That's why the cells in a Tesla batteries are individually fused. We're definitely doing a tradeoff here and it's possible to have an "internal" short - something the fuse can't catch.
LiFePO4 prismatic cell short circuit current and main circuit protection
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That's why the cells in a Tesla batteries are individually fused. We're definitely doing a tradeoff here and it's possible to have an "internal" short - something the fuse can't catch.
pablo fiasco
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I'm sorry if I'm posting this out of context. I haven't gotten the hang of this forum. =
I got the SCCR for my batteries from the mfg. 400A. I expressed surprise saying that I had been lead to expect much bigger numbers and warned repeatedly to use a Class-T fuse because of their AIC. It seems there is ‘short circuit protection’ built into the BMS. There is the caveat that a protracted short circuit could cause the battery to catch fire. To round out the picture my batteries describe as LiFePO4 100Ah 12V 1280Wh. If someone out there wants to apply some insight and math I’d be very happy hear their conclusions. Meanwhile I am more confident that a MRBF will be fine for my system.
I got the SCCR for my batteries from the mfg. 400A. I expressed surprise saying that I had been lead to expect much bigger numbers and warned repeatedly to use a Class-T fuse because of their AIC. It seems there is ‘short circuit protection’ built into the BMS. There is the caveat that a protracted short circuit could cause the battery to catch fire. To round out the picture my batteries describe as LiFePO4 100Ah 12V 1280Wh. If someone out there wants to apply some insight and math I’d be very happy hear their conclusions. Meanwhile I am more confident that a MRBF will be fine for my system.
John Frum
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Personally I would never trust a fet based bms as my last line of defense.
The 280ah@25.6vdc battery assembly I'm in the process of building will have...
A class t-fuse off the positive terminal
A fet based BMS off the negative terminal
Plus there will be an mrbf fuse on the busbar where the positive wire terminates.
I really like mrbf fuses but the aic rating is 5000A @ 32VDC.
The class-t is 50,000A @ 160VDC.
pablo fiasco
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Sounds kinda paranoid extreme but what do I know. I'm really only concerned with my particular situation and feeling okay with what I've learned and the choices I've made. 160VDC!! Holy doodle!!
I have no problem with my FET based BMS being my last line of defense. If it was my first line of defense, that would be something to worry about.
I went with a Class T fuse on the positive lead coming off the battery. It's what was recommended, so I went with it.
I went with a Class T fuse on the positive lead coming off the battery. It's what was recommended, so I went with it.
John Frum
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My class t fuse cost roughly the same price as the BMS.
If things go sideways I hope the ~$10.00CAD MRBF takes one for the team.
pablo fiasco
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I was warned repeatedly that the sky would fall on my head if I didn't use Class-T also. I also run with scissors. I've been called an idiot. My answer is; What, me worry.?
John Frum
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If you're not worried, I'm not worried.
(I've also read the replies below).
Use a fuse. What if the BMS short-circuits? The point of a fuse is to protect against downstream failures and a BMS is a great example of something that can fail. A BMS with short-circuit protection is not a fuse. A fuse is simple. A BMS is not.
Do you have to? Of course not - it's your money and your house or whatever that you're risking and only you can determine the level of risk you're willing to take. We're just here to inform you so you can make an informed decision.
Who is the the manufacturer of your batteries? (note: if we are talking about a drop-in there is a big difference between the brand/assembler of the battery pack and the manufacturer of the cells in most cases)
It sounds like they probably just quoted the overcurrent / short-circuit protection specification of the BMS, which is not the same as what is being discussed with regard to AIC
ArthurEld
Solar Wizard
Do you have to fuse the negative side to protect the BMS? I'm wondering if a fuse on the positive side would always protect the BMS.
It is possible to short from past the BMS to the battery
ArthurEld
Solar Wizard
I installed a 225A class T fuse between my on/off and + busbar.
But now I am wondering if I should have a class T on the positive terminal as well or instead.
I am also wondering if I should fuse my battery pack. Since I have 5 custom busbars already that are longer, it would be easy for me to cut a piece out of the center and bolt on a class T fuse. The blue markings are where the 100A class T fuses could go. My pack has more space there because it has threaded rod going between the cells. What do you think?
But now I am wondering if I should have a class T on the positive terminal as well or instead.
I am also wondering if I should fuse my battery pack. Since I have 5 custom busbars already that are longer, it would be easy for me to cut a piece out of the center and bolt on a class T fuse. The blue markings are where the 100A class T fuses could go. My pack has more space there because it has threaded rod going between the cells. What do you think?
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I think that would be expensive. Added safety, sure. Necessary? Not sure.
Can you mount it to the wood frame next to the main + and bend a 90 deg busbar to connect it? I wouldn't hang it off the cell directly.
I think ABYC specifies up to 7" of unprotected wire between the battery and fuse.
That wire looks small for being able to carry 225A
I think ABYC specifies up to 7" of unprotected wire between the battery and fuse.
That wire looks small for being able to carry 225A
ArthurEld
Solar Wizard
Thanks, there is a 100A fuse in the MRBF fuse holder. I plan to connect a second battery to the MRBF busbar.Can you mount it to the wood frame next to the main + and bend a 90 deg busbar to connect it? I wouldn't hang it off the cell directly.
I think ABYC specifies up to 7" of unprotected wire between the battery and fuse.
That wire looks small for being able to carry 225A
And I will upgrade the cable.
I like your idea to attach the fuse to the wood in front. I will probably do that with a 100A class T fuse for each battery.
ArthurEld
Solar Wizard
I like it sduser. What size fuse is yours?
It's a 150A fuse in a Blue Sea 5007100 holder. In hindsight, I should have gone with a 110A fuse which is the smallest that will fit in that holder.
ArthurEld
Solar Wizard
I don't plan to go over 100A. It would be really easy to make a fuse holder but those covered ones are so nice.
By the way, the cable on my 225A fuse is 4 AWG which is acceptable. I will be using much bigger cable when I get my Solark 12K.
I am just messing around with a MPP Solar 3048 now. Max amps is 60 because I don't have solar connected.
But I can still short.
edit: silly me
I am just messing around with a MPP Solar 3048 now. Max amps is 60 because I don't have solar connected.
But I can still short.
edit: silly me
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I don't think this is accurate, even if you are using 105*C rated wire. (unless you meant 4/0?)
I've attached wire ampacity charts from the NEC and the ABYC
Attachments
ArthurEld
Solar Wizard
One thing to be aware of (general point, not directed at you specifically), many of the wire size charts out there (like the one in the screenshot you posted), only consider voltage drop and make the (dangerous) assumption that people will realize they have to figure out ampacity on their own.
At long distances or low voltages this is usually a non-issue because voltage drop is a constraint before ampacity, a voltage drop table on its own can lead to dangerous conclusions.
side note: you can tell from the power ratings, the table above is a 12v table.
At long distances or low voltages this is usually a non-issue because voltage drop is a constraint before ampacity, a voltage drop table on its own can lead to dangerous conclusions.
side note: you can tell from the power ratings, the table above is a 12v table.
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ArthurEld
Solar Wizard
Alt E put pre made 4/0 cables in my package. I looked it up and I couldn't make them cheaper.
I am anxious to get my hands on that stuff. But I have to wait until my permit is issued.
I hope to get things moving this week.
I am anxious to get my hands on that stuff. But I have to wait until my permit is issued.
I hope to get things moving this week.
ArthurEld
Solar Wizard
Everyone is down on the cost of class T fuses but 100A Little Fuse JNN-100 are available for less than $20.
It wouldn't be that bad to put a few of them on a battery.
The reason I am considering more fuses is because when I shorted my battery the fuse didn't help. A class T on the positive terminal probably wouldn't have helped either. There was another disastrous short by AussieInSeattle and the fuse near the positive terminal didn't blow.
If splitting up the battery with fuses will stop that kind of short it seems worthwhile to me.
It wouldn't be that bad to put a few of them on a battery.
The reason I am considering more fuses is because when I shorted my battery the fuse didn't help. A class T on the positive terminal probably wouldn't have helped either. There was another disastrous short by AussieInSeattle and the fuse near the positive terminal didn't blow.
If splitting up the battery with fuses will stop that kind of short it seems worthwhile to me.
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