Charge Controller Opinions - Small RV Install

Tallguy

Re: Charge Controller Opinions - Small RV Install

Cariboocoot wrote: »

I think you're still missing it. One MPPT charge controller can have an array consisting of many panels. In fact they're rather a waste of money on a small array. What you can not do is power two MPPT controllers from one array. A single array is considered to be made up of similar PV panels connected in serial, parallel, or a combination of both and having the same physical location/orientation (i.e. next to each other).

The other problem which i think is influencing your understanding is trying to power one MPPT controller off two arrays with different power values (either due to differences in the panel specifications or orientations). That also does not work.

Bill's description of them becoming "confused" is something of an understatement. :roll:

why does the cc get confused? the solar conditions are largely same for all the panels in the array....

i get the other stuff..

boB

Re: Charge Controller Opinions - Small RV Install

Tallguy wrote: »

why does the cc get confused? the solar conditions are largely same for all the panels in the array....
i get the other stuff..

What he means, I think, is that the max power point voltage for each array will be different from one another.

A CC should not get "confused" though... There is still a max power point . It should just find the max power point voltage of the combined arrays and operate there. If there happens to be two or more max power points that are exactly the same, which can sometimes happen with various arrangements and partial shading, the CC can just pick one of those points to operate at. Even in that case, there are certain rules to follow to pick the best one of those multiple choices.

boB

Cariboocoot

Re: Charge Controller Opinions - Small RV Install

Tallguy wrote: »

why does the cc get confused? the solar conditions are largely same for all the panels in the array....

i get the other stuff..

Maximum Power Point Tracking: it tries to adjust its output according to the specific Vmp/Imp point of the panel(s) under varying degrees of illumination throughout the day.

So if you have:
Panels in the same array with different Vmp/Imp specs - the controller won't find an "ideal" power point. It will either pick the point for the "least powerful" portion of the array, some compromise point, or the most powerful point which will leave the other panels output at less than ideal.
Arrays pointing in different directions (i.e. under different illumination) the ideal power point for each array will be different. Again, the controller won't find the best for either.
Two charge controllers drawing power from the same array cause each to see an "incorrect" power point. They'll each try to adjust accordingly, which will again alter the power point of the array and again cause them to see a new "incorrect" power point and try to adjust again. This will happen over and over all day and neither controller will find the right power point; they'll just keep readjusting.

PWM controllers don't suffer from this, as they are basically "programmed on/off switches". There usually is no reason to feed two from one array, however, unless you are charging separate banks of different capacities which cannot be tied together (a small bank charging quickly, a large bank taking more time, different Voltages, et cetera).

BlueSky makes (made?) controllers capable of charging two separate banks, btw. It's not a common need so most companies don't bother.

BB.

Re: Charge Controller Opinions - Small RV Install

What I was trying to relate was that one large array and two (or more) MPPT charge controllers...

One charge controller "sees" 10 amps at 35 volts = 350 watts as its "optimum" operating point.

The second charge controller turns on and tries to draw some current and see its optimum operating point... So, if figures out that 2 amps at 17.5 volts is its operating point.

The first controller sees its Voperating fall to 17.5 volts--so, it backs off and find that 8 amps at 35 volts is not optimum...

Second controller now tries to take 3 amps at 24 volts, etc...

Just a guess on my part, there are different ways to figure out Vmp at any particular temperature/operating point (different controllers, different algorithms). Each controller figures it is the "Master of the array" and that changes in output voltage are the result of sun or other changes in conditions--not that there is another controller pulling its own power and doing its own calculations.

Could you make it work--possibly... But it would not be straight forward figure out what mix of controllers to pick.

The other issue that boB and others are talking about, for example you have one string of Vmp=17.5 volts of panels and second string of Vmp=35 volts of panels.

What is the optimum operating point... You may have several peaks, 17.5 volts, 20 volts, 35 volts, etc... Which peak will the controller pick? Most controllers are not designed to pick the best peak-and they may end up picking a lower (local peak or maximum) vs the true peak in the middle.

And, it is not efficient anyway... With two Vmp strings on one controller, even the best peak will still be a trade-off.

-Bill

Tallguy

Re: Charge Controller Opinions - Small RV Install

BB. wrote: »

The other issue that boB and others are talking about, for example you have one string of Vmp=17.5 volts of panels and second string of Vmp=35 volts of panels.

What is the optimum operating point... You may have several peaks, 17.5 volts, 20 volts, 35 volts, etc... Which peak will the controller pick? Most controllers are not designed to pick the best peak-and they may end up picking a lower (local peak or maximum) vs the true peak in the middle.

And, it is not efficient anyway... With two Vmp strings on one controller, even the best peak will still be a trade-off.

-Bill

just to get it straight, i am NOT looking to put more than one cc on one array/panel.
also, i am not looking to split the output power to several battery banks.

however i am considering to build two arrays, both with same nominal V (probably 24V) but possible different I.mp
the million dollar question would then be if the MPPT would still be able to find the max output point?

BB.

Re: Charge Controller Opinions - Small RV Install

Tallguy wrote: »

however i am considering to build two arrays, both with same nominal V (probably 24V) but possible different I.mp
the million dollar question would then be if the MPPT would still be able to find the max output point?

What is your battery bank voltage?

Vmp=24 volts is a bit high for 12 volt bank on a PWM controller. And too low to charge a 24 volt battery bank (need ~29-30 volts to charge a 24 volt flooded cell lead acid battery + controller + wiring drop).

Yes, you can match panels in parallel if their Vmp match within ~10% or better.

And you can add panels in series if their Imp match within ~10% or better.

MPPT controllers with properly configured solar arrays will work very well as long as the array meet's the controller's input specifications.

If, for example, you have a 400 watt array with Vmp~17.5 volts, then it is probably a push to choose a MPPT (more expensive and more efficient in sub freezing temperatures) vs a PWM controller (might be a bit more efficient in hot weather and certainly much less expensive).

There is no one correct answer that I can give without being more confusing/frustrating.

When choosing Solar PV panels... Many of the >100 watt panels these days have Vmp that is not 17.5 or 35 volts and instead, have Vmp of 30-70+ volts... So, for off grid systems, you pretty much are limited to using a MPPT controller and connecting the solar array in Series/Parallel as appropriate for your controller and battery bank.

The >100 panels are designed for Grid Tied Inverters with operating voltage in the range of 200 to 600+ vdc... And therefore, because of the volume and higher voltage, are less expensive than the traditional XXX watt Vmp=17.5 solar panels... So getting a 200 watt panel + MPPT controller may still be the battery deal than two 100 watt panels + PWM controller.

What panels are you looking at (Vmp/Imp) and what battery bank (voltage, AH rating) are you thinking of using.

-Bill

Tallguy

Re: Charge Controller Opinions - Small RV Install

BB. wrote: »

What is your battery bank voltage?



What panels are you looking at (Vmp/Imp) and what battery bank (voltage, AH rating) are you thinking of using.

-Bill

my battery bank is 12Volt (4 agm each 60 A)

panels:
V.mp17.2V; Open Circuit Voltage: 21.6V; Maximum Power Current: 4.95A (two of these)

V.mp 17.2V; Open Circuit Voltage: 21.6V; Maximum Power Current: 2.91A (two of these)

problem or?

Prefer to stick with MPPT

boB

Re: Charge Controller Opinions - Small RV Install

Tallguy wrote: »

my battery bank is 12Volt (4 agm each 60 A)
panels:
V.mp17.2V; Open Circuit Voltage: 21.6V; Maximum Power Current: 4.95A (two of these)

V.mp 17.2V; Open Circuit Voltage: 21.6V; Maximum Power Current: 2.91A (two of these)
problem or?
Prefer to stick with MPPT

The situation you just stated won't matter when those strings or modules are connected in parallel. That will work fine for either MPPT or PWM.

Different Imp isn't the important issue when paralleling strings. Vmp is.

boB

Tallguy

Re: Charge Controller Opinions - Small RV Install

boB wrote: »

The situation you just stated won't matter when those strings or modules are connected in parallel. That will work fine for either MPPT or PWM.

Different Imp isn't the important issue when paralleling strings. Vmp is.

boB

i was hoping to put each pair together serial wired (to get 24V nominal), then connect parallel to the cc... will that work ok?

BB.

Re: Charge Controller Opinions - Small RV Install

Yep,

Put the two 5 amp panels in series.

Put the two 3 amp in series.

Connect the two strings in parallel and you are good to go with about 8 amps charging a 24 volt battery bank.

You can use either a pwm or mppt controller.

-Bill

Tallguy

Re: Charge Controller Opinions - Small RV Install

Thanks Bob and Bill!
I ended up going with the sunsaver mppt as it gets some rave reviews.
I decided to get also the remote meter, combined cost was slightly over $300.

Currently my set-up consist of 2x85W panels (see below) and I am now considering to add two of these panels called Unisolar 68 Watt 12V DC Flexible Solar Panel PV Laminate.
Does anyone have experience with these panels?

BB. wrote: »

Yep,

Put the two 5 amp panels in series.

Put the two 3 amp in series.

Connect the two strings in parallel and you are good to go with about 8 amps charging a 24 volt battery bank.

You can use either a pwm or mppt controller.

-Bill

BB.

Re: Charge Controller Opinions - Small RV Install

Personally, I would stick with tempered glass mono or poly crystalline solar panels unless you have problems with thrown rocks or get the panels for near free.

I think crystalline + glass panels can last longer.

-Bill

Tallguy

Re: Charge Controller Opinions - Small RV Install

thanks for everyone's assistance on this subject.
i have now run my two x 85 w panels for around 1 month using the sunsaver mppt controller. i get around 65-85 ahs daily from the panels, not too bad.
the controller works very well, however the documentation which came with the product was lacking a lot.
i sent an email to their customer service and got reply back to my initial question but not subsequent questions.

BB.

Re: Charge Controller Opinions - Small RV Install

Don't know about MorningStar--But other vendors seem to work better with telephone calls instead.

-Bill

GreenPowerManiac

Re: Charge Controller Opinions - Small RV Install

I'll vouch for MorningStar controllers. I have the TS-60 & 45 models with digital display. Both have been working beautifully for almost 2 years now. Well worth the $300.00 each. All the data can be sent via phone line to a laptop or computer database. The cheaper it is, the more maintenance it'll likely need.