Main Wires Fuse Protection

R

rg12

100 kW
Joined
Jul 26, 2014
Messages
1,591
Most BMS don't really have that protection or have it and it's light years from working properly.

If I don't have a fuse and touch the positive and negative together, when it goes bang and blows the tips of the wires, how much current is flowing there?

and how exactly their protection works?
I've had a friend crash his bike, the motor cable twisted, phases were shorted instantly and the motor blew a phase, controller went bang and the bms of his battery had his fets explode.
It was a 72V 40Ah Pack with a 150A BMS.
Now I figure that if it shorts permanently from the internals of the controller that the BMS will see an over current and should just cut power using the fets, so why did it blow?

Would like to know what is the most common scenario a fuse on the main + of the battery would protect from.

Plus, when I was researching fuses I found out that most use 12V fuses since they are the most common for high currents but the thing is that it seems like a fire hazard when you are dealing with around 80V most of the times with our high powered systems and that means that when using a 12V fuse with an 80V system that it's going to arch when it blows and create a fire.
Any ideas how to solve that?
 
The way you have described the scenario, I don't think you will get any protection from blowing the fets with a fuse inline with the positive battery wire. The phase wires are on a different trace then the power input to the controller and the fets will blow when the phase wires short out. A fuse on the battery might get over amped enough to pop, but the damage to the controller will probably already be done.

As for fuses, big fuses are available if you hunt around on the internet. Bussman 80vdc 100a fast acting fuses go on eBay for around $12.00 USA. I have always kind of wondered why people put underrated components into their systems. As for fires, just make sure the fuse is sealed enough to keep from setting anything on fire.

Sorry about your friend.

:D :bolt:
 
e-beach said:
The way you have described the scenario, I don't think you will get any protection from blowing the fets with a fuse inline with the positive battery wire. The phase wires are on a different trace then the power input to the controller and the fets will blow when the phase wires short out. A fuse on the battery might get over amped enough to pop, but the damage to the controller will probably already be done.

As for fuses, big fuses are available if you hunt around on the internet. Bussman 80vdc 100a fast acting fuses go on eBay for around $12.00 USA. I have always kind of wondered why people put underrated components into their systems. As for fires, just make sure the fuse is sealed enough to keep from setting anything on fire.

Sorry about your friend.

:D :bolt:
Click to expand...

About fets blowing I meant the fets of the BMS, why would they blow?

I just found some 80VDC for $7 on mouser.com but the question is which one do I need for the kind of protection needed for batteries?
Is it only used for shorts? because the BMS is used for over current situations.
There is also the thing with the Slow Blow/Fast Blow...

and yeah, he was unlucky enough to try to charge the pack and it blew the charger too :lol:
 
Fuses are usually to protect the outside wires, to prevent fires, mounted near the battery post.

At those currents a fuse is more expensive than the kind of BMS you're talking about.

Just buy spares.

Or replace the FETs when they blow

 
rg12 said:
......
About fets blowing I meant the fets of the BMS, why would they blow?

........
Click to expand...

That is a question I can't answer except to say that the fets were severely over-volted either on the positive side or the negative side. A post-mortem of the wires would have to be done to know for sure. What we do know is when phase wires shorted the wheel must have continued to spin enough times to pump high voltage back into the system popping fets as it went.

As for slow blow of fast blow, fast blow for sure.
 
So what is the rating I should get if the only protection of the fuse is against shorts?

This is a chart of the fuse I was looking at:
https://www.mouser.co.il/datasheet/2/240/littelfuse_boltdown_housing_fusestrips_48V-1839962.pdf
It seems that an 80A fuse will never blow at 80A and will even hold about 150A for 100 seconds.
Seems like fuse ratings are so misleading as they will just never blow at their rated current.
If I use a 150A controller I can use a 40A fuse even.

Oh and about fuse voltages, if using an 80VDC fuse, that means that I can use any voltage below that right? say 30VDC with and 80VDC fuse...
 
There is no useful "rating" for fuses that is a single number.

Because the effect it is guarding against

heat rising with current flow

is not one dimensional, the **time** a given current level is held is just as important as amps

and that often fluctuates, dozens if not hundreds of times in a second.

The detailed spec sheets for quality fuses will show the plotted **curve**

and the designer of a given system chooses the fuse type where that curve is appropriate for a given point needing protection in the system.

And note that at high rates these devices cost a lot of money to get such documentation and reasonably accurate ratings.

If you want something costing only say $50 each, for some mission-critical purpose, you need to do your own testing to destruction in order to really know their behaviour.

And cannot assume too much consistency from one batch to the next.

All these issues are multiplied especially cost, if you go to CBs rather than fuses.

Fusible links are much cheaper, but their behaviour much more "ballpark" and in need of testing within the IRL system.
 
I understand that it's more complicated than most people (me included) might think.
My question is, when used on the battery's main wire, if using a BMS that stops at the max current of the cells, then what is the remaining job left for the fuse? only for physical accidents and short circuits?

Question is, how much current flows when sticking the positive and negative together?
 
rg12 said:
I understand
My question is, when used on the battery's main wire, if using a BMS that stops at the max current of the cells, then what is the remaining job left for the fuse? only for physical accidents and short circuits?

Question is, how much current flows when sticking the positive and negative together?
Click to expand...


Secondary protection in case the first protection fails. NEVER rely on only one protection. That would be quite stupid, like having only one lifeboat or raft on an ocean going ship.
Standard electrical practice.


Here is an excellent fuse system for low profile and small footprint, proven fast. Goes on 900$~1000$ esc. Available pcb for mounting.
https://www.amainhobbies.com/kontronik-200a-fuse-kosmik-kon9060/p492629
Click to expand...

Kontronick fuse.jpg

Only other ones I would recommend are the CC class in the DIN blocks or the (mini) ANL fuses and fuse blocks...

How much current? Well that depends on the power of the cells.. the IR.. the SCC....

measure it.... or calculate it.

V=IR.

For example, the Chevy Volt cells, I like, SSC for over 2000A @ 25v. Round 24v they do 1800A. To give you an idea. The voltage affects the current.

Definition of 'short circuit current'
short circuit current in Electrical Engineering
(ʃɔrt sɜrkɪt kɜrənt)
Word forms: (regular plural) short circuit currents
noun
(Electrical engineering: Circuits, Electrical power)
A short circuit current is an overcurrent resulting from a short circuit.
If the short circuit current is excessive, then the electrolyte will heat up and either leak or cause the capacitor to explode.
With the ground connection the current would flow through the ground line back to the power source, and this short circuit current would cause the system circuit breaker to trip.
A short circuit current is an overcurrent resulting from a short circuit.
Click to expand...
 
rg12 said:
My question is, when used on the battery's main wire, if using a BMS that stops at the max current of the cells
Click to expand...
Most BMS do not do that.

Many just smoke.

I would not rely on a cheap BMS to do OCP even if they claim to.

Measuring the short circuit peak in a ms or two with accuracy requires very expensive equipment.

Why would you want to know that number?
 
john61ct said:
rg12 said:
My question is, when used on the battery's main wire, if using a BMS that stops at the max current of the cells
Click to expand...
Most BMS do not do that.

Many just smoke.

I would not rely on a cheap BMS to do OCP even if they claim to.

Measuring the short circuit peak in a ms or two with accuracy requires very expensive equipment.

Why would you want to know that number?
Click to expand...

Well, since my use for the fuse will be only for short circuit protection.

I checked LittleFuse's datasheet for their fast blow 80VDC fuses and the 80A BMS (according to the provided chart) seems to never blow at 80A.
It can hold 150A for about 20 seconds which is crazy when you think it protects an over current of an 80A pack.
So I just use a programmable BMS that does the job and want a fuse for short circuit protection but according to their chart it seems that it might make the battery's internals blow before the fuse as it's so durable with huge current 2-3 times over it's rating and I don't feel comfortable using a 35A fuse for a 100A pack just because the chart says so.
 
The fuse is there to stop the wiring (and in our case, battery pack) from burning, in the case of a catastrophic fault in the system (for example, battery or phase wiring short circuit, controller mosfet blows, etc). So by the time a fuse is required to perform, there has already been significant damage to get to that point. The fuse should protect from further damage (particularly fire and electric shock). So a fuse is still a good idea.

You've been given a couple of options for fuses.

But you're asking for solutions, and you haven't told us anything about your setup...
What battery pack is it ? Cells ? Brand/type ? Configuraiton ?
What BMS do you have ? Link to product ?
What current (continuous and peak) do you use ?
What gauge and length battery wiring ?
 
rg12 said:
john61ct said:
rg12 said:
My question is, when used on the battery's main wire, if using a BMS that stops at the max current of the cells
Click to expand...
Most BMS do not do that.

Many just smoke.

I would not rely on a cheap BMS to do OCP even if they claim to.

Measuring the short circuit peak in a ms or two with accuracy requires very expensive equipment.

Why would you want to know that number?
Click to expand...

Well, since my use for the fuse will be only for short circuit protection.

I checked LittleFuse's datasheet for their fast blow 80VDC fuses and the 80A BMS (according to the provided chart) seems to never blow at 80A.
It can hold 150A for about 20 seconds which is crazy when you think it protects an over current of an 80A pack.
So I just use a programmable BMS that does the job and want a fuse for short circuit protection but according to their chart it seems that it might make the battery's internals blow before the fuse as it's so durable with huge current 2-3 times over it's rating and I don't feel comfortable using a 35A fuse for a 100A pack just because the chart says so.
Click to expand...
You do not need to know how high it can get.

You should use a fuse that will blow much much lower than that.

In fact one that will blow just above

the highest current you plan to ever allow.

With a cheap BMS rated for 80A personally I would not allow higher than 70A.

That "not allowing" is the job of a good controller, or perhaps an add-on like CAv3.

Then a fuse could be selected that would blow at say 75A held for 10-30sec

adjust a bit to suit, or what you can find for under $100 or whatever your budget.

The only job of the fuse is as a failsafe if your primary protection (controller / CAv3) fails, or as you say a physical mishap, to prevent total meltdown / explosion etc.

Including that dead short scenario.
 
Thanks alot, that 10-30 seconds is what I was asking for as a kind of rule of thumb as the fuse charts kind of confused me.

From the chart here:
https://www.mouser.co.il/datasheet/2/240/littelfuse_boltdown_housing_fusestrips_48V-1839962.pdf

It seems that even with the fast blow, the fuse for an 80A limited pack would be the 50A fuse.

* I see that in mouser the 63A and 50A fuses by little fuse are only available in 48VCD while i need the 80VDC.
How bad is it if I use a 20S pack with a 48VDC fuse? most use 12V or an AC fuse.
 
rg12 said:
How bad is it if I use a 20S pack with a 48VDC fuse? most use 12V or an AC fuse.
Click to expand...
You should use a fuse with the correct rated voltage. You're installing a safety device. If you cut corners, why bother at all ?

Some people use lower voltage rated switches, and they justify this by only using the switch when there is no load (so there is no chance of a high current arc). But for a fuse ? That's just a big gamble.

84VDC is around the open circuit voltage for DC tig/stick welders. The ideal voltage for striking an arc. So consider the arc that a welder can generate with 80A+ (and your short circuit current will be significantly higher). Do you really want to trust the wrong fuse to break that arc ? Have you seen the photos on this forum of people with :kff: ?

WRT fuse current ratings: the rated current is usually where the fuse can continuously operate while dissipating a set limit of power (for example, BS 1362 specifies 1W). Some fuse datasheets list the power, or sometimes the voltage drop (@ rated current).
 
serious_sam said:
rg12 said:
How bad is it if I use a 20S pack with a 48VDC fuse? most use 12V or an AC fuse.
Click to expand...
You should use a fuse with the correct rated voltage. You're installing a safety device. If you cut corners, why bother at all ?

Some people use lower voltage rated switches, and they justify this by only using the switch when there is no load (so there is no chance of a high current arc). But for a fuse ? That's just a big gamble.

84VDC is around the open circuit voltage for DC tig/stick welders. The ideal voltage for striking an arc. So consider the arc that a welder can generate with 80A+ (and your short circuit current will be significantly higher). Do you really want to trust the wrong fuse to break that arc ? Have you seen the photos on this forum of people with :kff: ?

WRT fuse current ratings: the rated current is usually where the fuse can continuously operate while dissipating a set limit of power (for example, BS 1362 specifies 1W). Some fuse datasheets list the power, or sometimes the voltage drop (@ rated current).
Click to expand...

Am not looking to cut corners at all but it's so hard to find high dc voltage rated fuses for high currents and when you find them they have an 80A, 100A...but not 50A and below for 80VDC.

Was also concerned about resistance and voltage drop by using a fuse...
 
rg12 said:
How bad is it if I use a 20S pack with a 48VDC fuse? most use 12V or an AC fuse.


Was also concerned about resistance and voltage drop by using a fuse...
Click to expand...

Negligible.
 
Great so a fuse rated for 80VDC is for MAX 80VDC right? meaning I can use it with 48VDC?
 
Yes even an 800V fuse is fine for 80V, they just get expensive

but be 100% sure that is rated for ** DC ** voltage

AC fuse voltage ratings are in a different universe.

If you PM @methods he might link to one he reco's

just stick to the good stuff do not go cheap chinese unless you are prepared to buy a bunch in different ratings and test them yourself.
 
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