diy solar

diy solar

Can't find high current 24V to 12V converter. Or is 40A good enough?

SerTiminS

New Member
Joined
Jan 1, 2020
Messages
23
Hello All,
New to the forum and little experience in off grid design.
I'll be on a volunteer trip to Africa. It's an off grid women school in Niger. All their fluorescent lights and ceiling fans are 12V. They have a total of 120 lights & 150 fans. We will put 8 125Ah AGM batteries in series / parallel to 24V. Use 2 X 60A mppt controllers and Victron batteryprotect 100A for low batt cut off. Now we need to convert the 24V to 12V but I can only find <40A online. Or maybe I didn't look hard enough. Price is another issue. Or can I use 2 40A in parallel?

Thank you~
 
You can usually use power supplies in parallel, check the manufacturer/seller to confirm, but that's mighty high current. Is it practical to split the load into 2 circuits?
 
I normally would advocate going as high a voltage as practical, but a school in Niger doesnt sound like they can't just hop on down to the local solar mart and get a replacment part.

They need a extremely high reliability system so I'd ditch the converter idea altogether as it's a critical failure point - if it goes down all that 24V power is stuck unusable in the batteries.

I this case reliability and simplicity take precedence over efficiency. They'll need to be able to get the system to work with several fallback options and redundancies. What you'd spend on a high reliability converter and spares would be better spent on a extra MPPT controller.
 
Thank you everyone for your comments. I agree with Maast and has ditched the DC to DC converter idea and purchased 2 60A MPPT controllers on Amazon. They have 5 MPPT on site and will add 2 more for 7 MPPT on 50 150W solar panels. 8 batteries will not be in series. All in parallel 12 volts with 3 60A MPPT supplying power. No inverter, low 12V cutoff to protect the precious batteries. To give you the details in the school setup, there are 12 2V 1Kah gel batteries which will make 24V, going to 220V inverter for their 2 refrigerator, 2 freezer, TV / labtops and others... I'm currently drawing up the layout of the wiring which will be redone. With the 2 controllers added, I'll have to redraw again. I will upload when I'm done, I would appreciate any input of the layout.

Thank you!
 
I have attached my layout design for the Niger school project. Please let me know if there is any adjustments I should make. I learned all this from google and YouTube so It is hard for me to spot any errors. Thank you in advance.
 

Attachments

  • layout 010220.jpg
    layout 010220.jpg
    127.6 KB · Views: 12
Your wiring layout looks good, but that was the easy part :) Now you need to start thinking of the actual hardware to do all this, your actual connections, combiners, fusing/breakers and disconnects.

I'd recommend you take a very hard look at your mppt input limits, not just voltage but the allowable amperage and keep it under 80% of the units capabilities for longevity. In general you dont want to be routinely driving things over the 80% mark for voltage or amperage.

Then start at one panel group and on very large sheets of paper draw out in gnats-ass detail EXACTLY how the power is produced, combined, overcurrent protection for the high voltage wires, their disconnects, the input into the MPPT, the output, the output disconnect, the battery, the battery output to inverter, it's disconnect and overcurrent protection, input to the inverter. Draw out every wire, every connector, every fuse or breaker, the boxes things go in, the things those boxes need (grommets, strain relief, etc). How about cooling for the equipment? How to keep out bugs, dust, water and curious small fingers. What tools will you need?

Then duplicate that for however as many panel groups you have. This'll tell you how much and what hardware you'll need to take with you. Then add 25%. Remember, no solar mart down the road, and the stuff you can find locally may or may not work with the stuff you bring and you'll need to know that too.

There's enough power in a battery bank to cook a rhino, how are you going to keep from killing curious morons because there always at least 1 in the area.

I think that once you start detail-level planning you'll change your mind a dozen times or more about what and how you want to accomplish.

I see you've got 14A coming out of your top left set of panels, is that a series/parallel arrangement? If not those are some big horking panels then. I think the biggest Ive seen is 12A, if they are going to be parallel how are you going to parallel them? In a combiner box or a T adapter, will there be fuses at the panels in MC4 fuseholders?

Lots of stuff to think about and hours and hours on google/amazon/etc on figuring out how to actually pull off your plan.

Also, unless you're dropping money on top quality Morningstar, Midnite, Outback, Magnum gear expect a 10% failure rate on complex electronics and have a plan on how to deal with it. Electronics like this either fail within a couple weeks, or last for a long time as long as it's not being abused.

You will need to leave them with a spares package and educate somebody there how to maintain it.

You can do this, its just a matter of breaking the problem down to very small chunks and stepping/solving your way through it. Then DOCUMENT your plan as you go through it. Trust me you're just not going to be able to remember the sheer mass of details you end up with, and with a well documented plan you can hand it off to others and they'll understand what you're doing and how without you having to be in 3 dozen places at once during the install.

Good luck!
 
[/QUOTE]
Your wiring layout looks good, but that was the easy part :) Now you need to start thinking of the actual hardware to do all this, your actual connections, combiners, fusing/breakers and disconnects.

I'd recommend you take a very hard look at your mppt input limits, not just voltage but the allowable amperage and keep it under 80% of the units capabilities for longevity. In general you dont want to be routinely driving things over the 80% mark for voltage or amperage.

Then start at one panel group and on very large sheets of paper draw out in gnats-ass detail EXACTLY how the power is produced, combined, overcurrent protection for the high voltage wires, their disconnects, the input into the MPPT, the output, the output disconnect, the battery, the battery output to inverter, it's disconnect and overcurrent protection, input to the inverter. Draw out every wire, every connector, every fuse or breaker, the boxes things go in, the things those boxes need (grommets, strain relief, etc). How about cooling for the equipment? How to keep out bugs, dust, water and curious small fingers. What tools will you need?

Then duplicate that for however as many panel groups you have. This'll tell you how much and what hardware you'll need to take with you. Then add 25%. Remember, no solar mart down the road, and the stuff you can find locally may or may not work with the stuff you bring and you'll need to know that too.

There's enough power in a battery bank to cook a rhino, how are you going to keep from killing curious morons because there always at least 1 in the area.

I think that once you start detail-level planning you'll change your mind a dozen times or more about what and how you want to accomplish.

I see you've got 14A coming out of your top left set of panels, is that a series/parallel arrangement? If not those are some big horking panels then. I think the biggest Ive seen is 12A, if they are going to be parallel how are you going to parallel them? In a combiner box or a T adapter, will there be fuses at the panels in MC4 fuseholders?

Lots of stuff to think about and hours and hours on google/amazon/etc on figuring out how to actually pull off your plan.

Also, unless you're dropping money on top quality Morningstar, Midnite, Outback, Magnum gear expect a 10% failure rate on complex electronics and have a plan on how to deal with it. Electronics like this either fail within a couple weeks, or last for a long time as long as it's not being abused.

You will need to leave them with a spares package and educate somebody there how to maintain it.

You can do this, its just a matter of breaking the problem down to very small chunks and stepping/solving your way through it. Then DOCUMENT your plan as you go through it. Trust me you're just not going to be able to remember the sheer mass of details you end up with, and with a well documented plan you can hand it off to others and they'll understand what you're doing and how without you having to be in 3 dozen places at once during the install.

Good luck!
Hello Maast,
Thank you very much for your detailed suggestions.
You mentioned disconnects, fuse and breakers. Well, Let me know how critical it is? My though is, there is protection in MPPT, protection in inverter, solar panels have diodes so current can't flow in. I would invest in these but due to tight budget, I didn't think there were must items. It is convenient for maintenance but there is no one onsite who will work on the system. due to low budget, I even made some copper bus bar to parallel 8 12V batteries as I can't afford $$ on battery cables. I used copper plumbing pipes / Idea from internet. See attached pics. haha

how are you going to keep from killing curious morons because there always at least 1 in the area. The system will be inside a locked cage.

14A is due to 8 panels in series / parallel. One panel is ~7A. No MC4. Low budget. copper butt connectors and heat shrinks.

I am taking a risk on controller I picked from Amazon. We will see how reliable they are. They seem sturdy physically, in a metal box and heavy.

"You will need to leave them with a spares package and educate somebody there how to maintain it. " I'll have to write simple procedures and change fuse / reset controller. I also text with main guy at school but I really hope they don't mess with it as it can be dangerous.

Anyways,
Thank you so much!!
 

Attachments

  • battery bus.jpg
    battery bus.jpg
    174.2 KB · Views: 9
  • mppt controller.jpg
    mppt controller.jpg
    104.3 KB · Views: 10
Back
Top