Two 12V 280AH LiFePo4 batteries each with their own BMS in parallel

JAS

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It's for safety. I wouldn't rely on the BMS to be your primary protection against a fire caused by overheating due to a short circuit or overcurrent draw. The BMS's only job should be to protect the cells in the battery and the condition of the pack.
I understand that... I guess, what I'm asking is: why wouldn't you fuse it at the lower value of what just one battery can handle? IE: why fuse it at the higher value when you know you don't want to rely on the BMS tripping (or getting smoked if it doesn't)?
 

ereams65

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I think because the assumption is either battery can handle the load and the only reason to add a 2nd in parallel is to increase run time. Take my golf cart for example; either of the 36v batteries are capable of pushing the cart. By adding the 2nd one I've gained in miles driven ability. Now, had I added 1/2 of what the cart draws as my fuse on each, then odds are, both batteries would "trip".
 

K8MEJ

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To add to what ereams65 said, with a two battery system you should definitely build the system so a single battery can take the full charge or discharge of the system in the case of a battery failure or if you want to take one battery offline for maintenance or some other reason (N+1 arrangement). If you have three or more batteries, the most conservative design approach is to still have the ability for a single battery to accept the full charge/discharge, but you could also consider thinking of it as an N+2 system and plan to never have more than a single battery offline, which would then split the max load across two batteries and hence the cabling, fusing, etc. could be reduced.

Right now I have a N+1 system for a total of 560Ah of capacity. I will likely add two more batteries but my system is still designed to run off a single battery if needed - it would just have lower total capacity.
 

JAS

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OK, so in the case of a modest 12v setup: For example (2) 12v Battleborn batteries (which I think the BMS maxes out somewhere around 100A) in parallel, you would NOT want to install an inverter that can pull 200A (with the thought that it would only be pulling 100A from each battery)?

So in that case, if the size of the Inverter can't be reduced, then the better option would be to either get batteries with a larger BMS or move to a 24v system with batteries in series, correct?
 

Sanwizard

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Hi!

I'm a noob who's been following Will for some time now before taking the plunge and making my own setup. (Thanks, Will, for your excellent videos!)

I followed one of the beginner setup tutorials as you can see from the attached picture. It works great with one battery but I would like to add another to the setup.
So now, basically I have two 12V LiFePo4 280AH batteries each with their own 4S 12V Overkill Solar BMS. I thought I'd ask for your opinion before proceeding and I do something potentially stupid.

I know I can put two batteries in parallel and basically make a 4S out of 8 batteries if the are balanced well to begin with. But I'd prefer to have two separate boxes with each 4 batteries if possible.

I have read conflicting statements on the net with some saying it's not a problem to others saying that the BMSs could go into a state of repeatedly switching on and off.

Is this a good idea or will it destroy BMS and/or batteries?

Thank you.
I am using 2 x 16s strings with their own bms. Works fine. No worries. Others like DMI are using 4 powerwalls all tied together with busbars. He has no issues either. In fact, its probably better than adding batteries in parallel with one BMS, since each individual cell will be monitored correctly. The more BMS the better, but it does cost more.
 

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Steve_S

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Hey Steve,

Why would this be necessary? In this case, won't the 100A BMS trip well before it could get to the max amp potential of the system? (and wouldn't you want it to?)
This is not that simple. If using a Battleborn 12V/100 then it should be fused accordingly, this applies to any commercial battery and their recommendations. If building your own, we can far exceed 100AH capacities, most commonly sought after now is 280AH, so a 12V/280AH battery cab output 280A and take 140A for charging. It is relative.

The ability to have each Pack in a parallel bank act as a Standalone is not only as a safety & failsafe, it is common sense. Murphy's Laws always have a way of hitting you. Some things like Lightning protection, proper grounding and such are a given to prevent Murphy's laws from being applied.

With regards to 12V, do appreciate that @ 1000W a 12V inverter is pulling between 83-104A depending on efficiency. + any Surge potential capacity.
 
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K8MEJ

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OK, so in the case of a modest 12v setup: For example (2) 12v Battleborn batteries (which I think the BMS maxes out somewhere around 100A) in parallel, you would NOT want to install an inverter that can pull 200A (with the thought that it would only be pulling 100A from each battery)?

So in that case, if the size of the Inverter can't be reduced, then the better option would be to either get batteries with a larger BMS or move to a 24v system with batteries in series, correct?

Sounds like you understand it well. Currently, in my system, I do have to run both batteries for full inverter load because my BMS' are only rated for 120A continuous duty (Overkill Solar). For normal loads I have an N+1 system but for very high loads I don't. It's part of the reason I want to buy extra batteries. So, what I said in my post above isn't quite true at the full capacity of my inverter. Buying two more batteries give me the redundancy I recommend AND more capacity.

It's not for the feint of heart, but you might consider building your own batteries if you have the time (2-3 months shipping for just the cells) and some technical experience OR have someone nearby who can assist or mentor you. In the ham radio community this is how we do things. For roughly $1,200 in cells, shipping, BMS's, cables, and tools I have 5.5x the capacity as what I would have spent on a single 100A BattleBorn Battery.

Just something to consider. Originally I was set on buying BB batteries but once I learned a little bit about how these batteries are actually made, I just couldn't justify their huge price tag. Building my own gives me more capacity and redundancy for the less money.
 

JAS

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It's not for the feint of heart, but you might consider building your own batteries if you have the time
Funny you mention it... I had previously bought (1) BB battery and after learning and being more comfortable, I am currently waiting on cells from Michael's group buy ;-)
 

rboothe

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I used a Lynx distributor for the battery input and fuses inside the Lynx as the main battery fuses. Each battery (up to four) connects to its own associated positive and negative input. I have no other bus bars outside of the two Lynx distributors and a small 6-way fuse box on the far right side of the electric panel for small loads like the tongue jack, break away braking system, etc. I hope that makes sense.
It certainly does. I am also using a Lynx for my fuses. However, I only have one. I was thinking of putting an inline ANL fuse on each positive side. Then adding the lugs on top of one another on the left side the Lynx distributor where the wires come in. Do you think that would work fine too?
 

K8MEJ

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It certainly does. I am also using a Lynx for my fuses. However, I only have one. I was thinking of putting an inline ANL fuse on each positive side. Then adding the lugs on top of one another on the left side the Lynx distributor where the wires come in. Do you think that would work fine too?
Are you planning to use a shunt on the negative side to keep track of your state of charge? If so, having inputs and outputs attached to a single Lynx distributor is going to be problematic. You could fuse your batteries individually and parallel connect them to the left most side of the positive bus on the Lynx as you indicated, connect your shunt to the left most side of the negative bus on the Lynx, and then connect your battery negatives to the shunt. Then you could connect your other loads/chargers to the Lynx. Electrically I see nothing wrong with this. Aesthetically, however.... :)
 

curiouscarbon

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I'm also wondering if there is a way to measure if the pre-charge has been done and make a diode light up. I'm ok with doing an Arduino setup if that's what it takes.
dear @Phero thank you for posting this thread, parallel cloned pack build is very interesting to me.

consider the ACHS-7121 hall effect isolated current sensor to be in series with the precharge switch and resistor. it provides isolated current measurement by presenting a voltage of VCC/2 + 0.185 Volt per Amp. Range -10A~10A. https://www.pololu.com/product/4030

connect a 5V arduino to a regulator fed from busbar maybe. attach ACHS-7121 output pin to arduino board. log the current measurements while precharging your inverter, and post them here (if you feel comfortable). now, keeping in mind the presence of standby load of your inverter, establish the baseline current measurement.

program the arduino to repeatedly measure the voltage of the output pin of the ACHS-7121 and light up an LED when the voltage is below the threshold established above.

hope this helps!

you said you wanted light indicator that was dark when precharge is in progress and illuminates when precharge is complete and it is ok to turn on the inverter.

(by the way the PCB copper can handle 100A transient for 100ms so i think it’s safe to use in this application)
 

GypsyMike

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Screenshot_2021-06-26-02-29-28(1).png
I was wondering the same thing, i wasn't even sure if i could wire two 280ah lifepo4 banks together to make a 560ah 12v bank. Overkillsolar sent me this, and said they are planning on adding it to their pdf download. I asked them if i could make it a 24v as well, I've never done anything with solar and i don't want to go back to school to get a engineering degree to understand what a lot of channels are saying. Ie the mass of tech terms, math equations, and so in. I just want to have some off grid solar to run a mini deep freezer as a fridge, and have some power for a 5,000 btu ac for sleeping on a hot night that a fan wont be enough for comfort.
A couple if years ago any channel on utube i asked would say no it's not feasible, id have to run a generator. I pissed off a lot of people questioning that lol. That off grid garage guy and some battery sponsored guys told me i shouldn't be messing with electric period 😂, idk i never minded being asked a lot of questiins or being wrong, i guess not everyone is the same. If i don't know i ask, if i don't understand i ask more, it just rubs people wrong. But i like this forum, i see more than a few guy's/gals that don't mind admitting they know next to nothing. And unlike some solar channels there seem to be a lot of guy's willing to give advice that doesn't take a engineering degree to understand.
So thanks to the people with more knowledge willing to give advice and not be a jerk about others lack of understanding. I didn't see this schematic posted so i thought I'd share it, the nice people at overkill just sent it to me a cpl days ago so not sure if everyone has it. God bless! Screenshot_2021-06-26-02-29-28(1).png
 
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Steve_S

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the image because the wire lengths is all I can see.

There is no such thing as a stupid question, except for the one not asked.
If you don't know or are uncertain about something, especially something that has the potential to harm, ASK if someone has issues with that, then THEY have the problem and likely best avoided. Yes, sometimes it can be annoying to "seasoned vets" but an 18 year old learning cannot have the knowledge of a 50 year old in the trade. Ces't la vie, mon amis.

Slightly off but not topic.
12V@250A=3000W (25A AC), 24V@250A=6000W (50A AC), 48V@250A=12,000W (100A AC) for quick reference. the (50A AC) can be 120V/50A out to panel or 240V/25A split phase to panel. In simple terms. -- AC 120V/15A=1800W, 240V/15A=3600W uncorrected
? 250A. Because it's recommended to be the MAX draw limit for a standard"single" system. Single, meaning non-stacked inverters and similar.
! Inverter Wattage has to support the watts required plus surge capacity. Should never be run at the limit for an extended period of time (that's undersized).

Batteries in Series add voltage while in parallel they add storage capacity.
KISS applied is ALWAYS BEST, more complicated the more things that have the potentials to cause havoc. Plain & Simple ! Look at Will's diagrams and schema's KISS applied and for good reason.

IE if you know you will regularly pull 4000W or 120VAC/33A, you can do it with a 24V system BUT a 48V for continous is more reasonable.

Series Linking Batteries is legacy Lead stuff. Lead batteries are limited because of size/weight/voltage & AH capacities. They gotta be handled & managed and also modular.

Lithium removes MANY of those limitations. With LFP we can build a 12V to 72V 1000AH Battery Pack ! using 1000AH Cells no less. 12V you could carry but not 24V and up without help ! (1, 1000AH cell = 41kg/90.3lbs)
It IS best to build specifically for your target voltage be it 12V (4S), 24V (8S) or up and then add Battery Packs in Parallel to increase the AH stored. Battery Packs in Parallel split the charge/discharge relative to their capacity ratings and BMS settings if/when applicable.

Why Drop In Batteries can do "some" series ? Because these are in fact designed to be 12V Lead Acid/AGM Battery replacements used in common applications. Only a few do support Deep Cycle cranking and they are identified as such. Some do allow them to operate in Series as their BMS are designed to handle that but NOT ALL. This is also a source of Common Confusion about how battery Banks should be configured.

It is still "Early Days" with Lithium Ion Tech and the DIY'ers are "fresh" when viewed in the bigger pictures, there is still a lot of New Things to learn & understand, all the while the whole sector is evolving faster than any other tech previously. This further reasserts the need to apply the KISS principles.

PLEASE REVIEW This document from Victron starting Page 17. These people are experts in their field.

Hope it helps, Good Luck.
 

HRTKD

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As suggested in the prior posts, I would not - and did not - wire my batteries the way that the Overkill Solar document suggested. Each battery gets equal length leads run to the common bus bars.

Paraphrasing what Steve said above, if you need a 24v battery, build a 24v battery. Don't take two 12v batteries and put them in series.
 

GypsyMike

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As suggested in the prior posts, I would not - and did not - wire my batteries the way that the Overkill Solar document suggested. Each battery gets equal length leads run to the common bus bars.

Paraphrasing what Steve said above, if you need a 24v battery, build a 24v battery. Don't take two 12v batteries and put them in series.
Yeah well steve will have to do what's best for him, and you do what's best for you. And i mean no disrespect, for me i have had two surgeries already and should have another but don't like the odds of success, so my bad backs going to keep me doing what's best for me. And that's keeping the weight down to a minimum. I could get away with one 4s 12v battery bank for my needs of running a 5,000 btu ac, but since i will be off grid in the middle of nowhere, and wont have easy access to get parts, i figure it would be best to have 2 battery banks. That way if one goes down i won't have to worry as much about being without power, and if by chance i have more than expected days of poor sunlight, i will have extra power just in case. According to Will Prowse videos it's good to have a couple of days worth of backup.
I could be wrong since I've never done solar, but I'll take his word on it. So the fact one battery bank is all i want to lift in and out at a time, the ability to still have power if one bank goes down, and the fact I'll be going from michigan out west which won't be cheap on gas, I'd just as soon not have to end my month or two sightseeing trip because of a bank going down.
I won't be using them when im home in the winter, so I'll be pulling them in and out whenever i want to go somewhere. One for the cold, two because there are to many low moral character's in the world. And I'd bet a wooden nickel im not the only person who has weighed (pun intended) their options and came to the same conclusion, two batteries work better for me and my situation and needs.
That doesn't even get into the warnings on dealing with the extra potential for hazard that is given when dealing with 24v. It wouldn't surprise me if people haven't given up on solar because when asking for help on something they don't know, they either get a tutorial that would take an advanced electrical degree or years of experience in solar to understand, or told that's not how you should do it, you need to go 24v, I'd only do 24v, or even you shouldn't even mess with it because you don't know about it.
Me I'm pretty stubborn, I've been told things couldn't be done my whole life, or that i shouldn't do something because of my lack of knowledge lol. So I'll do what's best for my situation, and I'll go against what the "pros" seem to think is the only/right way to do something. I have no problem with what you or steve or anyone else will or never will do, i won't judge you or tell you your wrong to do it. Shoot i encourage you all to, from my experience doing what makes you happy is the key to happiness, it's implied in the word happiness lol. And to all the people on here being helpful and supportive i thank you, may God bless each and all!
 

HRTKD

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Mike, you are off base with regard to what I posted. I never said you HAVE to go 24v. In fact, in almost every case, I recommend that folks with existing 12v systems in RV trailers stay with 12v.

Further, I never suggested you go with a single battery. My comment on the wiring was to point out that the WAY the wiring is shown in the diagram is not how it should be done.

I have two 4s 12v batteries in my trailer. They work great and I have the redundancy that you're looking for.
 
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