Fuse between battery and shut off switch?

[bds70]

Have a look at Nigels Calders latest video on utube

Just did, and it's extremely informative. This is the link

I wonder if perhaps the moderators might consider linking that as a sticky for diy safety.

What he describes would mean my home would be a goner in the event of a dead short with only MRBF fuses at the terminals.

 

[bds70]

You want t-class fuses or very high AIC breakers on each of the batteries so they don't short into each other on failure. You want one larger t-class fuse or very high AIC breaker exiting the combined battery bus bar to the rest of the system to prevent the batteries from dumping high amperage into the rest of the system if something shorts on the rest of the system (not the batteries themselves).

I am flip flopping a bit here. Can U clarify.
All my pos terminals are in a single straight line joined by a 10mm x 20mm solid copper bar. (Ditto for the negs) The way I have designed the boxes there is a gap of 150mm between bars. If there is a physical barrier completely preventing any form of physical contact between those bars, how can a short occur?
Each battery has its own JK BMS which has its own short circuit protection that would cut current in the event of a internal short.
My original idea was to have a 500amp mega fuse at the end of the Pos bar and appropriate cable of 250mm to the lynx distributor which will have a further 400 amp mega fuse on that connection.

If there is no chance of a physical dead short why would I need additional expensive fusing on each batt POS terminal?

 

[J.H.]

No need to explain, I did some research and now understand. I wish I had this knowledge before doing an 80km round trip to pick up 4 MRBF 250A fuses. Now I shall have to take them back and get T fuses. . However it's good that I now know. Thanks for the knowledge

We all go through this… it’s part of the self induced Hazing Ritual to be in DIY Solar…

 

[cs1234]

I am flip flopping a bit here. Can U clarify.
All my pos terminals are in a single straight line joined by a 10mm x 20mm solid copper bar. (Ditto for the negs) The way I have designed the boxes there is a gap of 150mm between bars. If there is a physical barrier completely preventing any form of physical contact between those bars, how can a short occur?
Each battery has its own JK BMS which has its own short circuit protection that would cut current in the event of a internal short.
My original idea was to have a 500amp mega fuse at the end of the Pos bar and appropriate cable of 250mm to the lynx distributor which will have a further 400 amp mega fuse on that connection.

If there is no chance of a physical dead short why would I need additional expensive fusing on each batt POS terminal?

A batteries BMS is not to be relied upon as protection from an internal short from the battery. Nothing on most BMS's are generally capable of high AIC protection. That's what the fuse on each battery is for. A fuse for each individual battery protects against the batteries dumping current into each other during a short and causing them all to become unhappy.

A fuse at the busbar where the batteries are all grouped together protects against dumping current into the inverters and other connected devices. It does nothing to protect the batteries from each other.

EDIT: When I say fuse, I am referring to a high AIC fast blow t-class fuse.. something around 20,000 AIC or so for a typical 48vdc 100ah battery.

 

[robbob2112]

The typical BMS or inverter has a bunch of MOSFETs tied to them which is how they turnon and off. When a FET fails it fails as a short 99% of the time. So you can have a dead short at the output of either device. If you have a BMS that uses contactors to disconnect it that is a bit better if the MOSFET in some ways and worse in others. The contactor has to be right at the output andif say it is on the other side of the shunt you get threads of house burned down.

You really do need a high AIC fuse right at the output of the battery.

And if you are think of a breaker instead of a fuse because they AIC I'd way higher, like 10x higher, that is a thought. But, it operates about 10x slower than a fuse.

 

[bds70]

The typical BMS or inverter has a bunch of MOSFETs tied to them which is how they turnon and off. When a FET fails it fails as a short 99% of the time. So you can have a dead short at the output of either device. If you have a BMS that uses contactors to disconnect it that is a bit better if the MOSFET in some ways and worse in others. The contactor has to be right at the output andif say it is on the other side of the shunt you get threads of house burned down.

You really do need a high AIC fuse right at the output of the battery.

And if you are think of a breaker instead of a fuse because they AIC I'd way higher, like 10x higher, that is a thought. But, it operates about 10x slower than a fuse.

Damn it all, seems like I have to bite the bullet and go rob a bank somewhere. Only problem is I am too old to outrun the cops wonder if I have any rich long lost relatives to knock off.??? Hmmmm.
Thanks all for the patience and clarifications.

 

[J.H.]

Damn it all, seems like I have to bite the bullet and go rob a bank somewhere. Only problem is I am too old to outrun the cops wonder if I have any rich long lost relatives to knock off.??? Hmmmm.
Thanks all for the patience and clarifications.

Do like Americans do…. Finance it ..max out those cards …sue somebody .. Do a HELOC ..

 

[robbob2112]

You can use the NH style fuses - they are square with the same sort of sand filler in them to quench the arc and have similar time/current curves.... Or you can use BS88 fuses - they are round bolt-down ones. Look at the Mersen brand for both although eaton makes them too.

Either of those options even with the holder will cost about 1/3rd the cost of a class T fuse.

note - I got some of the NH style off aliexpress made by Chint - they are certified by a german equivalent of UL listing so I think they are ok.

The real downside of the NH style is they take up a lot of space. And if you get the cheap holder they hve a lot of naked metal exposed so would need a plastic shield over them that allows airflow. If you get the NH style make sure to get the puller handle at the same time, they are damned hard to get out of the mounts by hand with the help of a flat blade screwdriver.

You can also get a sealed mount for the NH style but they are pretty pricey.

And I resemble that remark - maxed my cards with my wife's help and the HELOC is going strong.

 

[bds70]

You can use the NH style fuses - they are square with the same sort of sand filler in them to quench the arc and have similar time/current curves.... Or you can use BS88 fuses - they are round bolt-down ones. Look at the Mersen brand for both although eaton makes them too.

Either of those options even with the holder will cost about 1/3rd the cost of a class T fuse.

note - I got some of the NH style off aliexpress made by Chint - they are certified by a german equivalent of UL listing so I think they are ok.

The real downside of the NH style is they take up a lot of space. And if you get the cheap holder they hve a lot of naked metal exposed so would need a plastic shield over them that allows airflow. If you get the NH style make sure to get the puller handle at the same time, they are damned hard to get out of the mounts by hand with the help of a flat blade screwdriver.

You can also get a sealed mount for the NH style but they are pretty pricey.

And I resemble that remark - maxed my cards with my wife's help and the HELOC is going strong.

So I am looking at both types mentioned. The AIC on an NH1 (Mersen) is quoted as 120ka AC

What does that convert to for DC break current?

 

[TimE]

So I am looking at both types mentioned. The AIC on an NH1 (Mersen) is quoted as 120ka AC

What does that convert to for DC break current?

You need to find an NH fuse rated for DC

 

[rhino]

Remember, the AIC is rated based on a given voltage. The lower the system voltage the higher the AIC rating of a given fuse/breaker is. This rating indicates how much current the fuse/breaker can interrupt. Voltage plays significant role here when dealing with DC current. The lower the DC voltage the easier it is to stop the current and prevent it from "jumping" a gap (like a blown fuse or breaker tripped). That is why with 12V and 24V battery banks people don't generally use class T fuses since the cheaper ones should suffice.

 

[TimE]

Remember, the AIC is rated based on a given voltage. The lower the system voltage the higher the AIC rating of a given fuse/breaker is. This rating indicates how much current the fuse/breaker can interrupt. Voltage plays significant role here when dealing with DC current. The lower the DC voltage the easier it is to stop the current and prevent it from "jumping" a gap (like a blown fuse or breaker tripped). That is why with 12V and 24V battery banks people don't generally use class T fuses since the cheaper ones should suffice.

Should or will?

 

[robbob2112]

So I am looking at both types mentioned. The AIC on an NH1 (Mersen) is quoted as 120ka AC

What does that convert to for DC break current?

Look for the gG series of them

An example
500vac
250vdc

B219122C - NH000GG50V100/3 | Mersen Electrical Power: Fuses, Surge Protective Devices, Cooling & Bus Bars

DIN NH Standard fuse-link gG Size NH000 500VAC 250VDC 100A live tags Double quantity 3 Body width 21mm

ep-us.mersen.com

 

[bds70]

This may be a heretic post, but after hours of frustration trying to find suitable fuses at costs that are less than the cost of my entire 16 cells and 4 BMS's a few thoughts come to mind.

Every decision in life is in some sense a risk/reward decision.

So what are the realistic chances of internal dead shorts on brand new lifepo4 cells, A grade, never used before. I can find no research that is not PhD level that even discusses this aspect.

Second, it occurs to me that introducing 4 fuses into my circuitry also introduces 4 more points of resistance and therefore potential heat and possible failure points.

Third, if precautions taken can 100% rule out any form of external dead short causes, leaving only internal failures then the risk becomes point one again.

So financially with all the permutations of T fuses and holders, NH fuses and holders, or even FH type I am left thinking "Is it worth it" particularly because my two previous setups (done by commercial 'professionals') of 8 AGM 250ah batteries and then 3 320ah lifepo4 batteries, none of which were individually fused gave me no problems over 7 years.

Back to risk/reward, how often in peoples experience have your batteries internally shorted and blown up?

Sorry to be so negative but I am very frustrated over this conundrum.

"What is the answer?" he cries into the howling night air.

 

[robbob2112]

Why the boggle? I suppose it looked me 5 minutes because I know what I was after


Some examples - find your needed value And the open holder

https://au.element14.com/siba/20-000-13-100a/fuse-general-line-nh-100a/dp/24860

https://au.element14.com/siba/20-001-13-125a/fuse-general-line-nh-125a/dp/248617

- Fuseco

These are popular fuses due to their reliable performance, low price and blown fuse indicator function.

www.fuseco.com.au

- Fuseco

www.fuseco.com.au

 

[bds70]

Why the boggle? I suppose it looked me 5 minutes because I know what I was after


Some examples - find your needed value And the open holder

https://au.element14.com/siba/20-000-13-100a/fuse-general-line-nh-100a/dp/24860

https://au.element14.com/siba/20-001-13-125a/fuse-general-line-nh-125a/dp/248617

- Fuseco

These are popular fuses due to their reliable performance, low price and blown fuse indicator function.

www.fuseco.com.au

- Fuseco

www.fuseco.com.au

I suppose because of this, the AIC of this 250amp fuse is 120Ka, no mention of the DC break current of 20,000amp required for lifepo4, according to the posts above.

So yeah I am confused and a bit grumpy.

 

[Luk88]

I suppose because of this, the AIC of this 250amp fuse is 120Ka, no mention of the DC break current of 20,000amp required for lifepo4, according to the posts above.

So yeah I am confused and a bit grumpy.

I'm very reluctant to advise anyone to use a wrong fuse, but I use large NH00 AC 500V rated fuses mainly because the DC versions are unobtanium and cost many times more. So if you were to decide f...k it, its too much hassle, I'm not putting any fuses. Consider these 500V AC rated fuses in NH0 (sometimes written as NH1 or NH00/NH2) size:


You will not get the 110kA AIC, but ~25kA is probably a reasonable estimate for the larger (NH2) size and 15kA for the smaller (NH1). I'd skip the smallest NH3 altogether.

They cost about $10 each and the holder around $20 here. I don't expect them to be much more expensive elsewhere.

Edit: If you do choose such fuses please pay attention to their characteristics. The one on the photo is gF (or fast) this is what I would use if I wanted it to trigger ASAP when the current gets exceeded 1.5x. Some people buy smaller amp rating, but with delayed characteristics. There are also fuses with aR (very fast), gR (still very fast, but a little slower), gS (fast, but slower than gR) all faster than "fast". Here is an internal drawing showing one of elements of these aR - semiconductor protecting (I'll believe it when I see it) fuses




This is how it is installed (sorry for text in polish):

 

[robbob2112]

I suppose because of this, the AIC of this 250amp fuse is 120Ka, no mention of the DC break current of 20,000amp required for lifepo4, according to the posts above.

So yeah I am confused and a bit grumpy.

Read the datasheet For the fuses. Most of the NH style are labeled in AC, but the datasheet lists the DC characteristics

 

[Luk88]

Read the datasheet For the fuses. Most of the NH style are labeled in AC, but the datasheet lists the DC characteristics

Can you link to one? I've read everything I can find about these including a huge catalog published by my manufacturer (ETI) and there is nothing about their DC characteristics.

How could they sell same fuses with different label for 6x the price if they just "gave away" this info?

But seriously, if you found a manufactyrwr that does list DC characteristics for AC rated NH2 fuses I'm very interested in it.

 

[bds70]

Read the datasheet For the fuses. Most of the NH style are labeled in AC, but the datasheet lists the DC characteristics

I have attached the full data sheet for the entire range of Fusemaster NH fuses. I can't see any mention of DC interrupt ratings. Can you have a look and point out what I should be looking for. It might help me a lot.
Cheers

 

[bds70]

I think at the moment I am tending toward using the 250A MRBF fuses I bought the other day, as they are clearly marked 10000A @ 14V DC AIC, one to each battery and then the 500amp T fuse between the batteries and the lynx distributor. I know that's not "technically" correct. But given the difficulty of finding out the DC interrupt rating of other fuses, and my "Risk Analysis" based on 7 plus years of full time bus traveling 100% off grid, all over Australia , without any issues of dead shorts blowing up batteries or my home, I feel that is the best way forward at this time.

However I am definitely still open to being pointed in a better direction as long as the solution is financially viable.

A long time ago I bought a car that had been written off by insurance Co because of severe hailstone damage. Here we can buy such a vehicle, have it assessed for being roadworthy and have it re registered. But you can't get fully comprehensive insurance because it had been a write off. So I have had to have only third party insurance. As a result I have saved about 5K in insurance premiums. I have not had any accidents although tomorrow I could. But the Risk Reward benefit has clearly been mine.

I think this situation falls into the same basket.

Happy to continue to receive advice

 

[bds70]

I'm very reluctant to advise anyone to use a wrong fuse, but I use large NH00 AC 500V rated fuses mainly because the DC versions are unobtanium and cost many times more. So if you were to decide f...k it, its too much hassle, I'm not putting any fuses. Consider these 500V AC rated fuses in NH0 (sometimes written as NH1 or NH00/NH2) size:
View attachment 233254

You will not get the 110kA AIC, but ~25kA is probably a reasonable estimate for the larger (NH2) size and 15kA for the smaller (NH1). I'd skip the smallest NH3 altogether.

They cost about $10 each and the holder around $20 here. I don't expect them to be much more expensive elsewhere.

Edit: If you do choose such fuses please pay attention to their characteristics. The one on the photo is gF (or fast) this is what I would use if I wanted it to trigger ASAP when the current gets exceeded 1.5x. Some people buy smaller amp rating, but with delayed characteristics. There are also fuses with aR (very fast), gR (still very fast, but a little slower), gS (fast, but slower than gR) all faster than "fast". Here is an internal drawing showing one of elements of these aR - semiconductor protecting (I'll believe it when I see it) fuses

View attachment 233255


This is how it is installed (sorry for text in polish):
View attachment 233256

Thanks for that, but the AIC ratings are again for AC current which requires lower amps to blow, than DC. So how do they perform on DC batteries.

 

[Luk88]

I think at the moment I am tending toward using the 250A MRBF fuses I bought the other day, as they are clearly marked 10000A @ 14V DC AIC, one to each battery and then the 500amp T fuse between the batteries and the lynx distributor. I know that's not "technically" correct. But given the difficulty of finding out the DC interrupt rating of other fuses,

I think one can roughly guesstimate AIC of a fuse by the length and the amount of quartz sand inside. This is a part of where my guesstimate of 25kAIC for NH2 comes from.

I believe you're are using 12V lifepo batteries. So 10kAIC is probably fine for a 100AH 12V battery.

My batteries are all 16S (48V+) and the smallest is 280AH. Thats a completely different ball came.

and my "Risk Analysis" based on 7 plus years of full time bus traveling 100% off grid, all over Australia , without any issues of dead shorts blowing up batteries or my home,

I saw a very interesting video yesterday made by another Australian about a guy who entered a lift while carrying an ebike battery (li-ion) and within 3s he was dead. (Venting, ignition and explosion all on camera).

The guy talking about this incident said one interesting thing. "The risk of having battery isues is very low, but potential consequences when you do can be huge" and "we should risk managed accordingly" (he is for more education of the public and signs not to bring such batteries in to enclosed spaces you can't escape or apartments). If add the difference between lipo,li-ion and lifepo to this.

This conversation about fusing is somewhat similar. Hopefully I'll never have to replace one of these fuses, but if I do, they better be up to the job!

 

[Luk88]

Thanks for that, but the AIC ratings are again for AC current which requires lower amps to blow, than DC. So how do they perform on DC batteries.

We don't know for sure. I can only guess as said before. I'm happy with such guess for myself, but I'll always advise others to get a proper DC rated fuse. There are DC rated fuses in the same form factor. They are usually rated at 25kAIC DC at this size (NH2). I never saw bigger ones.

 

[bds70]

I think one can roughly guesstimate AIC of a fuse by the length and the amount of quartz sand inside. This is a part of where my guesstimate of 25kAIC for NH2 comes from.

I believe you're are using 12V lifepo batteries. So 10kAIC is probably fine for a 100AH 12V battery.

My batteries are all 16S (48V+) and the smallest is 280AH. Thats a completely different ball came.


I saw a very interesting video yesterday made by another Australian about a guy who entered a lift while carrying an ebike battery (li-ion) and within 3s he was dead. (Venting, ignition and explosion all on camera).

The guy talking about this incident said one interesting thing. "The risk of having battery isues is very low, but potential consequences when you do can be huge" and "we should risk managed accordingly" (he is for more education of the public and signs not to bring such batteries in to enclosed spaces you can't escape or apartments). If add the difference between lipo,li-ion and lifepo to this.

This conversation about fusing is somewhat similar. Hopefully I'll never have to replace one of these fuses, but if I do, they better be up to the job!

I agree totally, but that's one of the reasons insurance companies are billion dollar conglomerates, they trade on fear of the "what if" yet only a tiny percentage of all insurances are ever claimed upon compared to the number taken out.

It comes down to risk factors. More people die on our roads every year, yet we keep driving faster and longer than ever.

People are still happily flying on Boeing aircraft despite the last few disasters... Risk Analysis is by definition - risky.

I wouldn't go down the no fuse road, but I think it's just as risky to "assume" the specs of a fuse, when that manufacturer doesn't specifically state the data you need to ensure the investment provides the safety you want.

There must be a reason most manufacturers will not print a DC interrupt rating. Perhaps it's because the fuse characteristics work differently between the two current types?

My bank is 4 x 280Ah 12v lifepo4 in parallel.

I don't know. But I know I am confused. Lol.