Off grid, with grid tie high voltage panels and mppt

[klloyd]

I'm working on a project that is coming together now pretty well. The one question I've got is on the higher grid tie panels verses the lower voltage off grid panels. I've researched the crap out of this, and cant find a good answer, so here goes:

Using the morning star 45 amp mppt, is it more efficient to use high wattage-high voltage panels, or high wattage low voltage panels?
Currently the battery configuration is 12 v bank @ 100 ah (two 6v 100ah batteries), but going to 24v as the system grows, then upward from there as needed. I have room for 48v battery bank, but hope to stick to 24v to grow my amp hours first. As my amp draw on the bank grows, I'll grow the bank voltage to drop off the amp draw to something manageable.

I've pulled my bed room off the grid completely with two 145 watt 17v panels, but I need more, and those are cheap, and rarely in stock... yet the higher wattage higher voltage panels are always in stock, ... upwards to 50 volts per panel. The controller will take it, so which is the most efficient best way to go?

[Naptown]

higher voltage is the way to go.
Less voltage drop, you can use smaller wires from panels to CC.
That CC can take 150V if I remember and will go from 12-48V all with a 45A output.
the only thing I caution you about is don't add new batteries to an old bank When you upgrade to a higher voltage replace all the batteries at once. Otherwise one old weak battery will bring down the rest.

[klloyd]

Thank you so much. Its not often I ask questions, I can read, and a lot has already been explained 100 times, but when I do, you guys are great, spot on, and kind. I appreciate you guys!
I really didn't think the batteries being trouble. I plan to build this current configuration to 24v now, dump them all to bigger better bank next year that tanks up less space.
Do u have any experience with the 36-48v forklift batteries? Huge power, in a small, HEAVY package! The idea seems sound. Two of them should hold my meger draw for a while.

[Naptown]

Thank you so much. Its not often I ask questions, I can read, and a lot has already been explained 100 times, but when I do, you guys are great, spot on, and kind. I appreciate you guys!
I really didn't think the batteries being trouble. I plan to build this current configuration to 24v now, dump them all to bigger better bank next year that tanks up less space.
Do u have any experience with the 36-48v forklift batteries? Huge power, in a small, HEAVY package! The idea seems sound. Two of them should hold my meger draw for a while.

No not any direct experience. However not as good as say a Rolls RE battery as # of cycles will be less.

[Wy_White_Wolf]

... I've pulled my bed room off the grid completely with two 145 watt 17v panels, but I need more, and those are cheap, and rarely in stock... yet the higher wattage higher voltage panels are always in stock, ... upwards to 50 volts per panel. The controller will take it, so which is the most efficient best way to go?

Completely depends on your situation.

While others may say the higher voltage panels, I don't agree.

Using an MPPT controller with a 150V max input would only allow you to use 2 - 50V panels in series (gotta allow for colder temps) for 100VOC from the string. Using a panel with a VOC of 35 to 40 might allow you to run 3 panels in a string. That would be 105V to 120V which would be more efficient. Your 17V panels most likely have a VOC of 20 to 23V so most likely you could run 4 to 5 of them in series.

WWW

[Naptown]

Completely depends on your situation.

While others may say the higher voltage panels, I don't agree.

Using an MPPT controller with a 150V max input would only allow you to use 2 - 50V panels in series (gotta allow for colder temps) for 100VOC from the string. Using a panel with a VOC of 35 to 40 might allow you to run 3 panels in a string. That would be 105V to 120V which would be more efficient. Your 17V panels most likely have a VOC of 20 to 23V so most likely you could run 4 to 5 of them in series.

WWW

Most modules I'm seeing these days are 60Cell modules (Everyone wants as many on a string as possible and enphase M215 compatable)
Voc on those are about 36v but keep temperature compensation in mind.

[Sunking]

I understand what WWW is saying and somewhat agree with him.

Assuming we are talking about the standard 150 Voc limit on MPPT charge controllers ideally we want to run th evoltage as high as possible, but that is not necessarily the best voltage to use. For example the Outback FM80 on a 24 volt battery system runs most efficiently at 34 vmp @ 98.5%. At 100 Vmp the efficiency drops down to 95.3%. However that is not taking line losses into account, and it must be factored in to find the most economical and lowest loss. If you have a long run from the panels to CC in most circumstances to keep the line losses to 2% or less best model is to use higher voltages.

So here is a little tip and secret I use. I ignore Voc limits. I take 1.25 as the correction factor because it is the highest factor which accounts for the lowest temperature conditions. So when you do that gives you 150/1.25 = 120 Volts. With that number in mind means the maximum number of cells in series is 240. That is real easy to do with standard 60 cell GTI panels. So on a

12 volt system use 1 panel per string
24 volt use 2 panels per string
48 volt use 4 panels per string.

It is a beautiful thing not having to think and just KISS it. All men should learn that lesson

[Mike90250]

I'll add: go with 48v

Buy gear 1x, use 4, 12v 100ah batteries, and when you need to grow it, step up to 6v 200ah batteries. Forklift batts have high self discharge.
Also look at the MS MPPT 60a