Am I getting the straight scoop?

sprintman · June 2011

I am starting out small and trying to utilize some equipment I already have. I plan on getting a good mppt controller. I already have a good 12v inverter. My idea is about 600-800 watts of power. So probably 3 panels. My idea is if I like this I would eventually add 3 or 4 more panels and go from a 12v battery bank to 24. So I want to get panels that will charge a 24 volt system eventually. Ive been talking to a nice rep who has recommeded kyocera 210 watt panels
KD210GX-lpu. In doing some research at different sites i've seen these listed as 16v nominal but here are its numbers @ stc:
Pmax 210W
Max power voltage(Vmpp) 26.6
Maximum power current (Impp) 7.9A
open circuit voltage(Voc) 33.2
Short circuit current (Isc) 8.58A

My question is I can tell these would work for sure charging 12v with mppt controller but will they work down the road if I change my battery bank to 24v.
Thanks so much......this is such a great place to learn!

Cariboocoot · June 2011

Re: Am I getting the straight scoop?

One of these panels or several in parallel will not charge a 24 Volt system. The reason being their Vmp is below the normal charge level of 28.4-28.8 Volts.

But, put two in series and feed the output to an MPPT controller and there's no problem. That means you'd have to buy an even number of panels for the 24 Volt system.

sprintman · June 2011

Re: Am I getting the straight scoop?

So that is part the I left out, I did specify that I'd be putting the panels in series. I had figured out how much of my power usage I wanted to cover at present with this system and the rep determined 500+ watts would be adequet.-- 3 panels. Can I use 3 in series for a 12v system?

sprintman · June 2011

Re: Am I getting the straight scoop?

Could you please also explain why the number of panels needs to be even for a 24v system when using in series and with an mppt controller......Thanks

Cariboocoot · June 2011

Re: Am I getting the straight scoop?

Yes, you can put three in series for either a 12 Volt or 24 Volt system with the right controller.
The Voc of such a string would be 99.6 Volts, so you need to use one of the more expensive MPPT controllers that can take an input of 150 Volts. These would include the Morningstar, Outback, Xantrex, and Midnight controllers. You'd be running about 35 Amps output current at most, and if you were planning on doubling the panels for a 24 Volt system you simply parallel another string of three in series. You'd still get 35 Amps, but at a charging Voltage suitable for the 24 Volt system. If that's what you want to do, the MS 45 Amp unit would probably be a good choice - rather than the other choices which are 60 Amp.

Typically MPPT controllers work best with an array Voltage that's no more than 2X the system Voltage, but it's not much of a worry really.

niel · June 2011

Re: Am I getting the straight scoop?

if you elect to use 3 in series then paralleled string expansions will be more pvs and more expensive. that is they will go 3, 6, 9, 12, etc.

the lowest number of those pvs in a series string that works is 2 and string expansions could go 2, 4, 6, 8, 10, etc.

icarus · June 2011

Re: Am I getting the straight scoop?

Be advised tht batteries of diffent ages don't play well together, so growing a systems battery bank is not recommended.

I suggest that tread the following:

http://www.batteryfaq.org/

http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

Tony

sprintman · June 2011

Re: Am I getting the straight scoop?

As usual great responses and information...I thank all of you---gives me some more to think on. My eventual plan was to grow to 6 panels (1260 watts). I didn't think I would probably grow it until the first set of batteries were ready for replacement. My more recent thoughts were to just go ahead and get the 6 panels now in part because of the federal rebate. I plan on getting a flexman 60 amp mppt (is this the better choice over the xantrex 60 mmpt controller?) can someone tell me what awg of wire I should run to my basement. (should be less than 30' one way) It would make sense to run wire heavy enough to go from the original 3 panels to when I double it to 6.. would 8 225 amp hour 6v batteries be ok? 675 amp hours at 12v or eventually 450 at 24v when I go to the six panels. I dont seem to be able to do a short post==thanks

BB. · June 2011

Re: Am I getting the straight scoop?

Difficult to make short posts with complex questions...

Even more difficult (for me :roll:) to make short answers with complex questions.

;)

Even if you purchase "enough batteries" to parallel connect for at 900 AH @ 12 volts and rewire later for 450 AH @ 24 volts... You still end up with needing new inverter(s), AC backup battery charger, new wiring and fusing (probably).

Picking the correct solar charge controller + solar panel configuration can be confusing right out of the box.

Rather than thinking about "growing the system"--There are two ways (typically) to proceed.

The preferred method is to measure/define your loads, then define the battery bank, then design the Solar Array + backup battery charger (and genset) for optimum sizing (and costs).

The second method is pick a battery bank (how much money you have or space for batteries) then pick an inverter+chargers+panels+genset to keep the battery bank "happy". Many times this is what is done with RV's which have limited space or a "first system" for educational value.

You have to put a stake in the ground (loads or battery rating) -- otherwise everyone gets confused. There are just too many variables to keep straight and things that change depending on what we are talking about at the time.

Picking the Battery Bank voltage can be "forced" by your loads (say you have a lot of 12 volt loads--so you cannot go 24 volts, for example). Otherwise, it is usually a "floating" value based on your peak loads (gets expensive to manage >> 100 amps of current).

-Bill

petertearai · June 2011

Re: Am I getting the straight scoop?

I plan on getting a flexman 60 amp mppt (is this the better choice over the xantrex 60 mmpt controller?)

Is that the outback fm60, If so I would go for the fm80 as a bit more grunt for not much extra $.

BB. · June 2011

Re: Am I getting the straight scoop?

The MPPT controllers to look at in that class are the Outback, Xantrex (Schneider), MorningStar TS MPPT series, and Classic from Midnite Solar. The MonringStar and Classic are newer designs and some have Internet connectivity and/or USB communications (if of interest to you).

For a smaller system (12/24 volt, 30 amp maximum), the Rogue (at least the previous version) was a very good/cost effective charge MPPT controller (with optional computer bus).

For 15 amp 12/24 volt systems, the MorningStar 15 amp MPPT controller is very nice.

Outback has an integration bus for connecting with other outback products. Xantrex has a comm bus for connecting with their XW product line. MorningStar has a remote bus too.

I do not own an off-grid system so I am not the best one to tell you which to pick--There are many people here that use the above charge controllers and can answer your specific questions.

-Bill

Am I getting the straight scoop?

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