Single vs. Multiple Class-T Fuses
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John Frum
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Get rid of the circuit breaker box between the lynx power-in and the batteries.
Use a 250 amp class t fuse as close as possible to each battery positive terminal
Get rid of the class t fuse between the power in and the shunt
Connect the the backplane of the lynx power-in to the lynx_shunt
Use this fuse in the lynx shunt
Littelfuse 0CNN800E.V CNNE Series, Very Fast Acting Fuse, 800A | Waytek
Littelfuse 0CNN800E.V CNNE Series, Very Fast Acting Fuse, 800A, 80VDC is in stock and ready to ship. Order today!
www.waytekwire.com
If you have specific questions I will do my best to answer them.
I hear ya! As a retired firefighter...well, you can guess it from there.
Problem is...I have no way to connect a fuse at the battery terminal. I was leaning towards the MRBF but you pointed out that its AIC isn't good enough. So I am trying to find something.
You can see in the topo I have a 125a CB but I would rather have a fuse...or a fuse in addition to the CB.
And I still haven't seen a system yet with a fuse right on the battery terminal. I also still haven't seen an explanation yet why a Class-T at each of two inverters vs a single Class-T at the busbar.
I am no brainiac, by far, but I can understand things...just no one is giving a clear answer or showing pictures.
Yeah, you can tell I am frustrated. I just want to get this right...and understand what I am doing.
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John Frum
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Kirchhoff's circuit laws - Wikipedia
The summary of this article is, current in = current out.
Nope I can't.
Just assume I'm not from this planet.
Use a small wire try for <8 inches.
I already told you to get rid of them.
I don't care what you have seen.
Its been discussed at length in this very thread.
re-read all my posts and all by @upnorthandpersonal
Yes they have repeatedly.
Help us help you by asking questions about the words and concepts that you don't understand.
Do you trust my advice on the matter?
LithiumSolar
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Please keep in mind those fuses can get pretty warm/hot under normal operation.
John Frum
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@the overall topology concept is called feeder and branch.
The lynx components are made to work with this topology.
The batteries connect to the power-in, which doesn't have fuses because the batteries are supposed to be fused at source.
The optional shunt tracks all current in and out of the batteries.
The distributor is for all the branch circuits which are fused at the busbar.
The lynx components are made to work with this topology.
The batteries connect to the power-in, which doesn't have fuses because the batteries are supposed to be fused at source.
The optional shunt tracks all current in and out of the batteries.
The distributor is for all the branch circuits which are fused at the busbar.
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Get rid of the circuit breaker box between the lynx power-in and the batteries.
Use a 250 amp class t fuse as close as possible to each battery positive terminal
Get rid of the class t fuse between the power in and the shunt
Connect the the backplane of the lynx power-in to the lynx_shunt
Use this fuse in the lynx shunt
Littelfuse 0CNN800E.V CNNE Series, Very Fast Acting Fuse, 800A | Waytek
Littelfuse 0CNN800E.V CNNE Series, Very Fast Acting Fuse, 800A, 80VDC is in stock and ready to ship. Order today!
If you have specific questions I will do my best to answer them.
First off...THANK YOU !!!! I am not a dummy, but I don't know the deep technical stuff like what we are talking about. You taking the time to help me out, and educate me, is incredibly kind of you.
I am using the CB box as a battery disconnect since 2 of the 3 batteries don't have a CB of their own. I wanted to be able to isolate each battery in the start up process. Can you tell me what I gain by getting rid of the CB box?
You have me convinced on the Class-T for each battery. I will have to figure out a mounting scheme to get it as close as possible to each battery. I hate to just "hang" the fuse holder off the cable. Not real professional, probably not safe either.
And it makes sense now to connect all the Lynx components since getting rid of the Class-T downstream of the PowerIn.
Right now, I already upgraded the 325a to a 400a CNN fuse. What is the benefit to an 800a? The 325a/400a was to protect the 4/0 cable. I know I wouldn't need that amp rated fuse now since the 4/0 is eliminated, but why 800a? Because the Lynx busbars are rated to 1000a? And since each battery would now have a 250a fuse...ok, my head hurts.
So basically you are just substituting a Class-T where the MEGA would go. Did you have to modify the class-t holes, etc?
Where did you source your fuses?
Sorry to have bothered you.
And FWIW...I do trust your advice, I hope you didn't think otherwise. If I offended you, I apologize.
John Frum
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The batteries have smart BMSs which you can use for administrative disconnect.
The circuit breakers add significant resistance, significant expense and just add clutter.
This guy said it best
A quote from Airman's Odyssey
Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.
www.goodreads.com
It stops that fuse from getting in the way.
The backplane is rated for 1000 amps continuous.
A 400 amp fuse is a potential bottleneck for no good reason.
Blowing that fuse would be a pain in the ass.
The lower the fuse rating the higher the resistance.
Understood.
see above ^
Yes.
4 batteries each with 250 amp fuse is 1000 aggregate fault amps.
The busbar is good for 1250 fault amps.
We hope the 800 amp fuse never gets in the way.
BTW we can go back and forth between continuous amps and fault amps by using the .8 reciprocal.
continuous amps / .8 = fault amps
fault amps * .8 = continuous amps
Using 2/0 in from batteries and 4/0 out to inverters. Most the fuse should see is less that .4C (40 amps)
Do they get any hotter than MEGA fuses?
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Had to drill those center holes. Easy, soft metal. Didn’t touch other holes.
I bought fuses last year when they were hard to find and expensive. Got some from Alibaba. KingSun Microelectronics CO., LIMITED
I am thinking of buying more to have some spares.
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John Frum
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@AHTrimble are your batteries made by trophy?
They have a different breaker in them.
If yes, I suggest you talk to the owner of trophy and finding out breaking capacity of those breakers.
Remember breaking capacity is voltage specific.
We are looking for > 4000 amps at > 60 volts.
Ideally > 10,000 amps at > 60 volts.
They have a different breaker in them.
If yes, I suggest you talk to the owner of trophy and finding out breaking capacity of those breakers.
Remember breaking capacity is voltage specific.
We are looking for > 4000 amps at > 60 volts.
Ideally > 10,000 amps at > 60 volts.
John Frum
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@AHTrimble what does you being a firefighter have to do with anything?
As long as you are using an appropriate insulation (RHW or equivalent) and you have under ~6' of cable... and aren't doing something stupid like running it in a length of EMT with sharp edges your chances of a bolted fault are near-zero. A lower-level fault might be possible, but the protection from that is really in the BMS.
Personally, I view the string protection as being there to prevent a backfeed fault more than anything else, beyond what the BMS provides. The BMS makes the battery essentially a current-limited source. The battery's UL listing is what makes it "safe" as a component. When you do a DIY pack that is where the AYBC rules make sense, even with lead acid.
John Frum
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Bottom line is I don't trust BMSs, and I see any un-protected wire as a danger that can and should be avoided.
The feeder/branch design pattern is to the best of my knowledge a time tested best known method.
In a dead short scenario, if the BMS fails I think its between possible and probable that all the FETs will fail nearly simultaneously.
And if they do, they can fail closed.
The question is...
q: How much current can be passed through that current limited source?
I can't agree, at least when it comes to server rack batteries.
IMO the signature solar manual makes it clear they don't stand behind their breakers.
Everyone is free to call it as they see it.
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John Frum
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Do you mean 6 inches?
If you mean 6 feet, isn't that a bit arbitrary?
This isn't very pretty, but I use t class fuses to separate each server rack tower from each other. So each one is handling 30-50kWh of LiFePO4 each:
Two were prototypes, the black case one I ordered for another project. But you get the idea. If there is a dead short on one tower, it will be stopped pretty quickly.
When you parallel massive batteries, you need that high AIC rating to avoid 10,000's of Amps of current from flowing. This is how I do it.
I really need to make it more pretty and use a proper bus bar for distribution. This is why I never put this in the videos.
Two were prototypes, the black case one I ordered for another project. But you get the idea. If there is a dead short on one tower, it will be stopped pretty quickly.
When you parallel massive batteries, you need that high AIC rating to avoid 10,000's of Amps of current from flowing. This is how I do it.
I really need to make it more pretty and use a proper bus bar for distribution. This is why I never put this in the videos.
John Frum
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Does that mean you trust the server rack battery breakers to protect the individual batteries?
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