PotusGobbleBottom
Corn Pop was a bad dude.
But its called a trace. not. a. wire. yes, same function
House burned down
- Thread starter Jejochen
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robbob2112
Doing more research, mosty harmless
Perhaps if you were a bit more tactful you wouldn't rub people the wrong way,... there are ways to express your opinion politely and still get your point across.
"but you can learn in the next disaster."
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42OhmsPA
What's in a title?
What if someone is using 2/0 wire with 125A fast acting Class Ts and 100A BMS's for multiple (over 5) packs in parallel?
How would that protect the wire and not the battery?
No, I'd use contactor based JBD's.
How is any BMS overpriced and useless if it does its job?
Lighthouse Beacon
Following the Light
Well maybe, but that’s not a very nice thing to say
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_voltmeter_
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look at the first post of the thread
the bms protect the battery not the fuse.
if the charging current of the faulty battery is lower than the trip current of the fuse your battery will burn or explode causing other banks starting fire and your fuse cant do anything about it. only a bms can per paralel bank.
it does not matter how low your fuse is, the charging current can be even lower and overcharge cells over several hours.
a batrium used in first post of this thread cant do anyting in this case. maybe it can make an alarm sound but what if youre not at home.
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marionw
Solar Enthusiast
I would suspect that in may cases it is simply a misunderstanding of cultures. I just spent the better part of the last two weeks in communications with SunGoldPower over technical specifications with respect to "open loop" vs "closed loop" communications between batteries and their SunGoldPower 10kw inverter. In every instance they responded to each and every email I sent (yes at least 24 hours delay due to time zones). Each response to my emails reflected courtesy and politeness on their part and I hopefully responded in kind. I suspect that some of the hinderance in our communication was my lack of Chinese and their lack of my somewhat "English" vernacular. In the end they answered my questions. Patience, courtesy and respect will ultimately win the day for all concerned.
Lighthouse Beacon
Following the Light
well said
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42OhmsPA
What's in a title?
I've read the entire thing. Nowhere does the first post point to a BMS failure.
Until it doesn't.
Not necessarily.
Again no mention of BMS in the first post.
........
Good luck with your system.
_voltmeter_
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you can ignore this but then things in first post happen. as you can see.
robbob2112
Doing more research, mosty harmless
Oh my! You spittin fire! Not sure many here know much about surface mount fuses on pcb boards....aka no wires
Im gona get in trouble with you.
As discussed earlier in this thread, semiconductor fuses are simply designed with faster time/current curves. But from looking at those most are very similar to standard class T fuses. They seem to be mostly just a different package style and size.
The problem with just depending on the standard BMS to disconnect the battery is it uses mosfets to disconnect. They are very fast, on the order of 10x faster than a class T fuse (0.000001s to 0.0000010s). But when you overcurrent a mosfet it breaks down the semiconductor junction and becomes a short 99% of the time. That short will typically only last a few seconds the blow open.
But, in a situation like the OP that few seconds would have been enough to melt the sheath from the wires and cause shorts and more trouble.
Using class T fuses on each string will disconnect the batteries from each other. But they work in 0.01s to 0.10s. The Mega fuse works on 0.1s to 2s. The ANL is a bit slower than that. All the different types have different time/current curves.
The other aspect is the AIC rating. The class T , filled with a sand mixture, can extinguish an arc 5x to 10x more than a Mega fuse or ANL. This all depends on the voltage involved when the arc starts.
In the case of a BMS based battery with mosfets that are shorted the current can be VERY large, on the order of several thousand amps. This goes for DIY or factory built batteries.
Contactors in the range we would use can switch open in 0.001s to 0.003s. They are like a relay, but the load contacts are larger and they are sealed and filled with a gas that extinguishes an arc at a much higher current than a relay. The larger the contactor the larger the arc they can break.
Not exactly sure what a shunt trip is, but I assume it is a contactor like device that opens when commanded or when excess current is detected.
Fuses can arc across the burned away links. Contactors can have their internal load contacts welded shut. MOSFETs can short. Everything has a failure mode.
The batrium in this case uses fuses between batteries and a shunt trip upstream. The current recomendation is a shunt trip per string, no idea what it was when built.
There are other BMS that use contactors instead of mosfets. So there is nothing inherently wrong with the design.
When I build a battery from scratch I will probably use a traditional BMS that can use contactors and oversize them. And I will probably also use a contactor and class T on the connection to the inverter. The contactor controlled by a shunt. I am also considering a shunt per string as a second method of turning off string contactor in the event the BMS is damaged. I have a lot of thought to put into it before I am ready to design this.
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I am beginning to understand the case for an oversized BMS and undersized fuse/wire to increase system impedance in highly parallel systems. (Say design load per string is 50A-- set your BMS parameters for 50A, but size it at 100A, and fuse it at 60-70A as an example.)
robbob2112
Doing more research, mosty harmless
Sorry, a bit long -
You can do that but it would still be treating the BMS as a fuse/OPD (overcurrent protection device). The other reason not to do it is efficiency higher impedance would mean higher heat loss. Also if a set of batteries dump into another the extra impedance would be like light bulb filaments, burn hotter and hotter unless they had enough air cooling they would eventually melt the sheath.
When the mosfet on the BMS is most likely to fail is where there is a dead short across the terminals or wire between the terminal and the bus bar.
The other case which is similar to thread most likely cause is the two or more strings dumping current into a single string.
The cause can be hooking up a low SOC string to a set of two or more other strings that are fully charged. It can be a shorted cell causing lower impedance in a string. This causes the whole string inpedance to be less than other strings so they dump current to try and equalize. But no matter how much they dump the voltage and resistance never comes up so they keep dumping.
Assume all strings are fused with class T
With one other string the dump doesn't ever exceed either fuse OR the BMS max current - things get hot and cells may or may not vent.
With two other strings the current will exceed the short string fuse and BMS - The BMS tries to shutdown the current and it is at 200% capacity - no bet on if it succeeds or the mosfets on it short. If it succeeds then current is turned off in 0.0001sec or so. If it fails then the fets short and the class T blows after a period of time.
With three other strings the current can be 3 times rated current of the bad string the class T lasts 30 seconds.
The more strings the more times rated current can flow - Ohms law applies and you can do the math to figure out how much depending on a dead short internal or 1 or more shorted cells.
At 2 times rated current the class T will last 5 full minutes - long enough for the heating to cause a problem
At 5 times rated current (short) it blows in a very reasonable time.
Just a sample of 100amp JJN fuse chart
The OP had 7 strings using Mega fuses - 125v 58v - here is the chart - Should blow in 0.1 sec
But, as it blows the fuse element draws an arc across the empty space it leaves behind. Very small voltage 2.45v if it was a single cell, but at 600 amps the arc would continue even though the fuse element is gone. Arcs are very very hot - we weld steel with it and do other things. Now the AIC of that type fuse is 2500amps - so in theory it should have gone out while the element was still burning away.
But we have seen with testing a dead short a lot of times blows apart the cheaper and plastic body fuses. Usually with some fire and melted metal fuse element.
So we are back to not enough left to know the actual cause of the fire.
To extinguish an arc the higher AIC rated fuses are filled with sand or other things that block the arc physically. They are also physically larger with a longer distance the arc would have to fill to keep on once the element is burned away. They also typically have a ceramic or reinforced fiberglass body to contain the arc and resulting heat.
So to my way of thinking the ideal method to be safe would be a combination of items - fuses, BMS, shunts, controller.
One idea - a batrium style BMS with shut trips on all strings watching for overcurrent in or out and a contactor to turn a string/all strings off.
-- this is basically how batrium recommends installing today, not sure about 5 years ago -
Also a fuse on each string in case the contactor fails shorted - I haven't looked at the docs to see if batrium recommends this.
The other idea is to substitute a standard BMS for the batrium per string and have a shunt that trips a contactor if overcurrent is detected for more than X number of milliseconds - X to be determined by nuisance trips verse high loads kicking in and drawing current.
Trip at the wrong time and the compressor stalls and you loose your airplane ... uhm - wait, wrong window -
But for real, if you trip when an AC is starting it stalls the compressor and can't start again until that bleeds off, they have a pressure switch for this.Follow the shunt/contactor/controller combo with a fuse per string.
YES, it will cost a bit more, but it covers all possible faults that I can think of other than being in a fireproof detached room to start with.
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windsurfer
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Sorry, but I disagree. Technical support needs to be both accurate and unambiguous, otherwise it risks making the matter worse. The answer shown failed on both counts.
I dont know or care what nationality the author is or what their native language is. My response was based purely on the quality of the work product.
Poor technical support is a huge problem in this industry.
Additionally, non-redundant mosfet switches should never be relied on as the only shutoff in a high power safety critical system.
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DPC
Solar Enthusiast
One time at work we had to get the plant operator and an engineer interpret a chain of email from a part supplier that used Google translate to order..
Went from English to Fin to Russian and back again.
A LOT was lost in translation and the parts did not fit lol
Thats the kid of stuff that can lead to serious accidents and death...
Should be a law if you sell anything that instructions need to be easy to read and understand.
robbob2112
Doing more research, mosty harmless
I absolutly agree on written instructions - they should be clear and proof read AFTER translation by a native speaker of the final language.
For email and voice tech support where you can ask for clarification on what they are saying having to do so can be frustrating, but is ok.
The only times I really have an issue is when the accent is so thick I can't understand the words or if they get offended when you ask questions.
And of course the curse of all tech support is the new person that is just reading a script and won't hand you off to someone that knows what they are doing. The "script" seldom covers my issues if I need to call in.
This is one (of a few) reasons I like the chat online option vs a phone call. Their accent can be as bad as possible, but as long as they type it OK, I can understand it!
(Also nice not having to sit on hold with a phone in my ear for an hour, listen to elevator music, if there's an address or whatever they need from me they don't misspell what I tell them, and several other things)
robbob2112
Doing more research, mosty harmless
The most frustrating thing is when you fill out their intake form or tell them your problem, then you get disconnected or handed off and the next person doesn't know anything abour your issues.
Snd I HATE IVRs that are more than 2 levels deep.
_voltmeter_
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there are bms with contactors if you like this more
the 200A jk inverter bms is rated for 350A 2 minutes peak and 550A short current this is enough to blow a 200A or 250A fuse so there are two parts one is the fuse protecting the wire and the bms protecting the battery.
a fuse is always needed for wire protection not for battery protection
i have mega fuses in my lynx power in, i can fit littelfuse L15S250 fuses if i want to. have to do a little modification.
i will do this if i upgrade to 4 batterys, now i have 2 not because the mega fuses ar bad but i do not trust the plastic of the lynx
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robbob2112
Doing more research, mosty harmless
I agree wth this.
What about the OP case where the current was probably 700amps at a guess? I assume it would blow a standard BMS. Which may mean it stays on and may mean it would cut current after a few seconds.
Now the BMS that do use contactors - Do they use NO contactors or NC? If NO then if they blow up the contactor should open if able. But what about a fault that surges current into the contactor control? Would that fry it so it stayed in the current state regardless of type? I don't know enough about that type BMS or contactors to answer either question?
Seems like the safest bet would be a shunt/contactor combo that is independant of the BMS - If overcurrent for long enough or if overcurrent is high enough it just trips? Would be pretty easy build with ardiuno or RPi if something doesn't already exist?
And in any case I would class T or equivalent fuse the line. The only downside of more elements is more resistance in a high current line.
What do you think of that sort of setup?
_voltmeter_
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there is always a risk use a contactor or mosfet i am using mosfet jk bms. havent read about that they fail.
the jk bms has relay contacts to drive some external load so there can be added a NO contactor this will reduce the risk if somebody want to install this.
or install a 2nd bms with a NO contactor.
you have choose the wire and fuse above the maximum load your battery/bms can handle
so in the jk case i would use a 250A fuse with a very flexible 70mm² H01N2-D welding wire, dont know the american gauge or cable type
if a 70V mega fuse is used it will blow at 200% current in 7 to 15 seconds so in this case at 500A
a german guy has tested this. watch it starts at 13min7seconds in the video (activate englisch subs)
maybe a class t is faster so it is a nice to have
he compares the victron mega fuses with the noname. and the noname fuse is better than the victron. the victron fuse does not blow fast its a very bad fuse and very expensive.
well if all fails, the fuse, the bms and the 2nd contactor then your battery build has to past a extreme test if all non flamable materials used in the build can stop the thermal runaway.
