RV 24v vs 12v redundancy...

Short_Shot

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If you have a smart bms you can alternatively change the setting to reenable the battery as well. Assuming it gives you access to do so anyways.

Regarding the finer points, I've only used smart bms offerings so I don't know about "jump starting" them when locked out of those settings.

With my overkill units I just put a charger on it and off I go. Whether it's a mains charger, dc to dc, or literally just plugging it in to the tow vehicle or seems to work fine.
 

MattiFin

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I just realized there might be another important advantage of having 2x 12V batteries instead of 1x 24V.

I read here (at par. Other BMS-Induced Peculiarities & Problems) that after a low voltage disconnect, the BMS turns off and there's no way of turning it back on other than with an AC charger that can do “dead battery charging”. What does that mean?
In the case of a single cell going below the low voltage disconnect, we can't turn it back on with an MPPT or with DC-DC charger? That would mean having to find an AC outlet, which could be a problem if we are in a remote area for example.

In the article, it says that alternatively the battery can be jump-started with another battery. So heaving a 2x12V battery would solve the issue or it can also be jump-started with the starter battery of the vehicle if it's 12V.
"dead batttery charging" means that the charger supplies voltage even if it doesn't "see" existing battery voltage.
(With lead-acid batteries you needed sometimes charger that started to charge even if it looked like the charger is connected to shorted battery (near 0v))

If your MPPT or DC-DC charger doesn't output any voltage unless it "senses" connected battery the BMS won't wake up from hibernation.

If you end up with BMS shutting down and MPTT refusing to output any voltage so that the BMS would re-connect the battery you can probably "jump-start" the BMS even with two 9 volt batteries. Or 18v cordless tool battery and lightbulb in series. Or small 20 usd non-smart dc-dc power supply.
 

12VoltInstalls

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With lead-acid batteries you needed sometimes charger that started to charge even if it looked like the charger is connected to shorted battery (near 0v))
Automatic/smart chargers are like that. I have had to ‘bump’ batteries or use an old power-dump transformer battery charger at times to get my “smart” 30A charger to work. A bad event (broken down car with eflashers going while parked on side of road for a long period) with a battery at 10 or 11 volts sometimes reads as a 6volt and then shuts down as ‘full.’
The biggest nice things about new automatic chargers is being able to leave them 24/7 and that mine will hit 15V but if you need to actually charge a dead battery? Notta.
 

jwelter99

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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)

The only reason to not use 12V is:

1) You are starting from scratch and can purchase all the major consumers in the voltage you choose. This is typically the air-conditioner and fridge as the main ones; and then pumps (for slides) and anything like electric heat being secondary.

2) You need a lot of AC power off grid and therefore current requirements on a 12V system become difficult. This is around 3600W of AC power.

But in both of these situations you are better off to go right to 48V and purchase accordingly.
 

filippomasoni

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If you have a smart bms you can alternatively change the setting to reenable the battery as well. Assuming it gives you access to do so anyways.
So with a JBD/Overkill bms there's an option to actually reactivate the battery? If the bms shuts everything off wouldn't it also shut itself off since it runs from the battery?

I'll have all of the "smart" things, like Victron smart MPPT and smart DC-DC, I'm not sure if those need to sense a battery to output current. Also if the battery is "shut off" what's going to power the MPPT and DC-DC electronics? bluetooth etc to display data on the app? Is everything shutting down when the battery is powered off by the bms?

If you end up with BMS shutting down and MPTT refusing to output any voltage so that the BMS would re-connect the battery you can probably "jump-start" the BMS even with two 9 volt batteries. Or 18v cordless tool battery and lightbulb in series. Or small 20 usd non-smart dc-dc power supply.
If two 9V batteries are enough to wake up the bms then that would be a great backup for such an event. Or also the 18V battery tool which I'll definitely have around.
 

Short_Shot

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The bms will still work even though it's shut the battery off. It's draw is only a few milliamps.

At least that's been my experience. There may be units which just turn off entirely.
 

childcarepro

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I just realized there might be another important advantage of having 2x 12V batteries instead of 1x 24V.

I read here (at par. Other BMS-Induced Peculiarities & Problems) that after a low voltage disconnect, the BMS turns off and there's no way of turning it back on other than with an AC charger that can do “dead battery charging”. What does that mean?
In the case of a single cell going below the low voltage disconnect, we can't turn it back on with an MPPT or with DC-DC charger? That would mean having to find an AC outlet, which could be a problem if we are in a remote area for example.

In the article, it says that alternatively the battery can be jump-started with another battery. So heaving a 2x12V battery would solve the issue or it can also be jump-started with the starter battery of the vehicle if it's 12V.
I never thought about this, that solar charge controllers don't simply feed voltage without reading a battery voltage. The Victron 150/85 manual does state:

"The controllers will operate only if the PV voltage exceeds battery voltage (Vbat). PV voltage must exceed Vbat + 5V for the controller to start. Thereafter minimum PV voltage is Vbat + 1V."

So it seems even more important to use a Battery Protect to shut off the loads before the BMS shuts the battery off.

But yes, I agree that the redundancy of multiple 12V batteries is multifold, including this new wrinkle, to jump-start a "sister" dead battery so it can be charged. (Who knew?!?) I'm also putting in an Amp-L-Start to maintain the chassis battery charged, as my experience has been the chassis battery is never good after a few days of boondocking. I just replaced it with a new AGM battery, so hopefully that plus the Amp-L-Start will keep it charged up.
 

Geokilroy

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I never thought about this, that solar charge controllers don't simply feed voltage without reading a battery voltage. The Victron 150/85 manual does state:

"The controllers will operate only if the PV voltage exceeds battery voltage (Vbat). PV voltage must exceed Vbat + 5V for the controller to start. Thereafter minimum PV voltage is Vbat + 1V."

So it seems even more important to use a Battery Protect to shut off the loads before the BMS shuts the battery off.

But yes, I agree that the redundancy of multiple 12V batteries is multifold, including this new wrinkle, to jump-start a "sister" dead battery so it can be charged. (Who knew?!?) I'm also putting in an Amp-L-Start to maintain the chassis battery charged, as my experience has been the chassis battery is never good after a few days of boondocking. I just replaced it with a new AGM battery, so hopefully that plus the Amp-L-Start will keep it charged up.
I am out in the desert a lot. I carry one of the newer LiFePo battery jump starters with me at all times. Small package but powerful unit. Can jump 5 or 6 cars on a single charge. It would probably jump start your discharged battery if needed. Cheap one from HF cost me $50 a few years ago. Still working great. Have used it for others in my Prospecting club when their chassis battery would not start their rig after too many days boondocking. One of the best investments I have made.
 

childcarepro

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I am out in the desert a lot. I carry one of the newer LiFePo battery jump starters with me at all times. Small package but powerful unit. Can jump 5 or 6 cars on a single charge. It would probably jump start your discharged battery if needed. Cheap one from HF cost me $50 a few years ago. Still working great. Have used it for others in my Prospecting club when their chassis battery would not start their rig after too many days boondocking. One of the best investments I have made.
Yet another great application of lithium jump starters! Great tip. Thanks. We already use them for powering 12V/5V items on long trips (our boy overheats without a car seat fan!).
 

filippomasoni

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I never thought about this, that solar charge controllers don't simply feed voltage without reading a battery voltage. The Victron 150/85 manual does state:

"The controllers will operate only if the PV voltage exceeds battery voltage (Vbat). PV voltage must exceed Vbat + 5V for the controller to start. Thereafter minimum PV voltage is Vbat + 1V."
In this case if the battery reads 0V because it's shut off, PV will be more than +5V so the controller will start and operate, therefore start the battery again and charge it. Or am I reading it wrong?

But yes, I agree that the redundancy of multiple 12V batteries is multifold, including this new wrinkle, to jump-start a "sister" dead battery so it can be charged. (Who knew?!?) I'm also putting in an Amp-L-Start to maintain the chassis battery charged, as my experience has been the chassis battery is never good after a few days of boondocking. I just replaced it with a new AGM battery, so hopefully that plus the Amp-L-Start will keep it charged up.
Never heard of Amp-L-Start and can't seem to find it outside the US. Is that something like a Victron Cyrix?

I am out in the desert a lot. I carry one of the newer LiFePo battery jump starters with me at all times. Small package but powerful unit. Can jump 5 or 6 cars on a single charge. It would probably jump start your discharged battery if needed. Cheap one from HF cost me $50 a few years ago. Still working great. Have used it for others in my Prospecting club when their chassis battery would not start their rig after too many days boondocking. One of the best investments I have made.
That works only on 12V batteries, right? Can we use two of those in series to jump-start a 24V battery?

I was decided to go with a 24V battery until I saw this new thing. I'm not too concerned with cells going bad because if they are Grade A and properly balanced will last a long time, but it's the electronics around them that can render the battery inoperable or "dead". And leave us stranded without power.

Since this problem seems to be the BMS, could heaving a spare one mitigate this? Is it possible to start a BMS with a battery gone in undervoltage? So that it can start to charge with an MPPT?
 

Zwy

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In this case if the battery reads 0V because it's shut off, PV will be more than +5V so the controller will start and operate, therefore start the battery again and charge it. Or am I reading it wrong?


Never heard of Amp-L-Start and can't seem to find it outside the US. Is that something like a Victron Cyrix?


That works only on 12V batteries, right? Can we use two of those in series to jump-start a 24V battery?

I was decided to go with a 24V battery until I saw this new thing. I'm not too concerned with cells going bad because if they are Grade A and properly balanced will last a long time, but it's the electronics around them that can render the battery inoperable or "dead". And leave us stranded without power.

Since this problem seems to be the BMS, could heaving a spare one mitigate this? Is it possible to start a BMS with a battery gone in undervoltage? So that it can start to charge with an MPPT?
Your battery shouldn't go into "undervoltage" as you refer to it provided you have the BMS low voltage cutoff properly programmed. The purpose of the BMS on a LFP battery is to protect the cells provided you program it correctly.

You will be fine with a 24V LFP battery with a properly programmed BMS. What is being referred to is jump starting FLA or AGM batteries used for starting a vehicle that have been severely discharged.
 

filippomasoni

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Your battery shouldn't go into "undervoltage" as you refer to it provided you have the BMS low voltage cutoff properly programmed. The purpose of the BMS on a LFP battery is to protect the cells provided you program it correctly.

You will be fine with a 24V LFP battery with a properly programmed BMS. What is being referred to is jump starting FLA or AGM batteries used for starting a vehicle that have been severely discharged.
From reading this (at paragraph "Other BMS-Induced Peculiarities & Problems") it's talking specifically of Lifepo4 BMSs. I don't know if this is correct, I've never tested a lifepo4 BMS, but it says the only case the BMS won't turn on by itself when going into protection is undervoltage of a cell. As I understand it, the bms shutts off all loads when one cell reaches the undervoltage cutoff, to protect the cell. No harm done, it's what it's supposed to do. But how do we turn it on again? Do you know of any bms that will stay on or start off by itself when a charging current from an MPPT is applied for example?
Of course, we should try to never get to that situation, but if it happens, I want to be able to start the bms with what I have in my camper, without having to search for an AC outlet to plug a charger. That's basically the purpose of this thread, redundancy to make sure we're not stranded.
 

filippomasoni

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Reading through Victron manuals, I noticed the Victron DC-DC chargers, Smart Orion, can be set in two modes, charger and power supply. It only says the power supply outputs a constant voltage instead of regulating bulk-absorption-float.
If that can act as a power supply can it actually awake a shutoff BMS and start charging? If that's the case then we would have a solution to this problem.
 

Zwy

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Reading through Victron manuals, I noticed the Victron DC-DC chargers, Smart Orion, can be set in two modes, charger and power supply. It only says the power supply outputs a constant voltage instead of regulating bulk-absorption-float.
If that can act as a power supply can it actually awake a shutoff BMS and start charging? If that's the case then we would have a solution to this problem.
To answer both of your replies. One, if the BMS did shutdown, one can always disconnect all loads and run a jumper for a few seconds across the BMS to initiate the MPPT to begin charge. Even a multi meter on voltage setting will accomplish this. This is why certain Daly BMS's wake up when putting a multi meter across the B- terminals.

Two, really what most BMS's look for is an increase in voltage coming to the BMS to turn back on. It doesn't care where it comes from, MPPT or a battery booster or even an AC charger. As you have common positive and neutral busbars in the system, the increase in volts would be present if the PV array is producing.
 

filippomasoni

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To answer both of your replies. One, if the BMS did shutdown, one can always disconnect all loads and run a jumper for a few seconds across the BMS to initiate the MPPT to begin charge. Even a multi meter on voltage setting will accomplish this. This is why certain Daly BMS's wake up when putting a multi meter across the B- terminals.

Two, really what most BMS's look for is an increase in voltage coming to the BMS to turn back on. It doesn't care where it comes from, MPPT or a battery booster or even an AC charger. As you have common positive and neutral busbars in the system, the increase in volts would be present if the PV array is producing.
Got it so that's basically not an issue at all then. Why did that article say solar charge controllers need to see a battery voltage to function?
 

Short_Shot

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Got it so that's basically not an issue at all then. Why did that article say solar charge controllers need to see a battery voltage to function?
Some actually do require it as a safety thing. Very rarely hooking up an scc without a battery can possibly nuke the scc. But not all models are all that subject to it.
 

filippomasoni

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I decided to go with 12v. The 2400w/3000va Multiplus seems to run fine for many using 4/0 cable. Everything is located next to one another so the runs are very short. I will work on the install this fall. Shake down the system on some east cost trips summer of '22 and have time to make any adjustments before our trip west in the spring of '23. Current state of the plan below. I will share what I learn along the way.
What software are you using to make the diagram? It looks really good.
 

emerge411

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What software are you using to make the diagram? It looks really good.
It's just bunch of squares in Lucid Chart (lucid.app). I can share the source file if you are interested.

I did some updates as seen below. My batteries finally arrived! Thankfully Amy Wan packs the cells well, they had a rough journey. One had a 1" deep gash but the foam was 2" thick all the way around. Getting close to pulling the trailer into the barn to start work.Solar - Overview(1).jpegPXL_20211020_181441516.jpgPXL_20211020_180142749.jpg
 
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filippomasoni

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It's just bunch of squares in Lucid Chart (lucid.app). I can share the source file if you are interested.

I did some updates as seen below. My batteries finally arrived! Thankfully Amy Wan packs the cells well, they had a rough journey. One had a 1" deep gash but the foam was 2" thick all the way around. Getting close to pulling the trailer into the barn to start work.

View attachment 71562View attachment 71499View attachment 71502
Thanks, the source file would be helpful thank you.
I know there are many different software to do that but this one looks really nice, chromatically as well as a way of color-coding things, makes it very easy to understand. I could do it with illustrator which is something I'm used to since I work with it often, but it won't be easy to modify and I'll waste a lot of time. It doesn't have to be pretty but practical, but if it's good to the eyes as well makes it more pleasurable to work with I guess.

Glad Amy's packaging worked well. I should receive my cells in the next couple of weeks 🤞

Also keep us posted with your build, I'm looking forward to see how it turns out
 
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