24v system with 12v step down

[Concretekona]

Hey everyone, this is my first solar build in a 5th wheel camper for my parents.
A 24v system using 2 12v 230Ah batteries. I planned this out with expansion in mind for both the batteries and panels later down the road.
I've attached both diagrams for the plan I'm going to put in place and the current battery system that is in the trailer now.
I looked around and saw some topics related but nothing that answered my question.
The current setup has four wires coming from the harness of the trailer connecting to the positive battery cable. I'm thinking these wires are setup for 12v DC power since they are connected this way.
My question is, where do I connect these wires in the new setup? Am I able to go directly to the charge controller? Or should I get a 24v to 12v DC converter?
I haven't come across a post on the forum that discusses this but if someone can point me in the right direction I would really appreciate it.
Thank you.

 

[Zwy]

You need a 24V to 12V converter. I ran cable from the 24V system to the converter which is mounted right at the 12V fuse panel.

 

[Concretekona]

You need a 24V to 12V converter. I ran cable from the 24V system to the converter which is mounted right at the 12V fuse panel.

This is great, thank you.
So my follow up question is how do I size that converter?
And do I connect it at the busbars or from the charge controller DC output?
I've been struggling finding any straight answers for these things.

 

[chrisski]

I think I misunderstand something.

Is the trailer loads 24 volts? The usual case like mine, a 24 volt to 12 volt converter runs the trailers dozens of 12 volt loads.

Where is this DC loads from trailer? Appears to me to be the electrical distribution panel with the AC circuit breakers and DC fuses. I'm asking because there could be other loads not on this panel like leveling jacks.

For those not wanting to click on links:

 

[Concretekona]

I think I misunderstand something.

Is the trailer loads 24 volts? The usual case like mine, a 24 volt to 12 volt converter runs the trailers dozens of 12 volt loads.

Where is this DC loads from trailer? Appears to me to be the electrical distribution panel with the AC circuit breakers and DC fuses. I'm asking because there could be other loads not on this panel like leveling jacks.

For those not wanting to click on links:
View attachment 221571View attachment 221572

Thank you for converting that to an image, I was having issues with that.

I'm going on the assumption that the wires coming from the trailer are 12v DC since they are currently connected to the 12v battery setup in the trailer now.

I used these pictures to draw up the diagram of the 2 batteries with all of the positive cable attached through the 2 busbar.

 

[Zwy]

This is great, thank you.
So my follow up question is how do I size that converter?

Determine the largest load, figure it in watts.

And do I connect it at the busbars or from the charge controller DC output?

Battery busbar system.

I've been struggling finding any straight answers for these things.

 

[chrisski]

I'm pretty sure this is the 12 volt side of the DC wiring.

I think you want to put a 24 Volt--> 12 Volt DC to DC to converter to provide 12 volt to the house. On the spare lugs on the busbars, hook the 24 volt side there, and then hook the 12 volt side to the that switch in the picture. With the 12 volt Negative, not sure if it goes to the switch or frame.

What are the two red wires on the switch hose two wires you have pictured, I don't know if those are two positives or a negative and positive. The only time I ever traced that switch in a motorhome, there was a black negative and a red positive and when the switched off, the cabin battery was shut off. I'm not sure if yours with two red wires is built for the battery and another 12 votl source with the negative wired to ground or what.

 

[Concretekona]

I'm pretty sure this is the 12 volt side of the DC wiring.
View attachment 221626
I think you want to put a 24 Volt--> 12 Volt DC to DC to converter to provide 12 volt to the house. On the spare lugs on the busbars, hook the 24 volt side there, and then hook the 12 volt side to the that switch in the picture. With the 12 volt Negative, not sure if it goes to the switch or frame.

What are the two red wires on the switch hose two wires you have pictured, I don't know if those are two positives or a negative and positive. The only time I ever traced that switch in a motorhome, there was a black negative and a red positive and when the switched off, the cabin battery was shut off. I'm not sure if yours with two red wires is built for the battery and another 12 votl source with the negative wired to ground or what.

This is a great plan, and thank you again. I will know more this weekend when I go out to the trailer.
Right now from what I can tell, the 2 red wires, the orange wire, and the black wire going straight down out of the harness are connected to the positive battery cable/busbar.
This is is why I believe those 4 to be positive.
Then there is the black wire coming from the ground off the trailer straight up, then into the harness, that I'm not sure if it is negative or ground.
I'm going to go with your suggestion for now. But I will trace the wires and see where they go.

 

[Concretekona]

Determine the largest load, figure it in watts.



Battery busbar system.

Thank you for all the info, looked into your build you linked in the comments.
Still need to read more.
Only issue I might run in to is the AC unit. I just need to figure out which wire it is attached to, but none of the step down converters seem big enough to run that load.

 

[Zwy]

Thank you for all the info, looked into your build you linked in the comments.
Still need to read more.
Only issue I might run in to is the AC unit. I just need to figure out which wire it is attached to, but none of the step down converters seem big enough to run that load.

AC unit should be 120V AC not 12V DC. The AC should run off an inverter, not a converter.

The 24V to 12V converter should be able to run lighting, 3 way fridge if equipped, furnace. Most won't run something like a power trailer jack.

 

[chrisski]

Only issue I might run in to is the AC unit. I just need to figure out which wire it is attached to, but none of the step down converters seem big enough to run that load.

If you’re running an AC, the wires should be 2/0, about as thick as your thumb.

Should go from the battery to the inverter. There’s a good chance the switch is not rated for that load.

Speaking of that switch, if it has four hot wires coming out not sure what the purpose is.

 

[Concretekona]

If you’re running an AC, the wires should be 2/0, about as thick as your thumb.

Should go from the battery to the inverter. There’s a good chance the switch is not rated for that load.

Speaking of that switch, if it has four hot wires coming out not sure what the purpose is.

I'm not completely sure myself on the switch and the other wires.
Going out to the trailer this weekend, I'll update this post on what I find.

 

[robbob2112]

I'll be shocked if the AC is running off straight DC at 12v. It is most likely off th AC inverter.

It is always best to just buy the voltage of batteries you need in the end rather than stacking to build it. But it seems you already have them so you will need to run a 12-24 active equalizer to keep the battery SOC in sync between them. This takes energy from one battery and feeds it into the other until they are balanced.

The passive balancers can either do that or just burn off the excess energy in the form of heat.... either way it isn't ideal so stil with an active balancer.

 

[Concretekona]

I'll be shocked if the AC is running off straight DC at 12v. It is most likely off th AC inverter.

It is always best to just buy the voltage of batteries you need in the end rather than stacking to build it. But it seems you already have them so you will need to run a 12-24 active equalizer to keep the battery SOC in sync between them. This takes energy from one battery and feeds it into the other until they are balanced.

The passive balancers can either do that or just burn off the excess energy in the form of heat.... either way it isn't ideal so stil with an active balancer.

I've seen a bit of content on the equalizer.
So far what I'm doing for the balance of the batteries is following in the manual to charge to the same voltage (13.33v suggested) then connect them in parallel for 24 hours.
I've also been in contact with the battery manufacture to confirm the process.

Is the equalizer in order to keep the batteries balanced over time?

 

[robbob2112]

Correct - with the active equalizer/balancer they will stay at the same voltage without doing anything special. They need to be in balance when they start.

 

[MattMan119]

Your attempting to figure out how big the 24v dc to 12v dc step down needs to be .. I know I had the same problem.. there is no real clear answer as it depends on the 12v pulls on your battery.. I am having a very close problem with my fifth wheel Solar upgrade to 48v battery bank.. I simply do not want to keep the old 12v battery bank.. now I have seen many setups with 30a step downs, or even 2 30a step downs,, I know that on mine there is a converter that will power everything from short power 120v ac —> 12v DC …(even with batteries disconnected) in most cases this converter is found attached to or behind the fuse box … so I suggest that you check the size of your and use that as a basis for your dc to dc sizing.. I needed to 40a converters, 80a total.. but this is very dependent on the RV ….

Home this helps —
MattMan

PS .. you have to disconnect this converter anyhow to prevent a loop of power loss.. 24v dc to 12v dc to 120v ac to 12v dc to 120v ac and so on …

 

[Concretekona]

Your attempting to figure out how big the 24v dc to 12v dc step down needs to be .. I know I had the same problem.. there is no real clear answer as it depends on the 12v pulls on your battery.. I am having a very close problem with my fifth wheel Solar upgrade to 48v battery bank.. I simply do not want to keep the old 12v battery bank.. now I have seen many setups with 30a step downs, or even 2 30a step downs,, I know that on mine there is a converter that will power everything from short power 120v ac —> 12v DC …(even with batteries disconnected) in most cases this converter is found attached to or behind the fuse box … so I suggest that you check the size of your and use that as a basis for your dc to dc sizing.. I needed to 40a converters, 80a total.. but this is very dependent on the RV ….

Home this helps —
MattMan

PS .. you have to disconnect this converter anyhow to prevent a loop of power loss.. 24v dc to 12v dc to 120v ac to 12v dc to 120v ac and so on …

I appreciate it
I'll be going out to the trailer this weekend to confirm wiring and the converter/inverter that is already in the trailer.
I've just learned a lot more about things since the last time I looked at the wiring.
I've ordered the Victron 80a step down converter. I figure that should be more than enough and I've seen good reviews on it.
In retro spec I would really have liked to go about things a more simple and efficient way but budget and time are limited.

 

[robbob2112]

That size converter will be overkill, which is not a bad thing.

The only way to know the size you need for sure would be with a clamp meter that reads DC amps. Then turn everything in the trailer on and see what comes up. Then double it for things you missed or that were cycled off at the time of reading.

Most people are good with a simple 30amp DC buck converter to step down the voltage. Lights and a fridge don't take much generally. Another good gustimate is to look at the wire size leaving your fuse panel and figure the amps for that and realize it is probably oversized.

 

[Concretekona]

That size converter will be overkill, which is not a bad thing.

The only way to know the size you need for sure would be with a clamp meter that reads DC amps. Then turn everything in the trailer on and see what comes up. Then double it for things you missed or that were cycled off at the time of reading.

Most people are good with a simple 30amp DC buck converter to step down the voltage. Lights and a fridge don't take much generally. Another good gustimate is to look at the wire size leaving your fuse panel and figure the amps for that and realize it is probably oversized.

I will probably return the victron one.
Ordered a second 30a converter, a lot cheaper.

 

[MattMan119]

I will probably return the victron one.
Ordered a second 30a converter, a lot cheaper.

Just make sure you check them slide out and jacks sizes first… these fifth wheel devices can take a lot, and without the battery to backup or support they may moan a little … but otherwise not a bad plan .. and you can alway add more then one if you find at some point that 30a is not enough

 

[Concretekona]

Just make sure you check them slide out and jacks sizes first… these fifth wheel devices can take a lot, and without the battery to backup or support they may moan a little … but otherwise not a bad plan .. and you can alway add more then one if you find at some point that 30a is not enough

We should be good for the time being on that aspect.
The trailer is parked and has been leveled, my parents are currently living in it. I don't think it needs to be moved for a couple of years.
Do the jacks needs adjusting as time goes on?

 

[robbob2112]

We should be good for the time being on that aspect.
The trailer is parked and has been leveled, my parents are currently living in it. I don't think it needs to be moved for a couple of years.
Do the jacks needs adjusting as time goes on?

Only if the ground settles under them. And most are simple sissor screw jacks with a motor attached. And the trailer should have come with an emergency use handle to extend/retract them by hand in case the batteries are flat

 

[Zwy]

I will probably return the victron one.
Ordered a second 30a converter, a lot cheaper.

I run a 720W converter in my truck camper. It is a cheap one but oversized by twice I thought it would need.

I tend to derate some of the cheap China stuff. It has worked flawless and I never even turn it off when not using the camper. The idle draw is basically nothing.