short circuit current is 8.69 amps. OK now this is at standard lab conditions (25C/77F). Despite this I have seen up to 21 amps on my LCD under clear and cool conditions when the sun is far from perpendicular to my panels! According to the nameplate specs I should never see more than 16 to 17 amps becuase my charge controller is not an MPPT...?

Now with 600watts solar and a MPPT I could reasonably expect to see a maximum of 25 amps when my battery voltage is at 24V and no more than 20A when at 30V under boost charge right? I've already seen 29.6V/20.9A with my cheapie PWM controller, thats over 600watts and we haven't even taken losses into account yet!

Now what I need to know is firstly this: I did my installation a month ago which here in the southern hemispere is just before the winter solstice - the weather is cool, sun is weak. In summer I can expect much hotter panels which according to the manufacturer will reduce their output voltage significantly and increase the current slightly. So in that scenario is an MPPT going to acheive that much? The batteries are going to need 31volts for their monthly equalization charges anyway and with cooking hot solar panels they may only just about reach that voltage, then of coarse there will be losses along the 60feet of wiring reaching from the roof to where the batteries are sitting.

I Paid $150 for the PWM controller but an MPPT will set me back about $500. Is it worth my while?

[Wy_White_Wolf]

Since you've already spent money on panels and everything else it depends on if you need the extra power or not. If you do then an MPPT could save you from buying more panels. If not then an MPPT would be wasting more money.

An MPPT may have saved in the intial system because you may have gotten but with only needing 400 or 500 watts in panels instead of 600.

WWW

[Dave3011]

Since you've already spent money on panels and everything else it depends on if you need the extra power or not. If you do then an MPPT could save you from buying more panels. If not then an MPPT would be wasting more money.

An MPPT may have saved in the intial system because you may have gotten but with only needing 400 or 500 watts in panels instead of 600.

WWW

Thank you - well of course I could always use more power but thats not really the point. I'm a little peeved at the salesman who sold me the panels and the charge controller as at that stage I didn't know that there were such cheap charge controllers and simply assumed all charge controllers would do what MPPT controllers do, but when I innitially didnt see the expected output I was a little dissapointed. I initially wanted to go out and buy another 300watt panel because the cheap charge controller is rated at 30 amps and could have handled the extra current (an extra panel at $450 would have actually been cheaper than buying and MPPT controller). But then seeing the high current produced on a cold bright day made me realise that a third panel would have fried the controller.

At the end of the day its down to maths for me - when I look at all the different types of weather I could expect to encounter over any given year, will the MPPT pay for itself?

D

[Wy_White_Wolf]

Well lets see, $500 for a new controller or $450 for another panel.

Another panel will also require more racking and the addition of a combiner box as you would now have 3 panels. It could also mean replacing the wire from the panels to the charge controller and battery to handle the extra amps.

Replacing the controller would mean rewiring the panels to series and possibly replacing the wire between the charge controller to battery to handle the higher amps.

I'd guess the new controller would be a cheaper improvement as the racking, combiner, and wire would be more than $50.

WWW

[Dave3011]

Well lets see, $500 for a new controller or $450 for another panel.

Another panel will also require more racking and the addition of a combiner box as you would now have 3 panels. It could also mean replacing the wire from the panels to the charge controller and battery to handle the extra amps.

Replacing the controller would mean rewiring the panels to series and possibly replacing the wire between the charge controller to battery to handle the higher amps.

I'd guess the new controller would be a cheaper improvement as the racking, combiner, and wire would be more than $50.

WWW

That's a fair point thank you. Guess it's time to go fight with the salesman about trading in an ill-advised charge controller for a MPPT!

[Mike90250]

in really good conditions, you could see Isc from the panels. Don't forget the first 10 years, Panels produce MORE than spec, so that they will be within 20% spec after 20+ years.

MPPT circuit produces the "extra power" only in bulk, and drops back to PWM mode in absorb and float, so it's helping you in the first 3 hours of charge.

[Sunking]

Warmer temps are not going to affect PWM because Input Current = Output Current. Temps only affect voltage, not current. Winter you get higher voltage, but less current. Summer less voltage but higher current. Its a wash.

However with PWM at the power levels you are running, you are screwed. PWM turns your 600 watt panel into 400 watts. You could have gotten the same results with only 400 watts of panels and a 20 amp MPPT controller. Would have cost you a lot less money than the route you went.

[Sunking]

MPPT circuit produces the "extra power" only in bulk, and drops back to PWM mode in absorb and float, so it's helping you in the first 3 hours of charge.

Mike you know how to get around that right? Set Bulk = Absorb. 3-stage charging for solar is useless. Not enough hours in a day to get all the way through the cycle.

[mschulz]

Mike you know how to get around that right? Set Bulk = Absorb. 3-stage charging for solar is useless. Not enough hours in a day to get all the way through the cycle.

Sunking so what you are saying is we would get the same "extra power" in both Bulk and Absorb? Would that allow it to go to float faster?

[Sunking]

Sunking so what you are saying is we would get the same "extra power" in both Bulk and Absorb? Would that allow it to go to float faster?

Yes much faster. Float charge is the kindest gentlest method, but takes up to 24 hours. 3-Stage charging is ole school and second best charging algorithm for FLA. 3-Stage is a trade off of speed and stress.. However for solar application 3-Stage still takes too long. You have been here long enough to hear countless user stating the SPG is too low, but system design and usage is within spec. The problem is there are not enough sun hours in a day to use 3-Stage as it takes about 6 to 8 hours.

Many of the battery manufactures are now recommending 2-Stage to combat the problem. For example Trojan used the term Daily Charge and Float. All Daily Charge is the old Absorb voltage. Still a Constant Voltage - Current Taper charge, it just gets you to Float quicker.

For a typical 12 volt battery 3 Stage voltages look something like Bulk = 14.1, Absorb = 14.7, and Float = 13.2 So for 2-Stage it becomes Daily Charge = 14.7 and Float = 13.2

[Mike90250]

And I have different charge setpoints for winter, than I do for summer. Saves lots of distilled water.

[LETitROLL]

short circuit current is 8.69 amps. OK now this is at standard lab conditions (25C/77F). Despite this I have seen up to 21 amps on my LCD under clear and cool conditions when the sun is far from perpendicular to my panels! According to the nameplate specs I should never see more than 16 to 17 amps becuase my charge controller is not an MPPT...?

Sounds to me like you may already have a cheap MPPT controller, if your readings are correct (20.9A) from 2 panels with 8.69 short circuit current, and you mention that the sun is not close to perpendicular to the panels, must be new math? Anyway you said clear and cool conditions so voltage would have been extra high, looks to me like that was already converted into current? The others here can chime in better than me with the math, but I did not think it was possible to exceed the SSC by that much under any circumstances. Would be nice to know if it does and briefly how.

[mschulz]

Yes much faster. Float charge is the kindest gentlest method, but takes up to 24 hours. 3-Stage charging is ole school and second best charging algorithm for FLA. 3-Stage is a trade off of speed and stress.. However for solar application 3-Stage still takes too long. You have been here long enough to hear countless user stating the SPG is too low, but system design and usage is within spec. The problem is there are not enough sun hours in a day to use 3-Stage as it takes about 6 to 8 hours.

Many of the battery manufactures are now recommending 2-Stage to combat the problem. For example Trojan used the term Daily Charge and Float. All Daily Charge is the old Absorb voltage. Still a Constant Voltage - Current Taper charge, it just gets you to Float quicker.

For a typical 12 volt battery 3 Stage voltages look something like Bulk = 14.1, Absorb = 14.7, and Float = 13.2 So for 2-Stage it becomes Daily Charge = 14.7 and Float = 13.2

I think I get it then. I guess I was puzzled as I have been working with my system design to use enough panel to replace my daily usage in 1.5 hours of sun having 2.5 hours of good sun in the winter while not exceeding C/8 on FLA battery bank. You all have said it might be a little over kill, but when I pencil it out I just don't find it costing me that much more because my system and daily watts are so low. I am finishing up some other projects that will allow power to be installed and then I might have a few more questions. Just when I thought I was getting it all figured out - I realize I am still aways away.

[Dave3011]

Sounds to me like you may already have a cheap MPPT controller, if your readings are correct (20.9A) from 2 panels with 8.69 short circuit current, and you mention that the sun is not close to perpendicular to the panels, must be new math? Anyway you said clear and cool conditions so voltage would have been extra high, looks to me like that was already converted into current? The others here can chime in better than me with the math, but I did not think it was possible to exceed the SSC by that much under any circumstances. Would be nice to know if it does and briefly how.

I'm pretty sure its not a cheap MPPT as that is something I doubt the packaging/manual would omit for the ovious reason of selling it short. Also I have never seen a difference between PV output amps and battery charging amps. I suspect Mike was correct in saying that the new panels are outperforming their specs somewhat, and that the panels were cool from a passing cloud before recieving more sunlight at that particular moment being reflected off bright white clouds and the atmosphere being extra transparent.

I'm beginning to lean towards the argument of getting an good quaility MPPT though as the idea of paying for 600 watts and only getting around 400-450 on most days is not good for morale...

I understand that it can only help when the PV voltage is above the battery voltage so the MPPT may be far more uselful in winter than summer - but thats really when you need the extra power anyway!

D

[Sunking]

Dave with PWM INPUT CURRENT = OUTPUT CURRENT 365 days a year. With PWM the panel voltage has to be 18 volts for every 12 volts of battery. So for a 24 volt system you are looking at a panel voltage of 34 to 36 volts. A 300 watt 24 volt battery panel should have a spec of roughly Vmp = 36 volts and Imp = 8.3 amps. With two in parallel should get you roughly 36 volts @ 16.6 amps. Assuming you battery is demanding full charge current and sun conditions are noon with panels facing directly into the sun they should recieve 16.6 amps and at 24 volt battery is 16.6 amps x 24 volts = 398 watts.

With MPPT Output Current = Panel Wattage / Battery Voltage. Using the same two panels we can wire them in series, or parallel if we wanted to waste money on copper, that same battery would receive roughly 23.75 amps or 575 watts assuming 95% efficiency in the MPPT.

You are claiming 20 amps which makes me think your meter is not accurate.