Help sizing 24v-12v buck converter

OnTheFlyPDX

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I will soon be installing a 24v lifepo4 200Ah bank and MPP LV2424 in our travel trailer. To replace the 12v lead acid batteries on the tongue, I plan on wiring a 24-12v buck converter into the bus bars that the lead acid batteries are currently wired into (located beneath a coroplast panel under the trailer behind the tongue). I’m a little confused on what amp size buck converter I’ll need and hoping someone can help based on this picture of the trailer’s DC fuse panel? I will remove the trailer’s current AC-DC charger to avoid sending power back to the buck converter.

I’m also attaching the wiring diagram for the project. My initial plan was to wire the buck converter directly into the DC fuse panel, but after looking inside the coroplast panel mentioned above with the battery bus bars, I think it’s better to connect the buck converter to those bus bars. It looks like the trailer’s break away system and other mystery wires are tied into those bus bars, and I don’t want to cut those out by wiring the buck converter directly to the fuse panel. I’m adding my thought process on my recent change of plans in case I’m missing something and someone can set me straight. Also noting it in case the trailer’s break away system and those mystery wires need to be factored into the equation when choosing the correct amp size for the buck converter. Thanks in advance!
 

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rmaddy

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The RV's DC fuse panel can handle up to 160A. The current fuses in there that have wires attached add up to 100A.

But the real question is how many amps will you use at any given time from those DC loads? Itemize the DC loads wired into that fuse panel. Determine how many amps each one actually uses. Just because an item has a 15A fuse doesn't mean the item actually uses 15A. It probably uses something like 10A. Once you have the list and each item's amperage, consider which of those would ever be on at the same time resulting in the largest amperage total. Then add at least a 25% safety factor and choose a converter that can handle at least that many amps on the 12V output.
 

smoothJoey

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Most RV's come with an ac2dc converter.
If your rig has one, check the amp rating on it.
If you have an inverter in your setup take care to avoid a power loop with the RV inverter.
 

OnTheFlyPDX

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The RV's DC fuse panel can handle up to 160A. The current fuses in there that have wires attached add up to 100A.

But the real question is how many amps will you use at any given time from those DC loads? Itemize the DC loads wired into that fuse panel. Determine how many amps each one actually uses. Just because an item has a 15A fuse doesn't mean the item actually uses 15A. It probably uses something like 10A. Once you have the list and each item's amperage, consider which of those would ever be on at the same time resulting in the largest amperage total. Then add at least a 25% safety factor and choose a converter that can handle at least that many amps on the 12V output.
Thank you very much for all this great info. I had a feeling that going off the total fuse panel value of 100A was a little high. I think where this is headed is that I will start with a 40A buck converter, and run a second 40A converter in parallel if one isn’t enough. I think that should work.
 

rmaddy

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You should still do a little inventory first. Why discover you regularly need 45A only after you fried the first 40A converter.
 
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OnTheFlyPDX

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Most RV's come with an ac2dc converter.
If your rig has one, check the amp rating on it.
If you have an inverter in your setup take care to avoid a power loop with the RV inverter.
Ahh good idea checking the amperage on the built in converter/charger. Our RV is old, so no built in inverter. I will remove the ac/dc charger to avoid the battery > MPP inverter > ac/dc charger > battery feedback loop. Batts will be charged with solar and shore power where available.
 

smoothJoey

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Ahh good idea checking the amperage on the built in converter/charger. Our RV is old, so no built in inverter. I will remove the ac/dc charger to avoid the battery > MPP inverter > ac/dc charger > battery feedback loop. Batts will be charged with solar and shore power where available.
How will you charge from shore power?
 

OnTheFlyPDX

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How will you charge from shore power?
The LV 2424 hybrid has a 60A utility charger that kicks in when plugged into shore power and batts are low and solar isn’t enough/producing. I think I can use that function by rewiring the trailer’s pigtail to power the MPP, and using the MPP’s inverter to power the trailer’s AC system:

Trailer’s 30A plug (pig tail) > 30A breaker > MPP AC IN.

MPP AC OUT > trailer’s main 30A breaker (which feeds the entire AC panel)
 

chrisski

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I had to go with 70 amps because I have a electric leveling jack. If you don’’t have that makes things easier.

Some of VIctron’s DC to DC converters are actually chargers, so you can leave the 12 volt in place.
IME, I’d get a 30 amp converter but would not expect to use more than 20 amps DC except for the leveling jacks. A worst case for me is the propane heater at 9 amps, 4 amps of DC lights, and two amps to charge USB devices, plus a little more.
 

OnTheFlyPDX

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I had to go with 70 amps because I have a electric leveling jack. If you don’’t have that makes things easier.
all manual so no worries with that here.

Some of VIctron’s DC to DC converters are actually chargers, so you can leave the 12 volt in place.
IME, I’d get a 30 amp converter but would not expect to use more than 20 amps DC except for the leveling jacks. A worst case for me is the propane heater at 9 amps, 4 amps of DC lights, and two amps to charge USB devices, plus a little more.
This is great real world advice, thanks. I think I’ll stick with a 40A because the price difference is pretty negligible when compared to lower amp units. However, converters over 40A start to get pretty spendy (at least the models I’ve seen), which is why I’m leaning towards trying a 40A first and paralleling a second if needed. Thanks!
 

chrisski

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Only thing to do is to compare the idle draw of those converters. Probably not much of a difference.

My 70 amp converter I’m using is 92% efficient, but has a “sleep Mode” of less than 20 ma. That 8% loss in efficiency I’m not that worried about because the battery is getting a bit bigger. Could cost me about 15 amp hours extra a night For worst case losses.
 

HRTKD

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Having dug into my travel trailer's electrical system, I have a good amount of experience with how mine is put together. This may or may not apply to yours.

You can attach the buck converter at either location. It's all the same circuit. The decision for where to tie the buck converter into the 12v system depends on where the largest amp draw will be. A dual axle trailer's emergency brake system will draw about 12 amps (3 amps per brake) if you have electric drum brakes. That could be your biggest draw right there. But it doesn't happen very often, if ever. Mistakes do happen and the pin for the emergency brake system can get pulled by accident. Letting it draw 12 amps for very long, across some questionable wires, could lead to less than ideal results. But, like I said before, that's an unusual situation.

One reason to tie into the panel under the trailer is that it will allow you to still use the battery switch that came with the trailer. If you tie into the main distribution panel then you have bypassed that switch.

Do check for loads that don't go through the main distribution panel. I went through every one of my loads in my trailer (a toy hauler) and found two loads that do not have a fuse on the main distribution panel. Definitely a WTF? moment. These loads tie into the panel that is located under the trailer where the battery battery cable from the tongue initially ties into.
 

smoothJoey

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Air compressor and fuel station. They have their own fuses. I found the one for the air compressor. Darned if I can find the fuel station fuse. I don't use it, so I'm not too worried about it.
What is a fuel station in this context?
I was expecting the break system to be unfused.
I think I recall reading here that its standard practice because stopping a run-away trailer is so important that the battery and wires and even the trailer are considered expendable.
 

HRTKD

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What is a fuel station in this context?
I was expecting the break system to be unfused.
I think I recall reading here that its standard practice because stopping a run-away trailer is so important that the battery and wires and even the trailer are considered expendable.

I have 30 gallon and 18 gallon gasoline tanks mounted under the trailer. There is a fuel station that can pump out of either tank and into a vehicle. The typical use is for pumping fuel into an ATV, motorcycle, etc. The electric pump for the fuel station is about $300. The rest of it isn't cheap either. Rebuilding the pump is on my list of things to do.

The on-board generator draws from only the 30 gallon tank and has its own fuel pump.

Here's a picture of a typical fuel station.

1630342439378.png
 
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