Ready to start building..

[Biofarmer]

Greetings and Sunny Southern Salutations,

This is my first post after a multi-year ban, plus a self imposed exile until some moderator changes occurred. I promise to play nice with everyone, and I'll try to be even tempered with the playground bullies this time..

With the price per watt of solar finally getting into the affordable range (for me anyway) and the fact that Home Depot actually has a pretty decent selection of Grape Solar mono & poly panels that are delivered free with no minimum order, I decided to go for it. I've been collecting parts and pieces for a while now, with the intention of fabricating a Traxle style tracking mount. I've acquired a small 24V 54rpm gearmotor off of ebay ($13) and have ordered a 24V LED tracking module from Duane at Redrock ($39). I have two of the 235W poly panels shown below that I wish to charge a small 24V NiCd bank (20 85Ah 1.2V cells) with. The only use of this bank will be to power a 12V 17A(max) pump roughly 15 minutes an hour for a small off grid aquaponics system. I had planned on tapping the bank for 12V at different locations every few days to keep the cells from too much uneven discharge, or else use a Pyle PSWNV480 24V DC to 12V DC Power Step Down 480 Watt Converter which is economical enough from a dollar standpoint, but apparently has a fairly high current draw when not in use, and I'm not sure if it would handle repeated start-up  current surges from the pump motor. I've cut n' pasted the panel's particulars below for reference, if anyone see's any sizing issues (or other potential problems) with this setup, I would be grateful if you pointed them out.   Thanks, Gus

Cell Size  (6.14” x 6.14”)
Module Dimension (LxWxT) (65.0” x 38.9” x 1.6”)
No. of Cells 6 x 10 = 60
Weight  (44.1 lbs)
Type of Connector MC-IV
urgesJunction Box IP65

Details
Max System Voltage 1000V / 600V
Max Peak Power Pmax 235 W (0% / +3%)
CEC PTC Listed Power 210.4 W
Maximum Power Point Voltage Vmpp 30.1 V
Maximum Power Point Current Impp 7.81 A
Open Circuit Voltage Voc 37.2 V
Short Circuit Current Isc 8.35 A

[tecker]

 Aright hock it up ! Not sure why you want to work with 1000 volts as most insulation is rated only fo 600 volt max . Push the details back of your plan and consider a user diary to account for some esoteric posting .

[Biofarmer]

Aright hock it up ! Not sure why you want to work with 1000 volts as most insulation is rated only fo 600 volt max . Push the details back of your plan and consider a user diary to account for some esoteric posting .

  Tecker- Maybe this was directed to a different post??

[mab]

If I'm understanding you correctly, you're wiring your 20 cells for 24v but wanting to draw off power at 12v?

if so I guess you could alternate between the high 1/2 and the low 1/2 of the pack at regular intervals.

As for the sizing issues: well I'm not familiar with ni-cd and don't know how much sun you get (or how short your winter days are), but if you're somewhere reasonably sunny (from your reference to 'sunny southern') then guess you should be OK. At least NiCds don't mind being run down to empty, though I wonder if you should have bypass diodes across the cells to prevent reverse charging.

couple of questions though:-

Can the nicds handle 16-20A charge when they're full (I don't know how to regulate for NiCd)?

Have you remembered blocking diodes?

[tecker]

Ok I saw 1000 /600 made a comment 

[Mary B]

Tecker that is pretty standard insulation specs for a solar panel.

[Mary B]

A small charge controller will be needed unless you like watching batteries swell up and maybe explode. Your converter is rated at 40 amps so I think it will handle the start-up current okay. I went with a Samlex step down converter to power my ham gear, little higher quality and maybe lower standby drain. Can find them on Amazon.

[Biofarmer]

@ mab- Each panel has 3 factory installed bypass diodes, and I have a 40A/600V stud diode screwed into a heavy copper plate for each panel's pos lead- but I'm wondering how any reverse charging would occur even without these. I don't believe the NiCd's are as affected by moderate overcharging as L-A's other than keeping up with the water, but that is one of the questions I was hoping someone here would have some insight to. I have plenty of extra NiCd cells to soak up spare charging capacity if that becomes necessary.

@ tecker- OK fine. Now, maybe you'd elaborate a bit on what these mean? "Push the details back of your plan and consider a user diary to account for some esoteric posting."

@ MaryAlana- No, I don't believe that I would especially enjoy watching that... A charge controller is an added expense, whereas adding another 85Ah of storage is free and on hand. Do you think the two 235W panels could top out a 170Ah bank in intermittent but continual use, or should I just bite the bullet and budget the expense of a charge controller?

[mab]

Sorry, I was thinking bypass diodes across the cells of the battery, so that if you run the battery down to 0v (which you can with NiCds I think), then the cells that run flat 1st will be reverse charged by the ones that are still going. Obviously if you don't run it flat that shouldn't be an issue.

[XeonPony]

2 * 235w pannels top off a 696Ah bank that is under constent use, so yes charge controller is an absalute, unles you got a battery replacing fetish! that and even nicads will die fast if not properly regulated in both discharge and recharge, they are tuff but not indestructable!

All so I'd point out my average Wh from my pannels is between 800 to 3KwH a day, Now NiCads are a bit lossy but not "that" lossy for a 300Ah bank to handle 540+-W with out a controller!

[tecker]

I wouldn't trust 1000volt specs 600 is iffy. That's a problem. The panels are not High potted for 1000 volts . 1000 Volts will arc over really easy and with current you need special building practices.

[dbcollen]

I wouldn't trust 1000volt specs 600 is iffy. That's a problem. The panels are not High potted for 1000 volts . 1000 Volts will arc over really easy and with current you need special building practices.

Those are the UL/Ce ratings for the max system voltage. With the extensive testing UL does, I bet they are safe at those voltages. Why are you hung up on the panel ratings anyhow? he is using 24v.

[Biofarmer]

XeonPony-
  Thank you! That was precisely the kind of information I needed.. Now to find a controller that is appropriate for this size system. No battery replacing fetish here, I assure you. o now I'm lookin at a Morningstar PS30M 12/24/48- any negatives on this one?

[XeonPony]

I have a saying, you can go cheap on the pannels, you can go cheap on the batteries, but never ever go cheap on the controller!

If you do not need the the higher level of monitoring and such I would go with the tristar Mppt 45, it will allow you to add another 540 watts if you chose to and you can hone in the charge profile very well.

you can do all the programing via the rs232 port, the 60a version has either net and rs485 port the web management is handy for expandibility.

but add all the batteries you can and go with the tristar 45 and see how it goes.

[Biofarmer]

-I'm afraid the 45A MPPT version is quite a way out of my current economic ballpark, however the 45A PWM version is just barely attainable at this time and reviews tout it to be pretty much bullet-proof. I realize MPPT is better and more efficient, and when I set up the main system for the house there won't be any skimping (there will also be a lot more $ to work with) but for this little greenhouse system I'm not convinced that I need that much zoot. HOWEVER- point taken about the expandability to two more panels with the 45A controller... Thank you!  Gus

[XeonPony]

well the pwm version will be a good starter then.