RV 24v vs 12v redundancy...

emerge411

New Member
I am finalizing my RV solar design and like many drifted back and forth between 12v and 24v. I was all set on 24v but swung back to 12v for the following reasons.
  • 24v makes each BMS a single point of failure for the whole system. I want to be able to remove the battery bank from the RV during the winter (stored in cold upstate ny) for storage and or maintenance. This means building 2x 4 cell batteries vs a single 8x cell battery which would weigh about 100lbs. I can still wire them in serial or parallel to get 12v or 24v but at 24v if one battery/BMS has an issue the whole system is down. Using DIY batteries mean I cant quickly replace a bad battery while on the road.
  • 24v requires a DC-DC converter (Orion 24/12-70) since everything but the inverter would run off 12v. I could buy two to make address this single point of failure but that would wipe out much of the savings on MPPT. I could also leave the OEM 55amp converter installed but disconnected and use 120v from the inverter to supply power to the 12v side of the RV as a fall back.
  • I like the decreased current and smaller wire requirements but all of my components will be house right next to each other. No long runs are necessary.
  • At 12v or 24v I will be using a small dedicated frame mount emergency brake battery to keep it totally separate from the rest of the system.
Purchased:
8x LF304 EVE cells from Amy
2x 4s 150a JBD Smart BMS

Planned:
3x to 5x - 415w Trina Tallmax panels
Appropriate SmartSolar MPPT - Likely 150/100 for 12v or 150/60 for 24v, panels will be flat mounted and used mostly in the northeast so I want to over panel decently.
Victron 12 or 24 / 3000 Multiplus (single leg 30 amp shore service will remain so no advantage for Multiplus II)
 

Short_Shot

Photon Sorcerer
If your system is designed for 24v and you lose one battery it's not so straightforward to simply run on 12v.

You've got it covered, but of course you'll be subject to rewiring a bunch of stuff if it happens.


At that point you're far better off just running 2x 12v in parallel and eating the amps for the inverter with thick wires and skip the 24v entirely.

Just keep in mind you'll need twice the BMS current rating.


Since you've already committed by buying the stuff I don't see why it's even worth posting it.

Especially since you haven't actually asked a question.

You've already set the plan in motion since you'll need more than 150a worth of bms to run fully 12v.


Frankly, I doubt I'll ever face an issue with the cells. Especially from Amy.


You also seem to have 12 or 24v multiplus there.
If it go with the 12v version your existing BMS modules won't support its maximum output. This may be fine if you don't mind the issues that presents, but you'll be relying on overcurrent protection from your BMS which may itself cause a point of failure as well.


In my opinion if you want redundancy on 24v then you need two 24v packs. Switching back to 12v because a pack failed is headaches waiting to happen.
 

eXodus

Solar Addict
24v makes each BMS a single point of failure for the whole system. I want to be able to remove the battery bank from the RV during the winter (stored in cold upstate ny) for storage and or maintenance. This means building 2x 4 cell batteries vs a single 8x cell battery which would weigh about 100lbs. I can still wire them in serial or parallel to get 12v or 24v but at 24v if one battery/BMS has an issue the whole system is down. Using DIY batteries mean I cant quickly replace a bad battery while on the road.
if you have only one BMS - no matter of 24V or 12V you got a single point of failure.

You can just have a second 24V battery, or carry a spare BMS.

24v requires a DC-DC converter (Orion 24/12-70) since everything but the inverter would run off 12v. I could buy two to make address this single point of failure but that would wipe out much of the savings on MPPT. I could also leave the OEM 55amp converter installed but disconnected and use 120v from the inverter to supply power to the 12v side of the RV as a fall back.

just keep the 12V fully intact in-place - including a 12V battery - no DC-DC required.

24 - > 120V Convert - > 12V not efficient but highly redundant.
 

Short_Shot

Photon Sorcerer
3000 ac watts * 1.5 low frequency factor / 12 volts low cutoff = 375 amps service current
2 x 150 < 375
1.5 seems a bit aggressive. Is there a reason for a factor that high? Typically a lifepo4 should be >13v also. Most of its capacity in fact. This helps a bit on current.

I usually figure 12.5v for the math there as a result, though you can go as high as 13v if you keep soc high. At 12.5v and 85% efficiency everyone quotes it works out to be ~283a.
Anything less than 12.5v and most of us would have the bms cut off by then.

Plus surge current of course. This depends on your specific bms.

The 24/3000/70 inverter is rated at 86.7% efficiency though @ 3kw. A bit higher with a lower load.

Assuming the same for the 12v version (it might not be) that's 276 amps.
At 25v, it's 138 amps.

If you add a couple percent for voltage drop between inverter and battery then you're looking at <300a still for 12v and <150 24v nominal (plus surge).
 

DerpsyDoodler

Photon Sorcerer
l
3000 ac watts * 1.5 low frequency factor / 12 volts low cutoff = 375 amps service current
2 x 150 < 375
He should also be able to configure the inverter to only draw x amps from (batt|grid|gen). At least, i know my samlex evo is configurable in this manner.
 

emerge411

New Member
Thanks for the input.

You are right about not asking a clear question. I am unfamiliar with the relative reliability of these components. Some failures are worth designing redundancy in, some are not. I think the suggestion of keeping an extra BMS on hand is a good option since outside of a catastrophic failure in the cells a BMS failure/issue is much more likely (just a guess, am I wrong in this assumption?). The cost to do so is also small.

Maybe a good question would be as follows: Am I silly to worry about a battery issue/failure with two 12v batteries in series? Is it silly to worry about that when they are powering a single inverter and being charged by a single MPPT controller both of which are more complicated and maybe more likely to fail?

I have already committed to building two 304ah 12v batteries with a 150amp BMS each. My understanding is I can configure them in parallel (12v, 608ah, ~300amp of BMS) or series (24v, 304ah, ~150amp of BMS). Although that is a little light on the BMS side it does not seem out of the norm for people running a 3k multiplus. The linear load max is 2400w and my steady usage should max out at less then 2000w.
 

Short_Shot

Photon Sorcerer
I'd be more worried about insufficient bms rating.

150 is basically the absolute minimum for that inverter at 24v and you'll definitely want overhead.

As for the cells, build the pack properly. Isolate everything with insulation and pad it a bit to prevent rubbing. Keep the wires neat and away from the terminals.
 

emerge411

New Member
I am not into the BMS for much $ at this point so I will look into alternatives. I am guessing I should look toward contactor based BMS for higher current. I have some time while my cells make their oversea voyage to make a change.
 

DerpsyDoodler

Photon Sorcerer
I am not into the BMS for much $ at this point so I will look into alternatives. I am guessing I should look toward contactor based BMS for higher current. I have some time while my cells make their oversea voyage to make a change.
Do you think you'll be needing to push your inverter to its limit? I suggest no rush. Use what you have, set the inverter's draw limits down, then see where you stand. If it's hitting your peak setting, you may need to upgrade right away. However, if you find you're only drawing a max of 100a on a regular basis, you can wait to upgrade BMS until you grow into your system and actually need those extra watts/amps.
 

Short_Shot

Photon Sorcerer
THe VIctron Multiplus as was pointed out to me is a 3000 VA, not watts. It is rated at 2400 watts.

If you plan on using this inverter to its limits, I recommend 24 volts. If you use this inverter to warm in a microwave, not cook, a meal three or four times a day 12 volts will be fine.
Yeah it gets weird when dealing with VA and watts. They're not exactly 1:1.
 

smoothJoey

Scary!
THe VIctron Multiplus as was pointed out to me is a 3000 VA, not watts. It is rated at 2400 watts.

If you plan on using this inverter to its limits, I recommend 24 volts. If you use this inverter to warm in a microwave, not cook, a meal three or four times a day 12 volts will be fine.
I keep forgetting that victron rates there stuff in VA. :oops:
3000VA assuming a power factor .8 = 2400 watts

2400 ac watts * 1.5 low frequency factor / 12 volts low cutoff = 300 amps service current
2 x 150 = 300
 

EricR

New Member
For our travel trailer upgrade I am currently considering the Aims Power AC Converter / Battery Charger 12V & 24V Smart Charger 75 Amps (CON120AC12/24DC), as it will charge at both 12V & 24V. If I'm thinking correctly, this (along with other details I'll not go into in this thread) should help with redundancy if one of the batteries in the 24V bank should fail.
 

chrisski

Photon Sorcerer
I have the 12/24, but have yet to install. I’m not happy with the float profile, but it’s the best choice I found. Just no good RV converter float profiles.

I do plan on shutting it off after batteries are charged.
 

EricR

New Member
The KISAE/Abso AD to DC & DC to DC chargers look interesting. They have selectable levels for charge voltage, float voltage, charge current, and charge termination (in amps).

Is this what we are looking for?

-Eric


1630979453149.png
 

chrisski

Photon Sorcerer
The KISAE/Abso AD to DC & DC to DC chargers look interesting. They have selectable levels for charge voltage, float voltage, charge current, and charge termination (in amps).

Is this what we are looking for?
Getting closer.

I want charge to be between 27.5 and 28. The lowest that will go is 27.8., so that may be OK. I'm think my float voltage will be 27, so that is the minimum.

Honestly though, I will only turn the converter on as a battery charger for cloudy days only, and will shut it off when its done. I've found that if I go to an RV park and use shore power, my DC requirements are so little, I charge the batteries off solar, and never need to turn the converter on.
 

filippomasoni

Solar Enthusiast
I am not into the BMS for much $ at this point so I will look into alternatives. I am guessing I should look toward contactor based BMS for higher current. I have some time while my cells make their oversea voyage to make a change.

I'm currently in a very similar situation, so I'm very interested in this thread.

I just ordered the 8 cells from Amy but I didn't get a BMS because I'm not decided between 12V or 24V.
As I'll be running an induction cooktop I was first considering 24V since the need for a high power inverter, but I'd also like the redundancy of 2x 12V batteries especially for the international overland travels that hopefully will do in the future.

My vehicle will also be 12V and many still advise to go with the same 12V even for the camper. Watching some overland trucks (24V) travel vlogs I also realized in the US and America, in general, is more difficult to find 24V components, since even big trucks are now all 12V. Both components and appliances will be much easier to replace anywhere in the world if it's all 12V.

Speaking of BMS, have you considered the Electrodacus? I still have a few things to learn about BMSs since there are many different ways to do things, but I like the idea of the Electrocadus SBMS0, and it could work for you as well since it's not limited by current and works with all the Victron stuff with remote on/off. It's made to work with 3-8 cells, and both 12 or 24V, it obviously needs to be wired differently, but as I understand it only in series, so 4S 12V or 8S 24V. In the case of a 12V, 8 cell system do I need 2x SBMS0 ?
 

DerpsyDoodler

Photon Sorcerer
I'm currently in a very similar situation, so I'm very interested in this thread.

I just ordered the 8 cells from Amy but I didn't get a BMS because I'm not decided between 12V or 24V.
As I'll be running an induction cooktop I was first considering 24V since the need for a high power inverter, but I'd also like the redundancy of 2x 12V batteries especially for the international overland travels that hopefully will do in the future.

My vehicle will also be 12V and many still advise to go with the same 12V even for the camper. Watching some overland trucks (24V) travel vlogs I also realized in the US and America, in general, is more difficult to find 24V components, since even big trucks are now all 12V. Both components and appliances will be much easier to replace anywhere in the world if it's all 12V.

Speaking of BMS, have you considered the Electrodacus? I still have a few things to learn about BMSs since there are many different ways to do things, but I like the idea of the Electrocadus SBMS0, and it could work for you as well since it's not limited by current and works with all the Victron stuff with remote on/off. It's made to work with 3-8 cells, and both 12 or 24V, it obviously needs to be wired differently, but as I understand it only in series, so 4S 12V or 8S 24V. In the case of a 12V, 8 cell system do I need 2x SBMS0 ?
If I were you, I would get 8 more batteries and do 2x 24v packs.
 
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