Fuse Questions

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rg12

100 kW
Joined
Jul 26, 2014
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1,591
I'm looking for a fuse that will blow when the current reaches 120A.
I know that fuses have slow blow, regular etc and have a rating for peak current and for how long.
Is there a fuse that just blows at the current it's rated for on the dot?
How accurate are they?
Can I count on the accuracy of those flat rectangle 120A fuses on ebay?
Do they burn faster/slower with different voltages?
AND how come nobody talks about performance? they are thinner than the wire so there is added resistance.
 
You'd need an electronic "fuse" for that kind of precision. For a regular thermal fuse a "quick blow" variety would probably be best if you can find one in the size and type you need. A fuse must be able to pass rated current continuously without getting more than warm, but to blow it must get much, much hotter so it can melt. That heat requires an appreciable increase in current.

Good fuses will come with a specification showing the time to blow at different currents.

Voltage doesn't make any difference to the blow characteristics, only current generates heat. Fuses do have a voltage rating though and if you go above this the fuse is not guaranteed to break the circuit - it might blow but continue to arc across the gap in the fuse which will usually result in a fire. Fuses with higher voltage ratings have features to extinguish the arc. So don't use 12/24/30V rated automotive fuses on a 72V ebike for example unless you or someone else has tested them first.

They are thin and they do create resistance in the circuit, but because the fuse element is very short the loss (heat, voltage drop) is small and acceptable.
 
Why would you need such a precise rating?
 
Here's a fuse tutorial. https://www.littelfuse.com/~/media/electronics/product_catalogs/littelfuse_fuseology_selection_guide.pdf.pdf
 
This is from the brand I'm interested in:
https://www.littelfuse.com/~/media/automotive/datasheets/fuses/passenger-car-and-commercial-vehicle/bolt-down-fuses/littelfuse_bf1_58v_datasheet.pdf

What does pre arching times mean?

This chart makes the fuse rating useless as a 50A fuse will blow at 100A after 10s and at 140A after 1s and will never really blow at 50A.
 
rg12 said:
This chart makes the fuse rating useless as a 50A fuse will blow at 100A after 10s and at 140A after 1s and will never really blow at 50A.
Click to expand...
that is how fuses generally work.

please elaborate on WHY you need such a precise trip point.
 
Can't trust a BMS, they always let you go above the cut off rating and the high end ones are huge and cost like the whole battery.
 
It's your controllers job to limit amps, not the bms except in a failure like a downstream short. Then it's faster than a fuse, and re-settable.
 
rg12 said:
Can't trust a BMS, they always let you go above the cut off rating and the high end ones are huge and cost like the whole battery.
Click to expand...

that is not how this works.

a bms or a fuse does nothing to limit power comsumption. your device (in this case a motor controller) regulates how much power is being converted into AC for the motor. nothing else.

a fuse or BMS is only there to intervene when something goes wrong, it does not limit power. and a half-decent bms can be set to have various trigger speeds on high loads (usually a few seconds) right down to milliseconds when detecting a dead short, faster then a normal megafuse could respond.

you are trying do the equivalent of using a airbag for slowing down instead of using the brake. you can do it but you might get tired of replacing the airbag at each intersection.
 
Some of my customers use a programmable controller or a controller that pulls more current then the pack allows.
I've worked with many different brands and not all cut off power where they should.
My current one is rated 150A and 300A peak and when I asked how long is the peak they said 0.38s which is ok for me but I've has a customer with an ASI BAC4000 who pulled 280A with only a few cut offs here and there but he managed to pull it for accelerations for a few good seconds.
It's hard for me to test BMS at those amps so I tried to find a solution for an extra level of safety.
 
Pretty much any old controller is going to be way more accurate than a fuse. If they go over a predefined limit, then program the controller to limit a little lower. Not too hard to verify this with a big ass shunt popped (poor choice of word) into the battery input even for amp draws in the several hundreds.
If you really want a fuse, at least use something resettable. It's really going to suck when a customer takes off at the lights and then pop dead 1/2 way to work etc.
 
you can program some bms to make a difference between having a dead short and just a set power limit and is exeeded for more then the set time.
but the trick is to properly callibrate those bms, some are off by a considerable amount in the current range.
you can also set the reset time after a overcurrent situation so you can just kill the power for a full minute as i do in some batteries to make it clear to the customer where the limit is and their device/vehicle just dies for a full minute (that is a long time on public roads)

this is a cheap bms that can be programmed and callibrated by yourself: https://www.aliexpress.com/item/32826469363.html

you might need to invest in a accurat current clamp and more high end multimeter in order to callibrate the offsets.

NEVER use a single blow (mega) fuse in these cases, or put its rating way beyond a reasonable use case so it only blows if the bms fails for example. a fuse must never blow under "normal" use and should be rated beyond the offical rating of the cells.
 
I know it's not the BMS purpose but I'm selling those packs and customers solder shunts or use huge ass controllers that can kill a battery within a few month if the max current should be X and they draw 1.5X

I guess the solution of a different programmable BMS is a good idea but the one in the link (very known one in AliExpress) seems like a great solution but it's shape is horrible, it's too long and too thick, especially the too long is the problem for the shapes that I'm building.
I usually use a 13.5*14*1cm.
Will look into more programmable ones I guess.
Thanks alot :)
 
yes, those stated bms is bigger, but they can take 300A from factory with the janky chinese mosfets and probably 1000A if you replace them with good ones. i modified one that runs at 700A peak with real mosfets and a small heatsink.
you can split the boards and probably remove the top alu plate as it only serves to protect the pcb and dissipate the balance heat.

and if you use the chinese app you can set a password to change settings so your customers cant frock around with your protection settings.
 
flippy said:
yes, those stated bms is bigger, but they can take 300A from factory with the janky chinese mosfets and probably 1000A if you replace them with good ones. i modified one that runs at 700A peak with real mosfets and a small heatsink.
you can split the boards and probably remove the top alu plate as it only serves to protect the pcb and dissipate the balance heat.

and if you use the chinese app you can set a password to change settings so your customers cant frock around with your protection settings.
Click to expand...

Nice!
Thanks for the info!
 
I see it's a problem if a customer abuses a battery and then tries to make a warranty claim. However, if you make absolute sure to use only genuine, good quality cells of appropriate performance (C-rate) and longevity and clearly state the operating limits for the battery (charge & discharge rate, maximum operating and storage temperature) then you can reject a warranty claim for any battery with premature ageing (capacity loss, excessive IR/sag).
 
rg12 said:
I'm looking for a fuse that will blow when the current reaches 120A.
Click to expand...
To know exact behaviour, currents and timings of a fuse, you have to study the curves in its datasheet. Fuses don't just "break", it's a matter of time and energy.
 
I've pulled apart lots of ebike batteries that have 'died' where the fuse has been so hot that all the surrounding plastic is heat affected, and the contacts are corroded out but the fuse is still patent. Long periods of just sub critical current. Must have been like a dim little light bulb in there....not good.
 
Punx0r said:
I see it's a problem if a customer abuses a battery and then tries to make a warranty claim. However, if you make absolute sure to use only genuine, good quality cells of appropriate performance (C-rate) and longevity and clearly state the operating limits for the battery (charge & discharge rate, maximum operating and storage temperature) then you can reject a warranty claim for any battery with premature ageing (capacity loss, excessive IR/sag).
Click to expand...


Still ain't a solution as I want to point out to the customer what he did wrong instead of seeming like this dodgy business trying to get away with providing warranty.
That's why I thought of a fuse (which apparently doesn't work as I thought) to point out that he crossed the current limit.
A mad customer can go on a public forum and say that I'm an a*hole or that I sell fake stuff because he ignored the warranty limits and killed the cells with over current.
 
If you want that kind of defense against every possible kind of abusive/lying/a***hole customer then you need a device (BMS or otherwise) that can log temperature, discharge & charge current and cycle count for the life of the pack.

A pack charged & discharged at the correct rates but charged when too cold will die early
A pack used in all ways correctly but deeply cycle 2 or 3 times a day will die early
A pack used in all ways correctly but kept in a bag or vehicle in a hot country will die early

Protecting against all such scenarios is quite difficult, but the response of high quality cells to such abuse is well established and predictable. Conversely these cells are so consistent in their manufacturing you can safely say if they have failed early they must have been abused.
 
programmable bms like the one i linked to can simply be set to refuse charging or discharging at certain temperature ranges, so it can be just disabled if you try and charge it at below freezing or above 40c for example.
setting the right voltages so it wont exeed limits is part of protecting yourself against undue warranty claims.

still, if customers mistreat a battery it simply means they have the wrong one for the task. generally i make a note in the final invoice that excludes warranty claims due to exessive use/wear in case customers dont want to take my advice and upgrade to a properly sized battery. this usually makes it also clear to customers that they are willingly killing their expensive battery and i am not willing to play their game and replace worn out ones.
some also tend to complain about this specific line even after i warned them about it that i can legally refuse warranty claims as they are planning to abuse a battery and EU laws allow me to refuse their claims. something that already happend by a specific customer that also tried to pull the same thing with another company and they did gave him a free replacement once after i asked them about this specific customer. turns out he was simply riding warranty claims for years as he was too cheap to buy a battery that was sized for the job. i countersued him for every penny i had to spend dealing with this guy and had gotten a signed affidavit from the other company proving he did it on purpose. in the end he was forced to pay me back and pay a fine for filing false claims.

so yea, ensuring you have a battery system that protects the battery well is also needed to protect you as a builder. and having logging and a huge option list to program the bms to protect both the battery and you is vital as warranty claims are simply not worth it due to the cost for you as a builder to make a replacemt battery. id rather spend more (that i simply add tot he final price) then get stuck with a bill for 500 replacement cells and no profit.
 
flippy said:
programmable bms like the one i linked to can simply be set to refuse charging or discharging at certain temperature ranges, so it can be just disabled if you try and charge it at below freezing or above 40c for example.
setting the right voltages so it wont exeed limits is part of protecting yourself against undue warranty claims.

still, if customers mistreat a battery it simply means they have the wrong one for the task. generally i make a note in the final invoice that excludes warranty claims due to exessive use/wear in case customers dont want to take my advice and upgrade to a properly sized battery. this usually makes it also clear to customers that they are willingly killing their expensive battery and i am not willing to play their game and replace worn out ones.
some also tend to complain about this specific line even after i warned them about it that i can legally refuse warranty claims as they are planning to abuse a battery and EU laws allow me to refuse their claims. something that already happend by a specific customer that also tried to pull the same thing with another company and they did gave him a free replacement once after i asked them about this specific customer. turns out he was simply riding warranty claims for years as he was too cheap to buy a battery that was sized for the job. i countersued him for every penny i had to spend dealing with this guy and had gotten a signed affidavit from the other company proving he did it on purpose. in the end he was forced to pay me back and pay a fine for filing false claims.

so yea, ensuring you have a battery system that protects the battery well is also needed to protect you as a builder. and having logging and a huge option list to program the bms to protect both the battery and you is vital as warranty claims are simply not worth it due to the cost for you as a builder to make a replacemt battery. id rather spend more (that i simply add tot he final price) then get stuck with a bill for 500 replacement cells and no profit.
Click to expand...

Thanks alot man, this is exactly what I'm going to do as I'm going through the exact same thing with dumb a**holes trying to get freebies through warranty.
I had one customer say he used an 80A controller, I said ok, that shouldn't happen from only 80A, then I recognized the bike from a for sale ad for the same bike that a friend sent me and he wrote that it's a modded controller (soldered shunt), so I sent him the photo from the sale saying that he lied when I asked him if he soldered the shunt so he said that he wrote it on the ad but it's not true, lame excuse, anyway he backed up and never contacted me again.
 
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