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Pull 12 volts from a 24 volt battery bank?

Nevadablue

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I’m building a trailer mounted backup power system. The trailer is a 2 horse thing, like an enclosed trailer really. I will be starting a thread on the build, but I have a (hopefully simple) question to ask.

I want to use 12 volt lights and fans in the trailer. Can I safely pull a limited amount of 12 volt DC from the 24 volt battery bank?
If so, how should I do it?
 
you can get Buck converter that converts 24V to 12V.
Do you know what the maximum power your 12V loads will be? You need to know that to get properly rated converter.
 
More than one way to do it. For lights, the simplest might be to just wire two 12V lights in series to make it a 24V load. If one bulb burns out though you have to take the time to figure out which one.

Two 12V fans could be wired the same way.

Alternatively get a 24V to 12V converter like this one.
1663559370928.png
 
thanks guys. I didn’t think of the buck converter.
No, I don’t know the load yet, but it will be small. I may just use an independent ‘shed light’ type of thing with a small solar panel. Still planning this.B2C5C94E-70DE-4D58-B64C-07B6A80C303E.jpeg

this is the trailer. I’ll start a thread on the project soon. Most of the parts are here, now to do the work.
 
Far better to use the converter instead of pulling power out of the middle of the battery and making the battery unbalanced.

My knowledge of electricity is sadly very limited.

Could you pull 12v off each of the batteries by using twice as much cabling (possibly a bus bar) and doing a 12v parallel battery off of them while still leaving them in 24v series?

I'm not asking if this is ideal, merely whether it can be done without causing a fire or damage to the batteries and other components.
 
No.
Batteries will become unbalanced and nothing will work right.
You are not the first to ask the question. But there's a reason nobody does it.
Better to protect the $1,000 battery by just buying a $25 converter
 
My knowledge of electricity is sadly very limited.

Could you pull 12v off each of the batteries by using twice as much cabling (possibly a bus bar) and doing a 12v parallel battery off of them while still leaving them in 24v series?

I'm not asking if this is ideal, merely whether it can be done without causing a fire or damage to the batteries and other components.

No. Result is two short circuited batteries:

Black indicates your two 12V connections:

1663593751093.png
However, the cable putting the batteries in series makes joins those 12V circuit and renders both batteries as shorted. The red lines show the effective connections, and as you can see, there is a direct unbroken path between each battery's positive and negative terminals.
 
this is the trailer. I’ll start a thread on the project soon. Most of the parts are here, now to do the work.
If panels are going to be put on the roof, most likely they will be laying flat? I commonly use a "fudgefactor" to compensate for lower then expected production. For common panels pointed directly at the sun, I normally use 85%. If laying flat, on a roof like this, I'd recommend as low as 60%.

What portion of time do you expect the trailer electrics to be on with the trailer just stopped and sitting there? Since this is an application on wheels, it makes sense to me to stick with 12V so the battery can be engine-charged.

Let's do the math for you. Let's assume you have two 100Ah 12V batteries in parallel. That would give you 200Ah at 12V. Assume you want to charge the batteries at 1/8th of C for optimal life.

[(200Ah/8) X 12.5Vcharging]/60% = (25A X 12.5V)/60% = 520W. Call that two 260W grid-tie panels

wired at 24V instead, the math would be....

[(100Ah/8) X 25Vcharging]/60% = (12.5A X 25V)/60% = 520W. See what the math works out the same.

Do this for whatever system voltage you decide on to know the best panels to charge your batteries at.
 
No. Result is two short circuited batteries:

Black indicates your two 12V connections:

View attachment 112506
However, the cable putting the batteries in series makes joins those 12V circuit and renders both batteries as shorted. The red lines show the effective connections, and as you can see, there is a direct unbroken path between each battery's positive and negative terminals.

At the risk of sounding even more stupid, but to make sure I'm understanding this correctly, you could pull 12v off of each one individually (unbalancing them because of uneven load), but they can't be put in 12v parallel because it ends up short circuiting across the cables connecting them in series?
 
At the risk of sounding even more stupid, but to make sure I'm understanding this correctly, you could pull 12v off of each one individually (unbalancing them because of uneven load), but they can't be put in 12v parallel because it ends up short circuiting across the cables connecting them in series?
Right. Cannot be in series and parallel at the same time. Use the converter. Life will be better.
 
So, simplest thing is to size the converter for the load since in all likelihood it will be a small load and the cost for that converter will not be prohibitive.

However, for anyone else who comes along and might have a slightly different scenario, one thing you can do if you have larger loads or loads with some kind of 'surge' that drives the cost for the converter way up, is just put a small 12v batt in parallel so that your converter just has to be more powerful than your loads 'on average', over time. Small 12v SLAs like UPS and scooter batteries, trailer breakaway battery etc are pretty cheap and easy to package, might add the flexibility to make a smaller converter work with a higher but short duration load, in some cases. You can get a pretty decent amount of 12v sla battery for <$30 and turn it any which way and put it basically anywhere. Only thing is you'd buy a 24>13.8v converter instead of 24>12, so it could keep the sla charged. Just throwing that out there.
 
While this is a trailer mounted system, it won’t be moved much at all. The reasons to use the trailer are because I have it, and so I can move it close to the house if necessary.
The panels will be mounted on a rack that tilts from side to side. That way I can park the trailer east/west and tilt the panel rack to suit the season.
I’ll go start the build thread. Is it OK to document a build?
 
Just buy a proper 24v to 12v converter.

Large trucks and busses commonly use a 24 battery, and there are lots of things that will only run off 12v, so its not an uncommon problem that you have.
Just do what everyone else does to solve this problem.
 
OK, for this issue, I have a solution. Thanks for the ideas and suggestions.

I just opened a box of ‘stuff’ that one of my boys gave me. Inside were:
Two of these lights with solar panel built in. They will provide light outside the trailer.

24EC749C-F07A-49A4-A053-870862CCD49B.jpeg

4 of these neat little solar panels: (measure 7.1 volts in the late afternoon sun)

D1A11127-825A-4936-85F2-3B325D5CB4F1.jpeg

And 4 of these lights: (they have 3 AA cells inside, a two level switch and charge port and pull cord on/off switch)

478C3363-F2D2-476B-A498-043C13BE43A1.jpeg

I will install matching plugs and sockets on the chargers and lights.

Light issues solved, I will look for a couple of fans and run them on 24 volts. Keep it simple.
 
You can sort of have series and parallel at the same time

Maybe I missed something but what do you need the 24 volt setup for?

Megalophobia is a type of anxiety disorder in which a person experiences intense fear of large objects. A person with megalophobia experiences intense fear and anxiety when they think of or are around large objects such as large buildings, statues, animals, vehicles AND THE LARGER WIRES NEEDED TO RUN !@ 12 volts.

I read it all the time but the wires, BIG wires ! Big Big wires at 12 VoLtS !
 
Please man no big wires Ill do anything up and down dc-dc converters, what ever it takes I promise I will complicate the crap out of everything JUST DONT MAKE ME USE BIG wires !!!
 
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