I'm a novice planning a new RV system. I'm trying to figure out where fuses are necessary and where they are not. I'm trying to understand the purpose of the "main" fuse in this system described by @Will Prowse . I think I understand the purpose of the 50 amp breaker between the SCC and the other components, since the input and output circuits are common. I also think I understand the purpose of the 25 amp fuse protecting the 24v->12v converter that Will shows in this picture. Finally, I think I understand why you would want a "main" fuse if the battery is lead-acid. My question is whether the "main" fuse is necessary if the battery output is from a BMS, and if so why. What protection would a "main" fuse provide that the output current overload circuitry of the BMS is not already providing? Also, if the BMS were of the separate-port variety with input current overload protection, would that also remove the need for the 50 amp breaker on the SCC? Thanks in advance for your help.
Is a "main" fuse or breaker required between a BMS and external loads/inputs?
- Thread starter mnally
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Yes, you need a big fuse between the battery and your common bus bar. The fuse would likely blow much faster than the BMS could react.
The way mine is setup, it's like this: Common Bus Bar <- Switch <- Fuse (Class T - 250 amp) <- BMS <- Battery
The way mine is setup, it's like this: Common Bus Bar <- Switch <- Fuse (Class T - 250 amp) <- BMS <- Battery
John Frum
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The fuse usually goes on the positive leg and the bms usually goes on the negative.
Yep, you're right. Not sure why I threw the BMS in there.
Thanks for your replies. I don't doubt your advice but I would like to understand better why it is true. I looked at the specs of a number of BMSes and they all advertised independent short-circuit protection and over-current protection. For some of them, at least, the over-current parameters are adjustable, which I assume allows them to simulate fast and slow fuses. I still don't understand why it is not correct to think of the BMS itself as a sophisticated "main breaker". Is the provision of an additional main fuse simply a "belt and braces" approach to provide some redundancy, or does the fuse provide a unique protection that the BMS cannot? I also don't understand why it would be significant that the fuse is on the negative or positive side; I imagine there has to be a full circuit from positive back to the negative for current to flow from the battery, so why would the position of the fuse in the circuit matter? I think the advice you are giving me is likely correct and it seems to be common practice, but I am still not understanding why it is so.
In theory a BMS advertised as that can be used as an e-fuse. But the main problem is that mosfets fail shorted more often than they fail open. And in case of cheap chinese BMS you don't know if it's actually capable of doing what the label says (trust me, it's not simple to interrupt a multi kA short circuit), and how reliable it will do it even if it is capable of doing it.
So even with good quality parts, good design, etc... I'd always put a fuse on the battery. The only places e-fuses are used AFAIK is in planes (because they have a lot of advantages directly translating to more safety for that application over traditional breakers) but they are triply redundant systems, extremely well designed and manufactured, and well tested (of course all that add a lot to the price...).
It doesn't really matters if it goes on the positive or negative side but it's usually simpler to put it on the positive (no BMS, shunt, etc... like on the negative side) and it's the most common way to do that.
That was my take also. By putting the fuse on the positive side it evens out the "legs" from the battery.
Given those explanations, would it not be a good idea for manufacturers of complete lithium batteries to put a fuse in addition to a bms inside the battery, rather than relying on users to add a fuse externally? I don't recall seeing an internal fuse in the battery tear-down videos I've seen. Any thoughts on why? Perhaps it is not practical because most battery cases seem to be sealed and can't be opened without destroying them.
Possibly. An internal fuse would be convenient and safe in some ways, but on the other hand, it would make it hard or impossible for a user to replace a fuse if it blows, and safety would still rely on the user selecting the right wire size for the fuse (while takig away the users ability to size the fuse to the wire). One option would be for a manufacturer to include a fuse sized for the peak discharge rating at the battery and stipulate very strongly that wire needs to be capable of handling at least that much current. But this would be inconvenient for those with only small loads, as they would be forced into using large gauge wire if they want the fuse to protect the wire. It would also mean in a large battery bank, there would be many fuses between parallel or series connected batteries (i'm not sure if this would be good or bad on the whole).
I like your thought process and your inquisitiveness and desire to understand the concepts. I'm not convinced this is a good idea, but I haven't thought about it before, and I like thought experiments. I'm curious what others think of it?
You can always add a smaller external fuse and just ignore the internal one. You can also imagine a removable cover over the internal fuse to be able to put a different fuse (and change a blown one, sloves two problems at once)
I don't see how it could be a problem. Better have too many fuses than too few.
True, definitely a solvable problem, but then there are trade-offs. I'm thinking of warrantied drop in batteries like the Battleborn for instance, it seems both the warranty (which I'm sure is based at least in part on the inability of consumers to meddle with the internals) and the weather resistance of the battery might be affected by altering the design so it was not sealed. Of course this is a solvable problem to, a separate compartment where only the fuses is accessible maybe. But again more trade-offs and mitigations. One of which is cost. A fuse capable of interrupting the current a lithium battery can produce (like a class T) is not cheap. If it might be improperly sized and require an extra fuse anyways, or if you have 4 or 8 batteries in your bank, that is potentially a lot of extra cost and a bit of extra complexity.You can always add a smaller external fuse and just ignore the internal one. You can also imagine a removable cover over the internal fuse to be able to put a different fuse (and change a blown one, sloves two problems at once)
I'm not saying it couldn't work, but with all factors considered (price, warranty, redesign) I wonder if the juice would be worth the squeeze, it might well be, this is just my initial reaction and it might be that i'm overlooking something or just plane not imaginative enough. There might be a market for it, I do like some elements of the idea for sure, an internal fuse definitely appeals to both my OCD and my tendency towards conservative design side
At the end of the day more choice and more options and more innovation successful and not is a good thing
Its a small problem maybe, but conceptually, the more connections, the more chance of poor connections, and greater voltage drop
I certainly think its workable if there is a desire for a product like that, the main question in my eyes is whether the marginal benefits outweigh the marginal costs
Yep, like on some DMMs.
Given the price of ready-made batteries even a 50 $ fuse is totally possible, they make a biiig margin... plus they would be able to buy fuses for less than we do as they can order in large quantities.
Yeah but your connections should be correct to begin with, else you'll have problems, fuse or not.
Yep, and when marketing has the final word over the engineers you get the current result: no fuse.
Hopefully marketing depts will not remove seatbelts from cars in the future... ?
Fair points
I've never considered this a problem or a marketing decisions (maybe it is who knows). The way I see it there are legitimate design (and cost) trade-offs to a modular approach or an integrated approach. Even setting the cost issue aside, there are usually legit engineering or design pros and cons between the two approachs (speaking generally, not just about batteries). And in many cases a simple solution is a good solution (though it could be argued either way which option is simpler depending on your perspective).
Good ones do. Have a look at the Discover Blue Lithium. Field replaceable fuses
Solarismaximus
New Member
True, definitely a solvable problem, but then there are trade-offs. I'm thinking of warrantied drop in batteries like the Battleborn for instance, it seems both the warranty (which I'm sure is based at least in part on the inability of consumers to meddle with the internals) and the weather resistance of the battery might be affected by altering the design so it was not sealed. Of course this is a solvable problem to, a separate compartment where only the fuses is accessible maybe. But again more trade-offs and mitigations. One of which is cost. A fuse capable of interrupting the current a lithium battery can produce (like a class T) is not cheap. If it might be improperly sized and require an extra fuse anyways, or if you have 4 or 8 batteries in your bank, that is potentially a lot of extra cost and a bit of extra complexity.
I'm not saying it couldn't work, but with all factors considered (price, warranty, redesign) I wonder if the juice would be worth the squeeze, it might well be, this is just my initial reaction and it might be that i'm overlooking something or just plane not imaginative enough. There might be a market for it, I do like some elements of the idea for sure, an internal fuse definitely appeals to both my OCD and my tendency towards v conservative design side
At the end of the day more choice and more options and more innovation successful and not is a good thing
Its a small problem maybe, but conceptually, the mor be connections, the more chance of poor connections, and greater voltage drop
I
I certainly think its workable if there is a desire for a product like that, the main question in my eyes is whether the marginal benefits outweigh the marginal costs
