24V panel, 10A CC, but want to run 12V lights etc.

[sl dl]

II have a very similar question, I think, except I leave the system in place so I do not have to bring them home to charge on my house solar system. I have done 2 other small systems which have lasted years but both were 12v and I am trying to confirm the math for a 24v PV array with 12v batteries.

I have been off grid at the cabin for 25 years but solar for 8 years with a 50 watt mono 12v panel and a 12v controller. It has been adequate but only marginally so. Weekend sees the batteries go from 12.6 to 12.2 and then recharge when I am gone. I was there 6 days in Nov and ran them down to 12.0 with very little sun. Not a lot of load.

I have recently received 2 175w 24v 2 year old panels. This would increase my capacity by 7 fold. The 2 batteries are 125 ah 12v agm. the panels and the batteries are a given for cost reasons.
I need a controller which will work for the panels and batteries either singly or in paralell @ 12v. each panel is Voc 44v, Vmp 35.2v Imp 4.95A, Isc 5.2A

Max load 4A to four 12v 13w each lights. usually more like 1A from lights and a cell phone charging. I run 1 of the 13w 12v lights for about 8 hrs. and another simultaneous for about 4 hours. Southern tier of NY. Need to know if I need a 30A or 40A controller to keep panels from blowing up the controller?

It looks like from the calculation above that I would need either 2 mppt controllers at 20A, (175/12= 15A) for two separate feeds to two 12v battery bank in parallel or 1 40A (350/12 = 30A) from 2 panels to two 12v batteries in parallel? I am considering two controllers from two separate panels to the batteries in parallel since i go there 10 weekends a year and I don't want to get there and have had 1 component go and have to go back to coleman lanterns for the weekend. Not sure if a 30A controller has enough fudge which I think should be about 25%.

If I went the series route and made 24v battery bank from 2 batteries could I use 1 15 or 20A pwm controller and a 24-12v dc-dc inverter without a lot of loss? i know mppt is better but cost is a factor here. 60% of 350 would be 3 1/2 times what I have been living with and more than adequate for now. I could always go to an mppt when the pwm goes down.

My question is how to size the controller(s) with the batteries and panels I already have in a cost effective manner.

thank you

[Mike90250]

.... This would increase my capacity by 7 fold. The 2 batteries are 125 ah 12v agm. the panels and the batteries are a given for cost reasons.
I need a controller which will work for the panels and batteries either singly or in paralell @ 12v. each panel is Voc 44v, Vmp 35.2v Imp 4.95A, Isc 5.2A
........
My question is how to size the controller(s) with the batteries and panels I already have in a cost effective manner.

You don't / can't It's like asking
" I have a VW Beatle, and tow a 300 pound rental cement mixer on weekends. Now my friend gave me a 5th wheel RV, How can I use the same tow hitch ?"

What is your existing charge controller ? Can it handle 50VDC from the new panels ? Can it handle 15 amps from panels?

The cheapest way is to get a 15A PWM 12v controller that can take up to 50V and use it with the 2 new panels (in parallel) to charge your existing 12V battery,
You will be delivering about 10amps to your battery, about half of your panel wattage because of the voltage mismatch.
Many cheap PWM controllers will smoke under these odd conditions.
here is a robust controller 60VDC input, 12/24V selectable 20A charge, 10A load $130
http://www.midnitesolar.com/productP...tOrder=1&act=p

Or go all out, 24V battery bank, 10A MPPT controller (Morningstar makes a nice 15A one) & DC-DC converter for the 12V loads.
http://www.morningstarcorp.com/products/sunsaver-mppt/ $195
Heck, it will even downconvert to a 12V battery, might clip a bit of power limiting at 15A

The Steca controller mentioned way above, has a 47V limit, which a 44V panel will exceed on a cool morning, and blow the controller.
read the specs before you buy

It seems you understand the limits of 12V living, and are not needing much more than a boost in maintaining the battery while less than perfect solar conditions prevail,

[sl dl]

Mike,
Thanks for the response. I think I should have said my question is how to size the NEW controller with the batteries and panels I already have. I was basically looking for an "Amperage" answer which would work for the 24v panels I have and delivering 12v to the batteries. I didn't know whether to size it at the panel output amps or the battery charging amps. I have no plans to use the original controller as anything other than a back up with the original panel or as a distribution box since it has fused 3v, 5v, 9v, 12v and usb outlets It is not adequate to handle more than 10A and is 12v which is why I am looking for a new controller. If I can keep the system 12v I could keep it hooked to the batteries and run with the new side by side if I had a heavy load or a long stay.
My takeaway is that I am going to have to go with 15A converter for two panels and then buy an inverter to go back to 12v. I was hoping to have fewer devices or redundant devices (2 controllers) because when I get there on a fri night and it is 20 degrees inside I want light and fire and I don't want to have to screw with the first to get the second started. In my head more devices means more chance of failure.
or maybe I could run the loads off the converter rather than direct from the batteries as I mostly do now.
or I could take a chance with a good 15A pwm and hope I don;t blow it up when I am not there.

Thank you for the good answer. Feels free to comment back since any advice is welcome.
S

[littleharbor]

I suppose I could simply things by asking the following:

Considering my panel and my CC, what battery options do I have to charge a battery pack that will be taken to a cabin to power a few lights and a laptop charger?

Furthermore, how would the wiring work?

Since you are charging at home and taking the batteries to your cabin what you can do is this, Series wire your batteries for a 24 volt bank when charging with your 24 volt panel. All good. When you take your batteries to the cabin connect them in parallel for 12 volt. Connect your loads to opposite diagonal corners of your battery bank and your going to draw down the batteries evenly. Now when you take the batteries home you should have equally discharged batteries to series wire for the charging cycle. Done.
This will work as long as you keep your battery bank at two batteries. Any more than 2 batteries in parallel will give you uneven discharging issues.

[Mike90250]

Mike,
Thanks for the response. I think I should have said my question is how to size the NEW controller with the batteries and panels I already have. I was basically looking for an "Amperage" answer which would work for the 24v panels I have and delivering 12v to the batteries.......S

Morningstar makes a nice 15A one
http://www.morningstarcorp.com/products/sunsaver-mppt/ $195
Heck, it will even downconvert to a 12V battery, might clip a bit of power limiting at 15A
You just need 2 golf cart batteries, 6V, 200Ah, wired in series, the Sunsaver MPPT 15A and wire the 2 new panels in parallel

[sl dl]

Mike,
Thanks again. I already have two 2 year old 125ah Deka agm 12v and two 4 year old same mfg,, model, specs, etc., which I use as separate banks. Any reason to not use them until their life is up then go to the 6v in series?

Stupid question #1: Is there any way to charge 2 separate 12v parallel 2 battery strings with the same controller? (I know I could get 2 controllers and keep them and the panels separate) I have a lot of excess PV power here which could be used. Currently I keep 1 set topped off on my house solar set up and swap them out occasionally when I see the cabin string needs a boost. I am semi retired and am spending a lot more time at the cabin so power consumption over time is going to go up.

Stupid question #2. If I run a resistance test on all 4 and they are close can I combine them to 1 bank, 4 paralell @ 12v. How close is close? I know I will sacrifice some life on the weaker set but if the trade off is small it might be worth the the convenience of having them all available all the time. Any other downside?
If I should post this somewhere else let me know.

thanks so much for the help with the controller.. I tried for a couple of weeks to get an answer and this is the only place I received on which made sense.

[Mike90250]

The sunsaver dual PWM controller is only preset for 12V systems, but does manage 2 separate battery banks

http://www.morningstarcorp.com/products/sunsaver-duo/

Otherwise, I'd say charge them all to the same voltage and make a series - parallel 24V bank and run them all, wired properly on the Diagonal. (method 2)

http://www.smartgauge.co.uk/batt_con.html