12v or 48v system in RV?

[wopachop]

The beauty of parallel systems (48v and 12v), is that the 12v system isn't pulling/supporting any huge loads, so it's componentry is small and dedicated to all the 12v loads that others have listed in this thread.

Does a toyhauler not lend itself well to a parallel system? Because the 12v loads can be quite large? The genny pulls over 100a and ive read my emergency trailer brakes pull 6a each and its a triple axle. So for that example you need to maintain a healthy 12v battery instead of a buffer battery. If you only need 500-1500w of inverter would it be best to have a single 12v battery bank?

Also in this example if i can ditch the 12v lead acid batteries on the tongue i could later put a mini split there. With lithium 12v batteries inside the trailer behind my distribution panel. Its under the fridge so i could put a fan into the fridge duct to expel heat during the summer. (again hope this info can help others and the OP too...down the road will make my own build thread with questions)

 

[Vigo]

Because the 12v loads can be quite large? The genny pulls over 100a and ive read my emergency trailer brakes pull 6a each and its a triple axle. So for that example you need to maintain a healthy 12v battery instead of a buffer battery.

So, if the 100a were a longer duration i would say you are right, but the starter circuit for the generator should probably never run for more than 5 seconds and the emergency brakes shouldn't be needed for much longer than it takes for the trailer to either come to a stop or crash into something.

For both uses, something as small and cheap as a group U1 'riding lawnmower battery' would be able to meet those short-duration needs and Walmart sells them for <$40. If you look at the small SLA batteries that are usually mounted to trailers for their 'breakaway brakes' system they are WAY smaller than a U1 is! So even then you are really only needing a battery 'as big as a u1' for purposes of cranking the generator, while it could be considered 'overkill' for the emergency braking.

Starting from $29...

 

[wopachop]

Very cool!! I was picturing that in my head after posting. I have some old 3s Lipos i could repurpose but i dont think it would be legal for trailer brakes. If i start the genny using other techniques, would a 24v to 12v converter be legal to use as trailer brake power source?

 

[Vigo]

Probably not a good idea (imo) because of the chance of the converter failing or going into some kind of fault mode and going from 100 to 0 on brakes. At least with a small battery in between the converter and the brakes, you're going to get brakes that gradually fade to nothing but should last the 5-10 seconds you need for emergency braking. I cant imagine a trailer that has left the tow vehicle at highway speeds aka worst case scenario, managing to stay on its wheels for more than 10 seconds without hitting something, even going past 5 seconds would surprise me a little. Considering the size of batteries we're talking about here are $20-40 and a small DC DC converter to keep it maintained is also no more than $20 for the size you'd need, i just don't see a good reason to eliminate the battery from the circuit. It doesn't take up much space, money, or add any real complexity to the circuit and buys you a decent safety factor in return. My .02.

 

[HarryN]

Please clarify what you mean by this. Example:

View attachment 169987

I see 1% gains going from 12 to 24 and another 1% going from 24V to 48V. A 1% gain from 93 to 94% is a 1.07% relative improvement, and a 1% gain from 94 to 95% is a 1.06% relative improvement. Smaller, yes, but it seems contrary to your claim. Sure, this is Victron specific, but I would expect similar ratios from other manufacturers.

If you're referring to the DC side, there's no difference. Going from 12V to 24V for a given power/cabling means a doubling of the voltage and a halving of the current, resulting in 1/4th the losses from wiring. But this is also exactly true for a jump from 24V to 48V.

Thanks for pointing out this chart.

They have definitely narrowed the efficiency difference between 12, 24 and 48 volt over time.

Historically (say 5 years ago) when I looked at the plots of efficiency vs output power, there were some pretty large gaps between 12 vs 24 especially in the 500 - 1500 watt range, but much smaller changes going to 48 volt.

____________

I tend to use US made inverters but also looked at and have done some testing of the victrons.

On this forum 48 volt systems are pretty common and I build them for people - but I am still not comfortable recommending it for a first time DIYer building up a system from scratch.

Anyone who can build a 12 volt system can use mostly the same parts to build a 24 volt system safely. This isn't really true for 48 volt.

With a little bit of care, nearly everything can be found in 24 volt so this makes the number of voltage conversions a very low number.

So I will stick to my suggestion of 24 volt as a base voltage to the OP.

 

[dwwells2]

I have a 48v/3000w All-In-One Solar Charge Controller/Inverter. There is 1400 of solar on the roof( because of the 48v battery setup I can go up to 5kw of solar on the roof down the road)

Sucking 2500 watts off the batteries to run the microwave and other 120 systems in my coach is no big deal.

Down the road I will be replacing the A/C and furnace with a Mini-Split.

I built my own 48v battery

View attachment 167411

What size breaker/fuse did you use between the victron 48-12 DC-DC converter and the battery box?

 

[chrisski]

What size breaker/fuse did you use between the victron 48-12 DC-DC converter and the battery box?

Looks like he used a 30 amp on the 12 volt side converter, so I’d use 10 or 15 amp 48 volt rated fuse or breaker.

48 volt fuses are limiting and get pricey. No longer using ATC fuses.

 

[mikes68505]

I have a 48v/3000w All-In-One Solar Charge Controller/Inverter. There is 1400 of solar on the roof( because of the 48v battery setup I can go up to 5kw of solar on the roof down the road)

Sucking 2500 watts off the batteries to run the microwave and other 120 systems in my coach is no big deal.

Down the road I will be replacing the A/C and furnace with a Mini-Split.

I built my own 48v battery

View attachment 167411

What type of breaker fuse did you use between 48V battery bank/box and 48V-12V converter? TIA.
-m

 

[mikes68505]

Question if I may, related to this 48V and 12V combination. BTW, I'm using Victron Lynx Distributor in between 48V battery bank (2 EG4 Lifepower) and 48V inverters (2 EG4 3K).

Question > What fuse is suggested between the battery bank (or from Lynx) to a 48V-12V 20A (Victron) converter? This 20A will be to provide 12V power to the existing WFDC distribution center for internal & indirect 12V needs (fridge, lights, water heater, water pump, furnace, … etc.)

I am keeping one 12V AGM battery (and spare) connected for the three external & direct 12V items that don't make sense to use 48V system and conversions (such as:
1- starting the 30A Onan EVAP 4K generator [needing 450 CCA],
2- the electronic RV jack [12V, 30A]
3- the trailer emergency brake [12V, ~14A]

Thanks in advance for your feedback. Other questions on output but will save that for another time/posting.
-m

 

[chrisski]

Question > What fuse is suggested between the battery bank (or from Lynx) to a 48V-12V 20A (Victron) converter?

I reccomend going above 1.25X the max current. 10 amps on the 48 volt side, and 25 amps on the 12 volt side. That’s the easy part. The harder part is getting a fuse rated for the voltage. Lower amperage 48 volt fuses are hard to come by.

The 12 volt side can have ATC fuses. Since fusing choice is limited at 48 volts, I can’t recommend a 10 amp fuse, but the lowest amp fuse I see for a 48 volt system is MRBF.

I have a 24 volt 35 amp to 12 volt 70 amp Converter. On the 12 volt side I used an 80 amp ANL fuse, and on the 24 volt side, I used a 50 amp ANL fuse. on the DC side. ANLs are rated for 24 volt and less applications.

 

[mikes68505]

I reccomend going above 1.25X the max current. 10 amps on the 48 volt side, and 25 amps on the 12 volt side. That’s the easy part. The harder part is getting a fuse rated for the voltage. Lower amperage 48 volt fuses are hard to come by.

The 12 volt side can have ATC fuses. Since fusing choice is limited at 48 volts, I can’t recommend a 10 amp fuse, but the lowest amp fuse I see for a 48 volt system is MRBF.

I have a 24 volt 35 amp to 12 volt 70 amp Converter. On the 12 volt side I used an 80 amp ANL fuse, and on the 24 volt side, I used a 50 amp ANL fuse. on the DC side. ANLs are rated for 24 volt and less applications.

Thank you. Now thinking that maybe this 48-12 converter should be directly connected to pos and neg bus bars. Then perhaps inline 30A fuse on pos line to WFDC distribution center. Thoughts?

 

[Rocketman]

Remember the fuse is to protect the wire - in case it shorts out. So if you have 12awg wire going to the converter- you should have that fused and the bus bar at 20amps. Because if it shorts out the entire battery bank amps can flow through it.

The 12v wires are limited because the max power that can flow through them is only what the converter can convert.

The way I think about it- anytime your main trunk line uses a smaller wire - that smaller wire needs fused.

 

[chrisski]

Thank you. Now thinking that maybe this 48-12 converter should be directly connected to pos and neg bus bars. Then perhaps inline 30A fuse on pos line to WFDC distribution center. Thoughts?

I have my converter connected to the main positive and negative line. The 30 amp fuse to the main WFDC distribution could work. I created a 24 volt bus in my trailer that I used a Blue Sea ATC fuse center for, and I fused the line into that with an ANL fuse, that is good to around 33 volts.

The only thing about thing about fusing the line to the WFDC, is sometimes trailers send power to other places. I have two things that have fuses that are not connected to the distribution center. The First is the RV leveling system, and the second is box that controls the lights and awning. That DC power to those two other devices is tapped elsewhere I have not found, I suspect under the floor or behind a wall.

I put my 12 volt fuse right next to the converter, that way all those lines are covered.

 

[RvNaut]

What type of breaker fuse did you use between 48V battery bank/box and 48V-12V converter? TIA.
-m

A dual pole... similar to the PV breaker, so it shut off both legs...