Looking at going 24v instead of 12v, what am I missing?

[enrious]

For the most part I've put together 12v kits to support my amateur (ham) radios and the normal "you need a battery in your car just in case" and will soon be receiving some some 3.2v 90ah batteries that I'll be assembling in a 12v circuit to provide field-expedient supplemental power for my radios, complete with a 12v 150w solar charge controller.

I then started thinking about building a home emergency power battery bank using 16 of the 280ah lifepo4 cells that are the new rage and my first thought was to go with the familiar, 12v, maybe a 40amp Rover Elite to provide solar recharge when suddenly math struck me like 3am chili. The main point of going to 24v (apart from smaller cable sizes) is the ability to double the charge compared to 12v.

Whereas before I could spend X amount of dollars and get a charge controller that'd accommodate 5 x 100w(ish) panels or roughly less than 4% of the total capacity (40a per hour of over 1100a @12v) , by going to 24v I could stay with the 5 panels allowed by that controller or I could throw money at the problem and double it to roughly 10% (by doubling the panels).

I'm ok with being a throw money at it kinda guy if it makes sense.

I looked - the 24v @ 150a bms is like ~$110-140 cheaper than buying a 12v @ 250a bms (Daly for both), with to me the difference being in a slight bump in getting a 3000w@24v vs 1500w@12v inverter and the roughly $100 for buying the step downs I want to go from 24v to 12v for the cig plugs, usb, and andersons.

So what am I missing? It seems to me that for roughly the same price or maybe a difference of $100ish, I get a system that is twice as upgradeable while having reduced costs in cable.

This almost seems too good to be true.

 

[Maast]

If you're looking at a 14.3kw battery bank you REALLY need to be looking at doing a 48V system. At those potential amperages the savings in cable will be considerable, and you can always upgrade your inverter in the future without having to rejigger your battery bank.

 

[Dzl]

If you're looking at a 14.3kw battery bank you REALLY need to be looking at doing a 48V system. At those potential amperages the savings in cable will be considerable, and you can always upgrade your inverter in the future without having to rejigger your battery bank.

48v is definitely worth seriously considering, and may be the best solution (particularly considering 16 high capacity cells).
I would point out though, that while OP's battery bank is large, the described max load (3000W inverter) is not beyond what a 24v bank is fully capable of. Accounting for inverter inefficiency 3000W @ 24v is about 150A. Of course @ 48v you can cut that number in half, and as you said, if power needs increase in the future its easier to scale up.

 

[Dzl]

So what am I missing? It seems to me that for roughly the same price or maybe a difference of $100ish, I get a system that is twice as upgradeable while having reduced costs in cable.

This almost seems too good to be true.

I don't think you are missing anything.

Below is my partial and muddled impression regarding voltage.

12v
The default for vehicle based builds, and small systems. 12v is popular and common partly because its the standard for cars and many other things, and the most readily available on amazon/ebay/etc, so its the path of least resistance and has the greatest selection and widest availability of components and gadgets. But 12v systems are limited in size, and higher powered 12v systems can cost more and are less efficient.

48v
This is probably the closest there is to a default for residential off-grid solar, residential energy storage, etc. It is no longer 'low voltage DC' (as defined by the NEC: <50V actual, ABYC: <50V actual, RVIA: <24v nominal), and is somewhat more dangerous to humans (still substantially less dangerous than household AC power). It is much more efficient at transferring energy, and is suitable for higher power systems than 12v and 24v. Automotive and marine components in this voltage will be limited, off-grid/residential components will be more common.

24v
Not sure if this could be considered the default for anything, it's a good compromise between some of the advantages of 12v and 48v though (depending on application). It is semi-common in the marine industry, and on some commercial vehicles, it is also semi-common for smaller off-grid residential systems. It is more efficient than 12v but less efficient than 48v, and suitable to medium sized systems. Decent selection of components available for both marine/automotive and off-grid/residential, but less availability of automotive stuff compared to 12v.

 

[Zil]

Selection should be sized to usage. How many watts will you use from the system? I didn't read anything that suggests more than 24 volts. The problems double at 48 volts.

 

[Trukinbear]

Selection should be sized to usage. How many watts will you use from the system? I didn't read anything that suggests more than 24 volts. The problems double at 48 volts.

What problems double at 48V? I had twin 12V systems (with 32 T-106a's) and going 48V cut my problems in fourth. I've never looked back and kick myself for ever doing anything but (48V).

 

[Maast]

IMO 48V is the preferred voltage unless you're forced into something else because its either a tiny system, or on a vehicle. 12V is just a PITA at anything more than 6 gauge wire (about 50-75 amps).

 

[Jeremiah]

I'll just chime in here that I used to use a 12v system with home-made panels and the change to 48v has been awesome. I actually wanted a 36v system but there just wasn't enough equipment out there to support that choice.
I still get a kick out of the thin thin wire I have coming up from the basement and powering several 24v (stepped down of course) fans with only minor voltage drop. After I'm up in the main area of the house I drop it down to 30v so I can use an automotive fuse system for the fans, usb chargers, and lights in various rooms, then I step it down to 12vwhere I need to drive modem, router, and other must be right at 12v hardware; most rooms get 13.8 v right near the plugins (so the fans go faster
I don't think your step-downs should be $100, you can get pretty hefty buck converters for a song these days.
I'm so glad I changed to a 48v battery; thanks to this forum.
Just be careful

 

[Dzl]

I personally would modify this to:

IMO 48V is the preferred voltage [for a home/structure]

I think 48v systems can shine where your setup is roughly: [Charge source --> Battery Bank --> Inverter --> AC wiring]. Particularly so with larger wire runs/bigger spaces, and medium to large system size. In this scenario the voltage is mostly irrelevant beyond the battery bank and core wiring, so going with the most efficient voltage makes sense.

For situations where some or all of the loads will be DC, 12 and 24 volt systems have some advantages. (1) no need or less need for stepdown converters (and associated efficiency losses), and (2) much greater availability of DC devices at those voltages (particularly 12v), and devices for specific use-cases (marine, automotive, offroad, etc, as well as just consumer tech gadgets) are much more available in those voltages. On the other hand wiring will either be thicker or have greater losses, and at some point you can't practically buy wire thick enough for a large load.

Because you (OP) are planning to use this for home backup, and because I think all your large loads will be AC, and your system size is moderate (large battery bank, but medium sized inverter), I agree (mostly) with @Maast 48v or 24v is reasonable, and 48v will have lower line loss/voltage drop, and more room to grow.

 

[EvRv]

48v is definitely worth seriously considering, and may be the best solution (particularly considering 16 high capacity cells).
I would point out though, that while OP's battery bank is large, the described max load (3000W inverter) is not beyond what a 24v bank is fully capable of. Accounting for inverter inefficiency 3000W @ 24v is about 150A. Of course @ 48v you can cut that number in half, and as you said, if power needs increase in the future its easier to scale up.

I have an RV that was inherited in the middle of being repowered. It has a 12.5kw diesel generator and two 24 volt 4000 watt inverters. There are also eight used 12 volt 255Ah agm batteries. The batteries are 7 to 8 years old and have not been maintained so they are no good. I am going to purchase LifePo4 Lithiums and a bms. The plan was for the inverter\chargers to feed separate 110v AC panels. There are 12 volt lithium and 24 lithium batteries available. I thought of getting four 12 volt lithium batteries for each inverter (2s 2p) separated banks. Someone has suggested to use 24 volt batteries. Would I need a separate bms to control a charging relay and a load relay for EACH inverter/load panel and the separate battery banks? Also how many Amp hours is minimum vs nominal for a 24 volt 4000 watt inverter? I know some bms units allow user programming. And the big consideration is could I buy just maybe half the batteries, get both inverters up and running and then buy the other half later? I could swap all the batteries to one inverter and then put a newly matched group on the 2nd inverter/charger. So what would be the minimum battery capacity that makes sense for a 24 volt 4000 watt inverter?

 

[Dzl]

I have an RV that was inherited in the middle of being repowered. It has a 12.5kw diesel generator and two 24 volt 4000 watt inverters. There are also eight used 12 volt 255Ah agm batteries. The batteries are 7 to 8 years old and have not been maintained so they are no good. I am going to purchase LifePo4 Lithiums and a bms. The plan was for the inverter\chargers to feed separate 110v AC panels. There are 12 volt lithium and 24 lithium batteries available. I thought of getting four 12 volt lithium batteries for each inverter (2s 2p) separated banks. Someone has suggested to use 24 volt batteries. Would I need a separate bms to control a charging relay and a load relay for EACH inverter/load panel and the separate battery banks? Also how many Amp hours is minimum vs nominal for a 24 volt 4000 watt inverter? I know some bms units allow user programming. And the big consideration is could I buy just maybe half the batteries, get both inverters up and running and then buy the other half later? I could swap all the batteries to one inverter and then put a newly matched group on the 2nd inverter/charger. So what would be the minimum battery capacity that makes sense for a 24 volt 4000 watt inverter?

First to clear up what may be a misconception, if you are planning to buy 12v or 24v lithium 'batteries' you don't need and cant use (as far as I know) a BMS bought separately. Drop-in lifepo4 batteries will have their own proprietary BMS (this is why many people like, and why many people dislike drop-ins). You use a BMS when you are building your own battery bank using raw cells (individual 3.2v lifepo4 cells).

I am not following this question

Also how many Amp hours is minimum vs nominal for a 24 volt 4000 watt inverter?

Will you clarify what you mean here? Are you asking how many Amps a 4000W inverter could draw or how large a battery bank would be needed at a minimum?

Based on 4000W maximum roughly 200A would be the max current therefore 200Ah would be the bare minimum battery bank size needed to provide the full 4000W without exceeding most batteries max continuous discharge current.

 

[EvRv]

I get it that there are some batteries that may have plug and play bms monitoring. Meaning, you still have to pay about $600 for the external bms module to provide relay disconnects to protect against overcharging and also over discharge. I know some inverter chargers protect batteries. But if I understand correctly, the bms is needed because if one cell in a lithium pack "pops" there just might probably be a fire. On some external bms modules, the supplier has to add his proprietary programming based on your specs. Some you can program yourself. This gives relay disconnect of the cells. The application engineers configure the bms to your specs and this enables control of up to 4 contactors. But the canbus monitoring, external control signals, isolation, and safety status would be safely implemented. It seems using 3rd party external bms controllers can still add some safety to a thoughtfully planned system.

What would be most helpful to me is if somebody would post an actual diagram of an RV system with shore power, generator, disconnects, breakers, relays, bms, lithium batteries, altenator, and inverter/charger. I have an onan 12.5 kw generator and two 24 volt trace/xantrex inverters. There has got to be someone that has a complete system diagram I can copy or at least model after. I would like to start with 200 Ah battery capacity on my inverter/charger.

 

[Zil]

OP Sep. 15 "I then started thinking about building a home emergency power battery bank using ---"
EvRv Oct. 16 "---post an actual diagram of an RV system with shore power----"
What are we building RV or earth base? What posts refer to what system?

 

[EvRv]

Class A diesel pusher.

 

[enrious]

OP Sep. 15 "I then started thinking about building a home emergency power battery bank using ---"
EvRv Oct. 16 "---post an actual diagram of an RV system with shore power----"
What are we building RV or earth base? What posts refer to what system?

Can't speak for some of the subsequent posters, but I'm still looking at home power in a duplex. I may end up building 2 24v systems using 8 cells each, one for me and one for a close family member who lives within walking distance. Historically we've gone 2 weeks without power thanks to ice storms killing power lines...that are still above ground.

I generally don't need a lot of power for my needs, as I've been able to lower my power consumption. For example, I got rid of my washer and dryer and replaced them with a low power washer only unit (https://www.amazon.com/gp/product/B081DG1BRW) and have an emergency DC 53Q refrigerator that draws about 2ah @ 12v per day in summer. While I may run several DC fans in the middle of summer, I don't anticipate running A/C and the same in the middle of a Missouri winter with heat.

So I guess I'm looking at this having two approaches: throw power at it or reduce consumption. I'm thinking about the former after having done the latter.

When I buy my house, I think I'll go 48v without hesitation.

 

[Samsonite801]

Class A diesel pusher.

I have some OEM diagrams I use for working on my '95 Safari Serengeti I could post for you to use as baselines (fairly typical type wiring scheme for this type of RV). They could be modded to suit custom changes... What make/model/year of motorhome is yours? Not hard to add in solar/lithium technology onto OEM design. I have lots more schematics too, these are just some example ones I pulled... You could start a different thread so we don't hi-jack this one too bad...

 

[enrious]

So spitballing here, is this sane?

Lets say I grab 8 of the 3.2v @280AH cells, attach a suitable 24v/200a BMS and decide to pair it with a MPP Solar 2724LV-MR with the following goals from the 2724: can charge via shore power (wall a/c), can charge via 600w solar, can provide up to 2700w a/c power thanks to the inverter.

At that point I toss a 24 to 12 step down 60a@24 to provide up to 100a@12v (I will limit via relay/circuit breaker) to accommodate reasonable 12v needs.

My ultimate goal for this would be to provide emergency power for a week with power loss due to ice/storm/whatever. In my mind I'd build an enclosure for the cells and 2724 out of PVC pipe, cutting board plastic, and plexiglass for the "lid".

In theory does this make sense?

 

[Maast]

So spitballing here, is this sane?

Lets say I grab 8 of the 3.2v @280AH cells, attach a suitable 24v/200a BMS and decide to pair it with a MPP Solar 2724LV-MR with the following goals from the 2724: can charge via shore power (wall a/c), can charge via 600w solar, can provide up to 2700w a/c power thanks to the inverter.

At that point I toss a 24 to 12 step down 60a@24 to provide up to 100a@12v (I will limit via relay/circuit breaker) to accommodate reasonable 12v needs.

My ultimate goal for this would be to provide emergency power for a week with power loss due to ice/storm/whatever. In my mind I'd build an enclosure for the cells and 2724 out of PVC pipe, cutting board plastic, and plexiglass for the "lid".

In theory does this make sense?

It sounds good except the MPP solar, they have such a high idle and conversion loss that you'd eat up a lot of your stored power just sitting there when the power goes out, especially critical at a time where every watt used counts. STRONGLY recommend a Samlex or Victron inverter/charger and have a separate solar and solar controller circuit, Epevers are a great value in MPPT controllers.

 

[Samsonite801]

Yeah I think if I just needed this as a backup solution (where I only needed 2700-ish watts of AC power available), and I had things that needed to run on 12v directly, I'd just skip the 24v > 12v converters and build a 12v system like you would in an RV application. Like for me would just consider building a system using a Victron MPPT solar charge controller, and for inverter use like a Magnum 2812, 2800w 12v inverter. They have a remote panel you can buy for them to easily turn the inverting on/off and such from far away. Xantrex has a comparable model for RVs. They act as an AC charger for batteries when on shore power, and auto switch to inverter when shore power goes away, the Victron will take any solar power and charge batteries, then you have raw 12v available to run 12v loads without any conversion loss. That's just my personal take on it. With 12v, just try to keep the wire run between battery bank to inverter shorter and use heavier cables.. Then you could wire the 8x LiFePO4 as 4x 12v x2 and use 2x Overkill Solar 120a BMS or something.

 

[schmism]

STRONGLY recommend a Samlex or Victron inverter/charger and have a separate solar and solar controller circuit,

At like 6x the price of an MPP or Growatt allinone. If your concern was just saveing a few watts you could throw more batts at the system for the money you would save over the Victron route.

If you need a buck converter for 12v anyway you might as well run 48v and save on wireing costs and have additional capacity for more panels should you find yourself able to hookup to ground deployed panels.