12v or 48v system in RV?

[MTC1014]

New to the forum and already learned a ton just by browsing. I wanted to get the expert's opinions on which route to take while adding lithium batteries to my travel trailer.

I'm considering going 48v because I've read that it would be more efficient than 12v, however my trailer already does have some 12v appliances (fridge, lights, fans, tongue jack).

would it be more efficient to go with a 48v system and just add a step down converter for the 12v loads, or just do a 12v system and eliminate the need for the step down converter? My AC loads would mainly be the TVs, microwave, and outlets for laptops/chargers/internet router, along with sometimes running the air con.

I'm still very new to all this and I may be overthinking it, but I'm wondering if the phantom draw on the step down converter would basically wash out the efficiency of going with a 48v system for the AC loads.

Thanks in advance for any and all input.

 

[DIYrich]

How many watts do you need? If more than 2,500, then 48v. Under that, 12v should simplify things.

 

[SamDeleted]

I'd stick with 12v personally, keep it simple

Doesn't look like you're going to need a big inverter, you only really need to go 48v if youre moving a LOT of power, either big inverter and/or big solar array

 

[sunshine_eggo]

What about 24V?

Inverter efficiencies are about the same:

From https://www.victronenergy.com/uploa...MultiPlus-inverter-charger--800VA-5kVA-EN.pdf

12V/3000W = 93%
24V/3000W = 94%
48V/3000W = 95%


Battery size is more about total power output and expandability.

Want to pull more than 2000W from 12V? your cables are going to get THICK due to your DC currents being high... 2000/12/.85 = 196A. 3000W is a practical upper limit for 12V.

More than 4000W from 24V? Same.

So, what is your peak continuous power need?

 

[Tomthumb62]

along with sometimes running the air con.

Can you explain this more? Some people think a “little bit” of air conditioning is running it full blast for 5-8 hours. Also it depends upon the energy needs of your ac unit and whether or not it will work with a soft start device.

How many ac units does your rv have? If more than one, you’re going to need a huge solar system as running even one can be challenging unless you size everything right and keep your wire runs short. Ie, place your battery and inverter as close to the ac unit as you can get away with.

 

[Rocketman]

In my MotorHome I went with a diy 544ah 12v battery - with Victron gear.

I have a Multiplus 12/3000 inverter- it can run one large item (microwave, hair dryer, A/C) and all the small items at once - computers, etc. or two medium items at once - toaster,etc.

Keeping everything at 12v is nice.

One BIG thing to watch for is the amount of standby watts the inverter uses. Victron sips watts, some use them like there is an infinite battery.

Yes a 12v inverter will have more losses when running big things - but you don’t run big things that often vs a converter that runs 24/7.

FYI- if thinking about air conditioning… you must have an easy start. With full-ish battery - I can run one A/C for about an hour- then I have to either start the generator or shut the A/C down. (Almost 2 hours if the sun is really bright on the panels).

Good Luck

 

[RvNaut]

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

 

[sunshine_eggo]

Concerning this:

One BIG thing to watch for is the amount of standby watts the inverter uses. Victron sips watts, some use them like there is an infinite battery.

This is one of "those" units:

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)

These typically consume 40-50W continuously even if you don't use any loads. Using the low end, that's 960Wh/day of wasted energy. Fortunately, that's only about 7% of the 280Ah battery; however, some folks try to just run one server rack battery @ 100ah, and that's 19% of the battery capacity - wasted and unused.

Additionally the first 200-300W of PV are unusable for loads as they're just feeding the inverter's idle consumption.

Hence, there's a hidden cost to the cheap AiO - you have to buy more battery and more PV to get the same usable energy as the higher end models.

As long as you're including the idle consumption of your inverters are included in your energy audit (link #1 in my signature), then you should be fine. If you neglect it, your system will be undersized.

 

[justinm001]

@DIYrich is correct it all depends on how much load you plan on using at a single time. I think its best to size around 100-150amps so 12v would be 1200-1800w otherwise upgrade

I have both 12V and 48V in my coach and 48V is so much easier, safer and just better in every way. One thing I didn't initially realize is 4/0 gauge is only rated at 230a which is under 3000w total. Wiring makes a huge difference in performance and cost but also when you go larger voltage you get much larger range of acceptable voltages. With 12V you cause all kinds of issues going above 14V or below 10V but 48V you can easily go 55V+ without issues.

One thing to mention is there isn't much 12v to 48v stuff yet but it seems 48V is growing a ton and I'm expecting more products to come out to support such.

 

[Tomthumb62]

One thing I didn't initially realize is 4/0 gauge is only rated at 230a which is under 3000w total.

Not always true. Some 4/0 is rated for much higher. It depends upon the quality/rating of the insulation. An example, this 4/0 is rated for up to 445A:

https://www.batterycablesusa.com/assets/images/battery_cable_compare.pdf

 

[wopachop]

You might find it all comes down to the air conditioner. Really have to decide if its an absolute must to run AC from the batteries.

 

[justinm001]

Not always true. Some 4/0 is rated for much higher. It depends upon the quality/rating of the insulation. An example, this 4/0 is rated for up to 445A:

https://www.batterycablesusa.com/assets/images/battery_cable_compare.pdf

Correct but the rating being higher is based on temps, that one is actually rated at 378a at 105c. The 445a is open air ambient temps like outside on a welder.

Running that cable at 105c vs 75c (230a) introduces a ton more issues I'd assume. I'm correct in assuming that a cable running at 75c vs 105c will be physically much hotter and those cables being rated that high means their jacketing can withstand the heat without melting. Doesn't mean they won't put off 105c temps and melt other components touching it or nearby

 

[sunshine_eggo]

Correct but the rating being higher is based on temps, that one is actually rated at 378a at 105c. The 445a is open air ambient temps like outside on a welder.

Running that cable at 105c vs 75c (230a) introduces a ton more issues I'd assume.

Only if you touch the cables or try to boil water on them or lay them on things that can't tolerate boiling water.

I'm correct in assuming that a cable running at 75c vs 105c will be physically much hotter and those cables being rated that high means their jacketing can withstand the heat without melting. Doesn't mean they won't put off 105c temps and melt other components touching it or nearby

NEC standard ratings also include 90°C @ 260A for 4/0. Note that these are for wire enclosed in a raceway. Where do you draw the line? 75 up from 60 is okay, but 90 up from 75 isn't? 105 up from 90 isn't?

These are simply design constraints.

 

[justinm001]

Only if you touch the cables or try to boil water on them or lay them on things that can't tolerate boiling water.



NEC standard ratings also include 90°C @ 260A for 4/0. Note that these are for wire enclosed in a raceway. Where do you draw the line? 75 up from 60 is okay, but 90 up from 75 isn't? 105 up from 90 isn't?

These are simply design constraints.

60c is 140f and 75c is 167f. With Victron inverters they derate at higher temps and will shutdown at 65c. I don't want anything over 60c to be used often and really don't want anything too hot to touch anywhere which is why I like under 150amps since you can oversize cable. IIRC these are max ratings under normal conditions and I don't want anything built to max

 

[sunshine_eggo]

60c is 140f and 75c is 167f. With Victron inverters they derate at higher temps and will shutdown at 65c. I don't want anything over 60c to be used often and really don't want anything too hot to touch anywhere which is why I like under 150amps since you can oversize cable. IIRC these are max ratings under normal conditions and I don't want anything built to max

Wire temperature is not inverter temperature.

 

[justinm001]

Wire temperature is not inverter temperature.

Which begs the question if the wire temp is 105c then what's the temp of lugs at the inverter there there isn't any insulation? The heat from the wire needs to dissipate somewhere and the inverter is a massive heatsink so it really depends on where the inverter temp sensors are.

 

[Zenithund]

The 12V system is relatively mature and the charging speed is relatively slow. If there are no requirements for charging speed during driving and parking, you can choose the 12V system.

The 48V system is relatively mature, and the parking charging speed is faster than the 12V system. If you do not pursue the driving charging speed but only the parking charging speed, the 48V system is the first choice.

 

[Samsonite801]

How many watts do you need? If more than 2,500, then 48v. Under that, 12v should simplify things.

For me, I'd personally prefer to stay with 12v on an RV, and just use two Victron Multiplus 3000 in parallel to get 6000w, if I needed to go over what one 12v inverter could do, because I wouldn't want to fuss with DC-DC buck converters on everything to get back down to 12v for the 12v accessories.

Or if I really wanted to go 48v (doubtful I'd ever decide that), I would just build it alongside (add in the 48v battery bank) and leave all the 12v stuff alone, and use one master 48v to 12v DC-DC charger to charge the 12v packs from the 48v pack, or something to that extent. I guess it depends on how much 12v stuff one uses on their RV..

 

[DIYrich]

These are simply design coconstraints.

Which includes the connectors being rated for the higher temps, and what the connectors are attached to...

I don't know if I would want 445 amps in an unprotected (free air) wire.

 

[50ShadesOfDirt]

I don't have a boat (still testing the idea of boats w/ dollar bill bonfires), but I believe they sometimes have "split systems" (12v & 48v).

Depending on the actual size & carrying capacity of your "travel trailer", it might be possible to have a full 48v system (inverter, battery-bank, panels) for all heavy-draw items, and a 12v "battery-bank, charger" system for all 12v items. The 12v side could be recharged via shore power, and the 48v system could step in as shore power to the 12v system. If done right, possibly no 48v to 12v converter needed.

Purely from a design standpoint, I'd do a planning diagram of two "bubbles" of components (48v & 12v). With these laid out on paper, you might be able to spot the relationships between all the components. A bit more planning, and you might be able to transfer very efficient wiring runs between devices and the 48v or 12v systems, directly onto your travel trailer spaces.

Hope this helps ...