MPPT Sizing Factor

vtmaps

Re: MPPT Sizing Factor

Scottg4001 wrote: »

We have had a record low of 36

ggunn wrote: »

Then that's the number you should use. Refer to the module specs for the temperature coefficient for Voc, calculate the array voltage at that temp, and compare it to the CC's max voltage. This is the number you have to be the most conservative with in PV design because it is the one that will damage equipment and void warranties if you get it wrong.

I agree that you should be very conservative with these numbers. But, a minor nit to pick.... I think its a good idea to plan for temperatures below the record low. The reason is radiational cooling. On clear nights we often have frost and ice on car windshields, solar panels, etc, even though the air temperature is a few degrees above freezing. I don't know how to quantify the effect, just be conservative and don't push the limits.
--vtMaps

Scottg4001

Re: MPPT Sizing Factor

Alright, so I hooked the panels in series and the batteries were fully charged in 6 hours from a 75% DOD. I am producing about 55.3 volts and 7.4 amps. Now, with the issue of a undersized charge controller with 75 volts Voc max, I have decided the best alternative is to downsize the panels to reduce the Voc. I was thinking of a pair of Sharp 180 mono panels rated at 29.6 Voc, 23.8v Vmp, and 8.4 Imp. This should give me around 44 VDC with around 8 amps for charging my (4) 6v batteries. I am running a 4 amp load 24/7. I assume that the should be adquate for charging. But I do have a second question:

Now with the short circuit voltage being too high with what I currently have for the charge controller, can I add overcurrent protection between the panels and controller so if a panel does short the breaker trips and the controller is protected from the short circuit voltage?

BB.

Re: MPPT Sizing Factor

You only (usually) need a breaker/fuse per solar panel string when you put three or more strings in parallel.

The fuse/breaker is there to protect the wiring, not the charge controller from Isc and a short circuit to ground/return. Normally, your wiring is more than heavy enough to manage Isc-array without a problem (you want low voltage drop/low power loss on your PV array wiring--so the wire is usually way over-sized).

A MPPT charge controller will not take more power/current than it needs/can use when charging the battery bank.

Since Isc is only a little above Imp (10-25% or so)--You really cannot install a breaker that will reliably break/open on Isc and reliably stay closed at Imp (plus, you can get >Imp in certain partially cloudy conditions which could cause a false trip).

-Bill

Scottg4001

Re: MPPT Sizing Factor

BB. wrote: »

You only (usually) need a breaker/fuse per solar panel string when you put three or more strings in parallel.

The fuse/breaker is there to protect the wiring, not the charge controller from Isc and a short circuit to ground/return. Normally, your wiring is more than heavy enough to manage Isc-array without a problem (you want low voltage drop/low power loss on your PV array wiring--so the wire is usually way over-sized).

A MPPT charge controller will not take more power/current than it needs/can use when charging the battery bank.

Since Isc is only a little above Imp (10-25% or so)--You really cannot install a breaker that will reliably break/open on Isc and reliably stay closed at Imp (plus, you can get >Imp in certain partially cloudy conditions which could cause a false trip).

-Bill

Thats what I figured but thought I may have found a loop hole. What is driving me insane, is why does all the stand alone sizing sheets say I need to wire my panels in parallel, when series wiring is the way to go?

BB.

Re: MPPT Sizing Factor

It depends... For PWM controllers Vmp-array must match the battery bank voltage (for a 12 volt bank, Vmp-array~35 volts; etc.).

Also, some charge controllers tend to be less efficient if Vmp is greater than 2xbattery charging voltage--So running the array near 2xVbatt is the most efficient for the charge controller (I think Outback is like this).

In the end, if you have Vmp-array~17.5 volts (for a 12 volt bank), there is really not much reason for running a MPPT charge controller... Just run a much cheaper PWM and you are done.

The great place for an MPPT controller is for large arrays (over ~200-400 watts) and arrays that are a fair distance away from the battery shed/charge controller--Using the MPPT controller allows to use much smaller gauge wiring, use "non standard" Vmp panels (>100 watt panels typically have Vmp<>17.5;35;70 VDC) which are much cheaper than using "battery sized" and typically smaller panels (Kyocera is one of the few >100 watt Vmp~17.5 volt panel vendors left).

Also placing strings in series allows you to use fewer (or no) fuses/breakers on the array (1 or 2 parallel connections do not need a series pass fuse per paralleled string)...

In some cases, such as a high desert where the battery bank may be quite cool (in-ground) and require a high charging voltage from a very hot solar array which can depress Vmp to below optimum for Vbatt-charging -- I.e., you may have trouble equalizing a cold battery bank on a very hot day with a Vmp~17.5 volt panel.

-Bill

Scottg4001

Re: MPPT Sizing Factor

I am gathering new info everyday, so here is my next finding and question. I just found out that my AGM batteries can accept a certain maximum charge voltage or it will damage my batteries. The charge controller does regulate this voltage at 28.5 volts, which is perfect. If the battery charge is only requlated at 28.5 volts and I have a input voltage of 55 volts, is there any reason to even have a MPPT charge controller and is there any reason to have my two panels in series? If I can't boost the voltage for my batteries, whats the point? I am thinking of just going back to parallel, with a input of 28-29 volts and 14Amps with a standard PWM controller.

ggunn

Re: MPPT Sizing Factor

Scottg4001 wrote: »

I am gathering new info everyday, so here is my next finding and question. I just found out that my AGM batteries can accept a certain maximum charge voltage or it will damage my batteries. The charge controller does regulate this voltage at 28.5 volts, which is perfect. If the battery charge is only requlated at 28.5 volts and I have a input voltage of 55 volts, is there any reason to even have a MPPT charge controller and is there any reason to have my two panels in series? If I can't boost the voltage for my batteries, whats the point? I am thinking of just going back to parallel, with a input of 28-29 volts and 14Amps with a standard PWM controller.

One reason to wire your modules in series and go with a MPPT CC is so that you can run smaller (cheaper) wire from your array. Obviously, the closer your CC is to your array, the less of a consideration that is.

Scottg4001

Re: MPPT Sizing Factor

This is a off-grid stand alone, the CC and batteries are 3.5'-4' below the array. Its only a 2 panel system so the wire is already small enough as is.

vtmaps

Re: MPPT Sizing Factor

Scottg4001 wrote: »

If I can't boost the voltage for my batteries, whats the point?

I'm not sure I understand your question. You won't lose anything with an MPPT controller. You may 'boost' the voltage by putting the panels in series and the MPPT controller will reduce the voltage and increase the amps.

Scottg4001 wrote: »

I am thinking of just going back to parallel, with a input of 28-29 volts and 14Amps with a standard PWM controller.

Only problem I see is that when your panels get very hot (and they will) they may not have enough voltage to get the batteries fully charged. Of course, if the batteries are also hot (not a good thing) they will require a lower voltage.
--vtMaps

Photowhit

Re: MPPT Sizing Factor

There will be some voltage loss at the charge controller, also in hot weather the charge controller won't produce it's panel rated voltage.

Not sure how your measuring the voltage, but if you've only seen 28-29 volt just by measuring the output with out a load (VOC) your actual VMP (voltage with a load) will be considerably lower.

With an MPPT charge controller, you won't loose the power from having the voltage higher than needed, as it converts the voltage to what's needed and increases the amperage (with small losses in the conversion)

Scottg4001

Re: MPPT Sizing Factor

vtmaps wrote: »

I'm not sure I understand your question. You won't lose anything with an MPPT controller. You may 'boost' the voltage by putting the panels in series and the MPPT controller will reduce the voltage and increase the amps.


Only problem I see is that when your panels get very hot (and they will) they may not have enough voltage to get the batteries fully charged. Of course, if the batteries are also hot (not a good thing) they will require a lower voltage.
--vtMaps

I was told previously in this thread that I should have my panels in series. This will increase my panel output from 28-29 volts (with load) to 55volts which will better charge my batteries. This will also give the MPPT the ability to lower the voltage and increase amperage. And it makes sense. So I did, the panel generates 7.4 amps output and the cc is charging around 11 amps. Great!

But today I read that the AGM batteries that I have can only charge at a voltage of 28.5 or the batteries will prematurely fail. And the cc does regulate the charge voltage to 28.5 volts. So basically when wired in series, the cc supplies the batteries with 28.5 volts and 11 amps.

So now I realize, if I can't supply the batteries with voltage greater than 28.5 volts, why wire the panels in series? If I reconnect in parallel I will get between 28-29 volts and 14 amps to the batteries. Since the voltage is not that high for the MPPT to modify the voltage to increase the current, what's the point of even having a MPPT?

ggunn

Re: MPPT Sizing Factor

Scottg4001 wrote: »

So now I realize, if I can't supply the batteries with voltage greater than 28.5 volts, why wire the panels in series? If I reconnect in parallel I will get between 28-29 volts and 14 amps to the batteries. Since the voltage is not that high for the MPPT to modify the voltage to increase the current, what's the point of even having a MPPT?

If you are seeing 28-29 volts now, you will see less than that mid-summer. Another advantage of MPPT is that it is constantly adjusting the load on the PV array to maximize power output.

Windsun

Re: MPPT Sizing Factor

Even in cool weather, solar panels can get pretty hot, especially if their is little or now wind/air circulation. I have measured actual panel temperatures as high as 120+ with an 85 degree air temp. That is why most of us here recommend going to a series setup whenever possible - no matter how hot things get, you will still get some charging.

SolaRevolution

Re: MPPT Sizing Factor

This thread seems to be getting a little bungled up and it has evolved from the original question. Here is a collection of some of the important issues (IMO) Scottg4001 has raised:
(I hope you don't mind my cut-and-paste job):D

Scottg4001 wrote: »

This is a off-grid stand alone, I currently have two 235 watt panels, 30.1 Vmp, 7.81 amps wired parallel. So I am getting about 27 volts and 14.4 amps on a sunny day. This is to charge (4) 6v 224Ah batteries.

...my four 6v batteries require 2.75 240 watt panels, 24 VDC nominal. Obviously that equals three 240w panels. With 25% extra...

So he is thinking of using 3 Trinas:

Scottg4001 wrote: »

Trina 235 watt panels. 7.81 Imp, 30.1 Vmp. Total of three panels

... I currently have a 15amp Sunsaver MPPT on a two panel setup.
... I also have a 45A MPPT TriStar CC still in the box.

Sunsaver is 75 volts max. Voc of panels is 37.1...

... I am in south Florida, so the system will never see freezing temperatures...We have had a record low of 36 ...

... I read that the AGM batteries that I have can only charge at a voltage of 28.5 or the batteries will prematurely fail...

So now I realize, if I can't supply the batteries with voltage greater than 28.5 volts, why wire the panels in series? If I reconnect in parallel I will get between 28-29 volts and 14 amps to the batteries. Since the voltage is not that high for the MPPT to modify the voltage to increase the current, what's the point of even having a MPPT?

...the CC and batteries are 3.5'-4' below the array....so the wire is already small enough as is.

I am thinking of just going back to parallel, with a input of 28-29 volts and 14Amps with a standard PWM controller.

The bottomline is my four 6v batteries require 2.75 240 watt panels, 24 VDC nominal. Obviously that equals three 240w panels. With 25% extra, do I need to spend the extra for a MPPT or would a standard PWM charger work fine ?

So,

I'm thinking that if the PV and batteries are subjected to high temps at the same time ( "3.5'-4' below the array") The low voltage of single PV module strings should not be a problem. Other than the fact that you already have the MPPT CCs, a PWM CC with (3) Trina 235s would make sense in your case.

With PWM and a 24 volt battery, your PVs will be operating very close to their MPP. You don't really need maximum efficiency once the battery voltage reaches the Absorb setting or for EQ. (I missed whether your batteries were sealed or flooded) You may gain a very slight boost when your battery voltage is low but you will have a charge current of about C10 anyway.

The option that really appeals to me is to use the 45A MPPT Tristar with all 3 PV modules in series. You mentioned how you liked the display and you would have a better set up for EQ. (if you need it)

Alex Aragon


BTW: If you need to have low voltage disconnect for your DC loads you can use your Sunsaver or a cheaper non-MPPT charge/load controller in parallel to protect you batteries from over discharge.