48 volt battery to 12 volt system

The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.

strugglebus

Well-known member
Joined
Aug 3, 2021
Posts
248
Location
Perkasie PA but love Florida
I'm thinking of using a 48 volt battery with a 48volt Victron Multiplus 2 to give some boondocking ability to the MH. Not super knowledgeable with this kind of stuff but always learning. Does the Multiplus need a step down converter after the battery and before the Mulitiplus or would it go after the Multiplus, if needed at all. Not sure if you can tell the Multiplus that you have a 12 volt system and it will manage the voltage.
 

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
2,306
Location
Albuquerque, NM
I guess it depends on what your goal is. 48V systems aren't common in this kind of off grid application because of conversion efficiencies. Most systems in an RV can operate on 12V directly, so to convert battery voltage to AC, then AC back to 12V is only costing you extra Ah for no useful benefit. More efficient to have an inverter just for those devices that actually need them, than to operate the entire house that way. 48V buys you some efficiency in higher power inverter applications but that implies a lot of power is being passed around which is usually not the case in RV boondocking. So maybe describe a bit what you're trying to do and from there pick the best storage and power conversion solution.

Mark B.
Albuquerque, NM
 

strugglebus

Well-known member
Joined
Aug 3, 2021
Posts
248
Location
Perkasie PA but love Florida
Looking to have enough power to use a mini split A/C during the night (or as much as possible) when a genny can't be run. A 9000-12000BTU mini split uses about 8-11 amps, roughly less than 200 watts all said and done. The battery has 5.1Kwh. I figure with the inverter tech on mini splits it will not be running at peak power continuously. My current A/C is pretty much on or off, but with slightly cooler temps at night, the lack of sun, and no other appliances running, it seems feasible for a few hours once the inside temp is down from the rooftop A/C. Recommendations always welcome. I love hookups but campgrounds get expensive day by day and I would love to visit a few national parks but many are boondocking only. I do plan on putting solar on the roof to help charge battery during the day.
I can get a 48 volt lithlum with BMS for $1500. Same in BattleBorn or other brands is like $4000
 

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
2,306
Location
Albuquerque, NM
You're off by a decimal place. ~10K Btu is 3kW. 8-11A is likely at 220V input, which also works out to about 2.5kW. That would take an impressive battery pack to run this for more than a couple hours.

Separate from the draw, whatever energy you use needs to be replenished and it would take a lot of solar panels and a lot of hours of sun to restore kWh's of power like that.

What you're proposing is certainly possible, and has been done. But I would spend some time critically reviewing the power budget as well as your financial budget as a system of this scale will be spendy.

Mark B.
Albuquerque, NM
 

strugglebus

Well-known member
Joined
Aug 3, 2021
Posts
248
Location
Perkasie PA but love Florida
I was using the 12 volts times the amps. My bad. It is actually a 115 volt mini split. The 220's use less amps for that size but I only have 30 amps into the RV so running it off shore power wouldn't work in that case. I figure with 115 volts I can plug into either a 15 or 50 amp with a splitter and run both A/C's. So 115 x 11 is 1265. I saw the same battery with a 1400 watt continuous draw from a heat gun providing 3-3.5 hours of run time. I figure the mini split would not be drawing the 1265 continuously, so maybe 4-5 hours run time? Still very early in the process and probably not getting around to everything this spring. Perhaps by next spring the entire system in whatever form will be up and running.
 

Henry J Fate

Well-known member
Joined
Jun 14, 2018
Posts
1,919
Lots of power will be needed to do what you propose. Mark has spelled this out for you. It does not make much sense and probably will be more of a headache rather than a comfort setting.

If you're looking for comfort at night for sleeping, consider sizing your air conditioning requirement to your sleeping area only. This will decrease the size of the air conditioning unit needed and reduce the power requirements.
 

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
2,306
Location
Albuquerque, NM
Part of your calculations needs to factor thermal efficiency of the cooled space. RV's have poor insulation so it's not just a matter of cooling X cubic feet some number of degrees, it will take a constant energy input to keep it there if it even reaches setpoint temp at all. Provided you come up with enough cooling and storage capacity to pull it off, restoring that energy with solar can be a trick as you may not have enough square footage on the RV roof for all the panels it would take. Consider too that during the day you're going to want to have cooling as well despite getting some shade from panels, plus whatever other house loads you operate so that only further reduces current delivery from the panels to battery. Odds are pretty good there will be some genset runtime in this formula so likely all this amounts to is having some night time battery A/C. Which is fine if that's what you really want but that comfort will be at a cost and complexity price. If it was easy everyone would do it, but if you're up to the challenge my hat's off to you.

Mark B.
Albuquerque, NM
 

Gary RV_Wizard

Site Team
Joined
Feb 2, 2005
Posts
75,890
Location
Looking to buy a new home
Last summer I got a new 8000 btu window unit (EER of 12) that has a max (surge) draw of 13A and a running amperage of about 8 @ 120v. Call it 1000W in round numbers, so it would be drawing 90-95A from a 12v battery bank via an inverter. I don't think a mini-split would be much different.

The problem in an RV is the one Mark cited - the RV interior heats up quickly so the a/c duty cycle is high, maybe 75% or even 100% in some cases. That's a lot of battery watt-hours, even overnight when it is somewhat cooler. A higher battery voltage helps the inverter efficiency, but doesn't reduce the watt-hours required.
 

Lou Schneider

Site Team
Joined
Mar 14, 2005
Posts
11,711
To answer your original question, you can either use a 48 volt - 12 volt converter between the 48 volt batteries and the 12 volt loads or keep your existing converter and house batteries in place and power the existing converter from the big inverter to recharge them whenever it's running. The drawback to the first scenario is unless you have 12 volt storage the 48 - 12 volt converter will have to run constantly putting a phantom draw on the 48 volt batteries.
 

strugglebus

Well-known member
Joined
Aug 3, 2021
Posts
248
Location
Perkasie PA but love Florida
Thinking more about this I don't have to tie it into the 12 volt system at all. I don't need it for the microwave or coffee maker. The current system can remain completely separate and I can just put in a panel to run off the inverter fed by the battery with a few other incidentals in there. I could install solar and keep it separate from the 100 watt panel on the roof trickle charging the 12 volt battery.
 

John From Detroit

Well-known member
Joined
Apr 12, 2005
Posts
26,119
Location
Davison Michigan
A thought or 3
First a second, independent 12 volt system might be the easiest solution.
Second a DC-DC converter will efficiently reduce 48 to 12 volts
A thought.... Would a solar charge controller be able to knock it down efficiently (Perhaps an MMPT) I do not know that the input limits are on those devices.
 

Lou Schneider

Site Team
Joined
Mar 14, 2005
Posts
11,711
Solar controllers only work with sources like a solar panel that produces constant current regardless of the output voltage. You'll likely burn it up if you feed it from something like a battery with virtually unlimited short circuit current.
 

strugglebus

Well-known member
Joined
Aug 3, 2021
Posts
248
Location
Perkasie PA but love Florida
So a solar MPPT controller that can convert to 48 volts from the panels going to the battery which goes to the Multiplus and then from there to the 120 volt breaker. Shutoffs, shunts, battery monitors along the way. Far less complicated than trying to tie into the current system
 

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
2,306
Location
Albuquerque, NM
48V is a common system voltage in a variety of commercial and industrial power applications. It's right at the threshold you have to accommodate any shock hazard, so wiring and connections are not as tightly spec'd. There is a ton of 48V bus compatible equipment out there, so there's some economy of size and scale already in place. For a given power, the current is lower than 12V so wiring and control devices are smaller and ostensibly less expensive. Since inverter/converter efficiencies at any voltage conversion are typically pretty good, then generally you want as high of a source voltage as you can get to minimize resistive losses. For something like an off-grid 120VAC home this bank voltage is a slam dunk and you will find lots of 48V solar/storage solutions for this. The fly in the ointment for RV's is that many things natively run on 12V and it's usually more efficient to run them directly off of a battery than through a converter. If one was super motivated you could replace all 12V equipment with 48V and enjoy a slight boost in efficiency but it would be a pretty specific application to make that degree of refit practical. 42V SLI batteries have been proposed in the auto industry for years, but even at that scale it's been tough to overcome the inertia of systems that are developed and proven. Even in an EV with a high voltage traction pack and any operating voltage you could want through conversion, you still find a 12V chassis battery and accessories. But if you have a chance to start with a blank slate with a power storage or distribution system, using 48V is what you want over 12V.

Mark B.
Albuquerque, NM
 

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
2,306
Location
Albuquerque, NM
I figured automotive would have transitioned by now, if there was ever a product where shaving a fraction of a cent from the product cost can drive a major change it's making millions of expensive things like cars. Especially when you start adding in things like start-stop, steer by wire and other power hungry equipment, it must be a pretty convoluted cost model that keeps 12V around. Just like LED has all but wiped out incandescent though, not hard to imagine a day when some product comes along using 48V, and it pulls everything else in behind it. Just wondering though what that would be considering all of the battery equipment that's already out there today and would benefit from the upped voltage.

Mark B.
Albuquerque, NM
 

Gary RV_Wizard

Site Team
Joined
Feb 2, 2005
Posts
75,890
Location
Looking to buy a new home
Gonna be a learning curve but this is the future vs 12 volt systems. With the popularity of Rving and boondocking 12 volt system are already antiquated, people and the tech just have to catch up.
As Mark says, the auto industry has wanted to make that transition for decades but still hasn't managed it. I did a lot of consulting work with the auto industry back in the 1980's and all the engineers said their vehicles would be 48v by the end of that decade. Never happened. Cheap, standardized 12v components and designs just had too much inertia. Hopefully electric-power vehicles will finally break through that barrier, but it's still going to take many years to totally eliminate 12v accessories.
 
Top Bottom