Wiring PTC Fuses

[redwolfe]

I have these fuses coming for my DNA20 to add some extra short circuit protection insurance and did a schematic drawing on how they might be wired and was wondering if it was correct. They are both in parallel. Also do I add the fuses directly to the tabs? I want to make sure I am doing this part right.

[CraigHB]

The wiring is good and you can solder leaded fuses directly to the tab.  FYI, you'd have to make a little PCB for the SMD type, but you could solder the tab to a pad on a PCB.

The tabs on those cells are a bit tricky to solder.  Scuff them well with ScotchBrite.  Apply flux to them and use a good rosin core 60/40 or 63/37 solder.  You must use a variable temp iron no higher than 600F.  A 30W soldering pencil is about 900F and could melt the insulators on the tabs possibly causing the cell to short out and ignite.  It's not a good idea to solder the tabs more than once, otherwise you risk damage to the cell.

[redwolfe]

Thanks Craig! I appreciate the advice. I've been continuously rethinking my design and how I want to do it. I think I've come up with something that will work how I want it.

[LukeTheDuke]

thanks for the info craig

[tvBilly]

Would you not want to wire the individual fuses to each of the battery's positive terminals, and then combine the other connections of the two fuses together, rather than just put the two fuses in parallel? Doing so would prevent one battery's failure from drawing excessive current from the other battery.

I understand that you have two fuses in parallel to drop their (total) resistance so as to minimize voltage drop during high current conditions, but if you wire each fuse to one battery, and combine the other side of the fuses together, half the current will pass though each fuse, which means (I think) the voltage drop will be the same.

Just a thought (and one I mentioned in the original thread from mamu).

[CraigHB]

Don't really understand what you mean.  This is for a single battery.  Two fuses in parallel cuts the resistance in half and doubles the trip current.  With these 20C cells, the trip current can be quite high and better to have lower resistance to minimize power loss.  There's no other configuration you can use with two fuses and a single cell.

If you're talking about putting a PTC fuse on each tab, that's going to result in a series configuration which is incorrect.  You'd only be doubling the insertion resistance and doubling the power loss.

[tvBilly]

Forgive me, I thought we were talking about two batteries in parallel.

[mamu]

I've been wiring 2 fuses in parallel using 2 18650 batts in parallel as shown in A below.  With li-poly batts in parallel, I solder the 2 positive tabs together, then attach one end of the paralleled fuses to the soldered tabs.

But now I'm wondering - should each batt in a parallel configuration have its own fuse (2x fuse wired in parallel on each batt) as shown in B below? 

Or does it matter?



ETA: this is how I wired the paralleled fuses in Milo with the parallel li-poly batts...

[Breaktru]

I use a fuse for each cell like your setup for your drawing "B". I got the idea from our resident EE, Craig when I was doing the parallel charging circuit.

I took this from the site "All About Circuits":
Batteries have been known to internally short-circuit, due to electrode separator failure, causing a problem not unlike that where batteries of unequal voltage are connected in parallel: the good batteries will overpower the failed (lower voltage) battery, causing relatively large currents within the batteries' connecting wires.

[mamu]

Thanks, breaktru.

I was wondering about independent fuses for each batt when I was working with the reverse polarity FET on this new DNA flip-top mod I'm doing that has parallel 18650 batts.  Each batt requires its own FET to have full protection in case one of the two batts is put in backwards, so thought hmmm... I wonder if that's necessary for each batt to have its own fuse too.

[CraigHB]

I was going to comment on that, but Breaktru already pointed it out.  For parallel removable cells, a fuse on each cell protects the pack from excessive equalization currents.  So, yes "B" is the preferable configuration.  For LiPos hardwired in parallel, you can go either way, whichever is easier to install.

You're actually doing double duty using a transistor for reverse polarity protection along with a fuse for short circuit protection.  The fuse can actually do the MOSFET's job.  The problem with reverse polarity is not that reverse voltage is applied, but rather the very high currents that occur when components are subjected to reverse polarity.  The fuse can protect those components from damage by limiting current even though it's flowing backward.

It's not a bad idea to use both the transistor and the fuse, but you can probably get away with just the fuse if you want to absolutely minimize part count, utilized space, and insertion loss.

[mamu]

...For LiPos hardwired in parallel, you can go either way, whichever is easier to install.

I've used a common fuse over this last year with my mods that have internal parallel li-poly batts.  So that's good news there that's it ok.

I've been leery to test the DNA for reverse polarity with just having a fuse inline lol 40 bucks down the clinker if it doesn't work out, but think I will set that up and see what's what.  Would be great not to have the extra part or to do that extra wiring with the FET.

Here's the issue with parallel 18650 batts with a common fuse like what I showed in A and what got me to wondering about the common fuse configuration I had been using over this last year - I tested this and inserting one batt in backwards and inserting the other correctly causes a meltdown.  So a common fuse is no go there, but a fuse on each batt prevents that. 

But I've only tested that with a common fuse/FET combo and independent fuse/FET combo, not with using just a fuse - always with the FET too because I thought the DNA needed the FET.

[Breaktru]

I worry about a single battery failure and causing thermal runaway in the other battery making it catch fire so I use a fuse for each battery

[mamu]

ok... I worked up the nerve to test reverse polarity with the DNA using just an external fuse without a FET in line.  It took me a while thinking about if I really wanted to try this though.  I was thinking 40 bucks down the clinker for a DNA if it doesn't work out. 

Adding an external fuse prevents overcurrent and reverse polarity for the OKR and Naos Raptor boards, but those are buck modules using batts in series while the DNA is a boost module using either a single batt or batts in parallel.  Didn't know if that made a difference, just wasn't sure, so I've been using a fuse/FET combo with the DNA for my mods that have removable batts to protect from overcurrent as well as reverse polarity.

So... here goes...I used the Efest 30A batts for testing - I didn't care if I blow those pos.   

Both batts inserted correctly...


Both batts inserted backwards... both red LEDs are lit indicating both batts are inserted backwards...


One batt inserted backwards, the other batt inserted correctly...red LED is lit indicating which batt is inserted backwards... the problem with this set up, other than a batt is inserted backwards  is that without the red LED indicator there's no system warning and the DNA works fine.


Many many thanks Craig for giving me the incentive to test this.   

Note1: for this to work correctly, you need a fuse on each batt if you're running batts in parallel - you can't use a common fuse for both batts else you'll get a batt meltdown from the short when inserting one batt backwards and one correctly.

Note2: these are the fuses (wired 2x in parallel) I tested overcurrent and reverse polarity with the DNA20D - 16R300GU and MINISMDC260F/16-2
 

[Breaktru]

Nothing like real world testing. Well done Mamu

[CraigHB]

Always good to hear that things stated in theory prove to be true on the bench.  I know with my own boosters a fuse alone will protect them from reverse polarity, but you don't know for sure until you risk hardware to test it on the bench.  Good job on that.

[CraigHB]

Oh, one other comment you might find helpful, by running a fuse on each cell you are accomplishing the same thing in terms of reduction of losses as running two fuses in parallel for the whole pack.  So in that case, you can get the same reduction of loss putting a single fuse on each as cell as you would two fuses in parallel for the whole pack.

[CraigHB]

And yet another comment, they keep popping into my head.  PTC fuses don't actually break the connection when they trip.  Rather they go into a high impedance state that limits current flow to a tenth of an Amp or two.  They are heat driven devices so when the fault condition is removed, they cool and return to their normal low impedance state.

That being the case, if you were to insert one of the cells backward and leave it like that, both cells would become overdischarged after an amount of time depending on the current flow of the fuse in its tripped state.  The user would notice the short run time of only a single cell and that would be a good indicator of a problem, but if left uncorrected, it would damage both of the cells with an over-discharge condition at some point. 

It's an issue with parallel removable cells that does not have a simple solution.  It's pretty much up to the user to ensure cells are not left in that condition for an extended period. 

I suppose it would be possible to use an LED indicator of some sort, maybe inside the enclosure visible when changing cells, but you can run into the same constant drain issue there.  Even when LEDs are off and not conducting, they still have some leakage.  Though it may be be do-able, would have to look at the reverse leakage characteristics of the LED.  If it's only a few microamps, it would not be a problem.

[mamu]

I was a bit nervous to try it, but so glad I did.   When I was working with the DNA12 and had it breadboarded I lost a dual parallel DNA12 to not paying attention when I inserted the batt.  Both the DNA12 and batt got hot hot hot and a bit of smoke from the DNA12 before it blew.  *poof* 70 bucks down the tube.

Oh, one other comment you might find helpful, by running a fuse on each cell you are accomplishing the same thing in terms of reduction of losses as running two fuses in parallel for the whole pack.  So in that case, you can get the same reduction of loss putting a single fuse on each as cell as you would two fuses in parallel for the whole pack.

That is helpful to know, Craig.  Thanks!

For the mod I just did, I put 2x 3A fuses in parallel on each batt (running 2 batts in parallel).  If I have just 1 fuse on each batt instead of the 2x in parallel, I would need a single fuse with a 6A hold current on each batt, correct?

In the mod I just did, I put a red LED in line on each batt to indicate reverse polarity. I don't mind the bit of drain from the LED when it's not lit vs not having some sort of indicator if one or both batts are inserted correctly.

Both batts in backwards...


Both batts in correctly...

[CraigHB]

I was a bit nervous to try it, but so glad I did.   When I was working with the DNA12 and had it breadboarded I lost a dual parallel DNA12 to not paying attention when I inserted the batt.  Both the DNA12 and batt got hot hot hot and a bit of smoke from the DNA12 before it blew.  *poof* 70 bucks down the tube.

Frying parts is something that just seems to happen at some point when working with electronics a lot.  I blew a whole build the same way I'm ashamed to admit.  In my case, the cell is normally hardwired so I don't need reverse polarity protection.  The MCU monitors current flow so there's short circuit protection there, but I don't like to rely on that alone as the only safety mechanism.  I use a temperature sensor on the converter's switching MOSFETs in lieu of a PTC fuse.  I'd rather use a fuse, but I don't want to afford the voltage drop or the additional PCB real estate for one so I rely on temperature monitoring as a backup.

Anyway, I use a clip-on cell when testing and I accidentally hooked it up backwards and fried the build.  That's about $50 worth of parts and a lot of my time for assembly.   I normally use a convention that makes it difficult to connect the cell backwards when testing, but I was doing something unusual that time, don't recall what it was. 

I looked at some of the data sheets I have for LEDs and none of them indicate reverse leakage characteristics.  Normally that's something listed in data sheets for other types of diodes, but they don't do it for LEDs.  I just grabbed one of my LEDs I have laying around and checked it.  The reverse leakage doesn't even register on my ammeter which has resolution down to 10nA.  The indicator LEDs should not result in any kind of drain when off with reverse polarity on them so forget about that comment I made before.

[CraigHB]

I would need a single fuse with a 6A hold current on each batt, correct?

Oh, I didn't answer your question. 

Yes, you can add hold currents like that.  One 6A fuse is equivalent to two 3A fuses in parallel. 

You can look at maximal outputs to determine fuse requirements.  If you're designing a mod for a maximal output of say 6 Volts and 5A, that would be 30 Watts.  In terms of loading, converters only care about power so that would be the same as 3 Volts and 10 Amps or any other combination that produces an output of 30 Watts.

A good converter is at least 80% efficient at maximal outputs so that means worst case it needs an extra 20% for losses.  Adding 20%, that's an input power requirement of 36 Watts.  You can find maximal current by using minimal battery voltage.  For a single cell, that would be 3V at the least, or 6V at the least for series cells.

For this example, maximal input current is 36 Watts divided by minimal input voltage so that would be 6 Amps for series cells and 12 Amps for a single or parallel cells.

With dual parallel cells, each cell carries half the total draw so it would be 6 Amps per cell requiring a 6 Amp hold fuse on each cell.  For a single cell, you would need a 12 Amp hold fuse or two 6 Amp hold fuses in parallel.

For the DNA20, you use 20 Watts maximal output with 80% efficiency requiring 24 Watts input.  Maximal battery draw will be 8A amps for a worst case input voltage of 3 Volts.  I think they state 7 Amps in the specs since the cuttoff is a bit higher and the efficiency a bit better for that board.  I usually pad things to some extent there anyway.

You can go under an amount on hold currents if you need to.  The hold current is the guaranteed amount of current that will not result in any change of fuse resistance.  They don't actually start to trip until they get to the trip current which is a good amount higher than the hold current.  Even then, the trip time varies with the level of overload current.  They trip pretty slow at the trip current and trip very fast when largely above the trip current.  You can look at the trip time charts in a part's data sheet to see exactly.

[mamu]

I've been choosing a fuse hold current based on the max current of the converter, or just a little under as I did with Raptor which has 20A max so I chose 18A for the fuse.  Plus there is that cushion of the trip current being a bit higher than the hold current.

My thinking was I didn't want the converter to see that max current going through it under load or the vaper to vape at that max, so a fuse would limit that. 

For fuses to trip that takes an extreme condition and only if they're pushed that far.  Most vapers, not all lol, don't vape anywhere near the max capable of the converter so they should never see a fuse trip condition as long as the batt is stable.

[Breaktru]

That's very informative Craig. Thank you so much. I didn't realize that converter efficiency plays a role in determining the fuse size.
I will now use your info to calculate fuse size in the future. I've always took the lazy way out by looking at the datasheet. You have opened my eyes my friend.

[CraigHB]

One thing that's really tricky about PTC fuses is they are heat driven devices.  I've run into an issue before were peripheral PCB heating can cause them to trip early.  So, you don't really want to rely on them as a means for over-current protection.  They're really most suited to protect against high current faults like short circuits and heavy overloads.  That includes reverse polarity which is essentially a heavy overload.

If you design for a trip current equal to the the maximum current allowed, you can get into a situation where a heavy load can trip the fuse if there's any external heating, which there likely will be when packed tightly with heat generating components. 

There is a good margin there since trip currents are typically double the hold current, but I don't think I would get too close to that trip current if possible.  An extra 50% is probably okay, but I wouldn't go more than that.  For a example, I would not subject a 3A hold / 6A trip fuse to more than 4.5 Amps normally.

[mamu]

Makes sense - thanks Craig.

I understand the reasoning for not using fuses as overcurrent protection from internal or external heating - we don't want them to trip for that purpose - but do want them to trip for heavy current loads such as an unstable batt or short circuit.

So here's my dilemma - I have like a kazillion of the 3A fuses (both the SMD ones and the leaded ones) that I've been using 2x 3A in parallel for the DNA20D.  I hate to waste those parts, but will if I need to.

The trip current on the leaded fuse is 5.1A with a 1s trip time.  The trip current on the SMD fuse is 5A with a 5s trip time.  2x in parallel doubles that trip current.

With using 24W/3V for the DNA20, hold current should be 8A.  For the DNA30, with using 36W/3v hold current should be 12A.

I've not had any issues with the fuse tripping with using 2x 3A in parallel while in normal operating mode with the mod, but that doesn't mean I might in the future.

And what I've realized with this new setup I have by running 2x 3A in parallel on each batt when running 2 batts in parallel, I now have a hold current of 12A with a trip current of 20A for the DNA20.  ach.  lol.

This is what I tested with though - 2x 3A on each batt for a total of 12A - for reverse polarity on the DNA20.  The fuse reacted fast enough to protect the DNA20 and I played with putting one or both batt in backwards several times with no apparent damage to the batts or the DNA20.

I reckon the good news is that with the DNA30, the setup of using 2x 3A for each batt in parallel for a total of 12A is going to be perfect.

[CraigHB]

All sounds reasonable to me.  With two 3A hold fuses, you have a 6A hold which should be fine for a load of 8A.

The reason PTC fuses don't do a good job at over-current protection is because trip times are very slow when fault currents are close to the trip current.  However, when currents are high due to a short circuit or heavy overload, they trip very fast doing a good job of protecting components from damage.

With PTC fuses, you're mainly concnerned with trip times for very high currents.  Since the batteries we use are typically low impedance, fault currents in the case of a short circuit are very high which means trip times are very low, plenty low enough to protect components from damage.  So, you could go pretty high on the trip current rating and still get good protection out of the fuse.

Specifically, you can look at battery internal impedance plus circuit impedance.  Using that with a short on the atomizer connector you can arrive at an amperage value.  For my own mods, they have a fault current of 45 to 60 Amps for a short circuit.  I have 25 mOhms internal impedance for the cell and about 45 mOhms in the circuitry between the battery and atomizer connector.  So, I would look at trip times for a fault current of around 40 Amps.

In your case, the Sony cells have about 12 mOhms each so together they have 6 mOhms.  However, the contacts and wiring do have some resistance, probably around 10 mOhms.  The resistance introduced by the DNA20 is probably in the neighborhood of 50 mOhms.  So, you're dealing with similar short circuit fault currents and should be looking at trip times in the area of 40 Amps.

[mamu]

TY for your help, advice, and guidance, Craig!!  Very very much appreciated.

It's so much easier for someone to tell another here just use this... but you always take the time to explain why and that's awesome!

[CraigHB]

Welcome, I do get a little wordy sometimes, wondering if that's a help or an annoyance

[Romelee]

I have been reading this thread with great interest but for the hell of my i cannot get my head around things (getting there slowly)

This is what i have at present:

Hold Current: 3A
Time to Trip: 1s
Trip Current: 5.1A
Voltage Rating: 16v
http://uk.rs-online.com/web/p/resettable-wire-ended-fuses/5176900/

Would i need 2 x of the above fuses in parallel for each battery?

If i was to try and use a single fuse per battery instead of parallel would the following fuse be ok?

Hold Current: 6A
Time to Trip: 3.3s
Trip Current: 10.2a (this bit i am unsure of)
Voltage Rating: 16v
http://uk.rs-online.com/web/p/resettable-wire-ended-fuses/5176944/


Or is it 1 x single fuse 12A per battery with the following details:

Hold Current: 12A
Time to Trip: 7.5s
Trip Current: 20.4A
Voltage Rating: 16v
http://uk.rs-online.com/web/p/resettable-wire-ended-fuses/5176994/

This is my battery configuration:

[Breaktru]

I have been reading this thread with great interest but for the hell of my i cannot get my head around things (getting there slowly)

With two 3A hold fuses, you have a 6A hold which should be fine for a load of 8A.

One 3A fuse for each battery

[Breaktru]

I have a question about fusing for the OKR-T/10
50W w/ 90% efficiency = 55W
55W / 6V (2 x batts low V) = 9.17A fuse.

The Datasheet says Input Amperage full load is 4.53A and recommends a 20A external fuse

[mamu]

Datasheets are meant to confuse. 

I wonder if that 20A fuse recommendation has anything to do with "Current Limit Inception 19 Amps".

For the OKT-T6, the datasheet recommends a 10A fuse and the Current Limit Inception is 9.8 Amps.  Calculating a fuse for the T-6 gives 32.7W/6v = 5.45A.

From my understanding, that Current Limit Inception is the converter's onboard protection for current limiting and short-circuit protection.  I've tested reverse polarity with the OKR without a fuse and when putting the batt in backwards the OKR is protected, but the batts are not.  But I don't know for how long the converter is protected as I pulled the batts when they got hot.

It seems the datasheet is recommending a fuse just an unch above (either that or they're just rounding up the fuse value) the Current Limit Inception value and an external fuse is just for heavy overload protection such as reverse polarity to protect both the batts and the converter from current greater than the Current Limit Inception.  I think lol.

The OKR datasheets also says 49Kohm pulldown to ground.  49Kohm is too high and won't shut the converter off - need a 10Kohm pulldown to ground to turn the converter off.

It also says "Vin must be 2V or higher than Vout for 3.3 to 6V outputs: Vin >= (2V + Vout)".  Maybe that's to maintain efficiency, but we're running these OKR modules less than that 2v input/output voltage difference and not having any hiccups in function.

[mamu]

I've been reading about fuses, breaktru, and maybe even if the OKR datasheet recommends a 20A fuse for the T-10, using Craig's calculation to select a fuse makes more sense for our purposes.  I dunno, but it makes sense to me especially since we want to protect the batts from meltdown as well as the circuit.
-As a general rule, select fuse types to provide the sizing of current (ampere) ratings as low as practical for a circuit without incurring unnecessary fuse opening for normal circuit operation. This provides optimum overcurrent protection.

Some interesting facts about paralleling fuses:

-Derating is necessary because two fuses in close proximity radiate heat less effectively than single fuses. As a general rule, a minimum reduction in rated current of 10% for 2 fuses in parallel (20% for 3 or 4 fuses in parallel).  Example: 2x 3A fuses in parallel should be derated from 6A to 5.4A and 3x 3A fuses in parallel should be derated from 9A to 7.2A.

-The voltage rating of 2 fuses in parallel can be less than the voltage rating of a single fuse. The derating factor can be 15%.

-Fuses for parallel connection must be of identical type and rating.

-For a given prospective short circuit current, the duty on parallel fuses is eased, as the current in each fuse is inversely proportional to the number in parallel.

-An ideal parallel arrangement will give equal current sharing, but even if completely balanced paths are not achieved, a small degree of self compensation will occur.

[Breaktru]

My main concern is to protect against battery failure. I was thinking maybe two parallel 4.5A hold fuses
Like this --> PPTC 4.5A 16V 0.011ohm Axial

[mamu]

I bought some of these that have similar specs to the ones you have POLYSWITCH PTC RESET 4.5A STRAP


I'm doing an OKR-T10 flip-top mod this week and want to write a tute for it and was planning to use 2x in parallel (9A) or maybe 3x in parallel (13.5A) , or I can use 3x 3A fuses in parallel (9A) or maybe 4x 3A fuses in parallel (12A).

decisions decisions lol

ETA: dang - I just noticed the price on yours, breaktru.  half of what I paid.  dang digikey and their markups. 

[CraigHB]

The Datasheet says Input Amperage full load is 4.53A and recommends a 20A external fuse

That input current data is based on a nominal 12V input.  In that case, the 50 Watts maximal output and 90% efficiency would indicate the 4.53A you're seeing in the ordering guide. 

I would expect that the OKR-T/10 can supply 50 Watts ouput over the full input voltage range.  That's something I'd take for granted, but it doesn't say specifically in the data sheet that I can see.  For a 6V input, you would see the 9A you arrived at before.

I think the "20A" fuse they're calling out is pretty much a number out of a hat.  Sort of a standard fuse you would use for short circuit protection in a typical application.

You get the best protecton when fuse trip currents are closer to the maximal current the system is designed to handle, but you don't want to be right on top of it either.  If you put the hold current on top of the maximum allowable current, you should be good, but you can also stretch that a bit if you need to.  I'd probably run my trip currents a little closer to the maximal current, but not more than 50% over the hold current.

[CraigHB]

Some interesting facts about paralleling fuses:

Yes, very good points.  There are some important considerations when running fuses in parallel.  That's why you don't really want to run more than two that way.  The biggest one in my mind is the fact you are splitting current between them so in effect, your fault currents are halved and trip times are doubled.

Two fuses in parallel gives you a tolerable amount of derating and halves the insertion loss which is a reasonable trade-off.  Any more than two in parallel results in unacceptable derating issues. 

I was actually going to comment on this before, but I forgot.  Glad you brought it up.

[mamu]

I'm glad you mentioned that, Craig.

Note to self: nix on running 3x or 4x fuses in parallel.   

One last question: when I put 2x fuses in parallel that have a hold current of 4.5A each and a trip current of 9A each (the trip current of a single fuse is close to the max current of the system), is each fuse tripping at 9A or are they acting as one unit and tripping at the combined total of 18A? 

If they're tripping at the total, then that's way over the max current of the system and I'd have to choose fuses with half that trip current which means the hold current would also be lower.

I had thought that when 2x fuses are in parallel, they now have 2x the hold current, 2x the trip current, 2x the trip time, and half the resistance - and they act as one unit by providing that combined total hold current and trip current just as a single fuse does.  Is this wrong?  Or is each fuse when using 2x in parallel still acting individually when the fault of the system is at their individual trip current?

I'm hoping that this is the last question I have about fuses and that maybe I now get it. 

[tvBilly]

With the two fuses in parallel, combined they should have 2x the hold current, 2x the trip current, and half the resistance, BUT they will still have 1x the trip time. And since the two fuses/batteries won't be identical, when the first leg goes (opens), the second one will have ~ twice the current going through it, and it should trip almost instantly.

At least that's what it looks like to me...

[mamu]

If the maximal current of the system is 9A and I choose a fuse with a 4.5A hold current that has a 9A trip current and put 2x in parallel, I was curious at what point does "the first leg goes (opens)" when running 2x in parallel  - does that first leg open at 9A or at 18A?  Or is there an in between gray area?

[tvBilly]

Keeping in mind that we're currently (no pun intended) talking about one battery and one load and two inline parallel fuses (since this thread has also been talking about two batteries in parallel, etc.), the first fuse to open in the pair will open at it's rated point, which is >9A in your question. At that instant, since the load resistance (be it the atty or a dead short) is remaining constant, and the voltage is remaining constant, the current through the remaining fuse will double. That remaining fuse is still rated at 9A trip, and hence will probably trip a few (?) milliseconds later.

As to the "between gray area" question, absolutely. But it's not in the "math", it's in the characteristics of the fuses and how they react to whatever kind of "failure" you're talking about (e.g., are we just drawing a little too much current for a little to long, or are we putting a dead short across the battery because our atty just had a delrin failure).

I feel better about letting Craig and the other's get into this part of the discussion though, as my knowledge of fuse characteristics is pretty limited. It hadn't even occured to me to take into consideration the thermal effects of having the two parallel fuses touching each other, and how that would de-rate them... 

[CraigHB]

Notice the mention of "compensation" in the points Mamu brrought up in post #32.  What that refers to is that no two parts are exactly the same.  One fuse will always have slightly less resistance that the other.  This consideration is something that pops up when paralleling diodes and transistors as well.  The one with lower voltage drop tends to carry the burden of the load.  It's especially a problem when using high power switches such as in automotive use where arrays of semiconductor devices in parallel are required to switch large currents.

In the case of PTC fuses in parallel, inevitably one will have a less voltage drop than the other so one will carry more current than the other.  The one that carries more current will trip before the one that carries less current.  However, as fuses have a much wider range of resistance in the active range than solid state junctions do, current will tend to balance out sooner than a diode or transistor would. 

So, you're not going to get exactly 2x the trip current with two fuses in parallel.  However, there's equal enough load sharing there it's not going to be hugely far away from that number.

If would be easy to test if you have a DC power supply and a couple lower trip current PTC fuses.  I haven't checked myself, but I'd be surprised if that's not the case.  I actually have some 100mA ones laying around from another project.  Maybe I'll check it at some point.

[mamu]

Thanks for the explanations Billy and Craig.

I decided to use these fuses 2x in parallel for the OKR-T10: AGRF500-2 (5A hold, 9.4A trip, 2.5s trip time).

And these fuses 2x in parallel for the DNA30: 1210L450SLWR  (4.5A hold, 9A trip, 2s trip time).

I can also use the fuses I chose for the OKR-T10 with the DNA30 if I have the space for a leaded fuse in the mod.

I searched and searched for a fuse with a bit of a higher hold current and trip current than those that I chose for the DNA30 (33/3 = 11A) that was also surface mount, but no luck.  I need the tiny surface mount size for use in some of my DNA mods, the leaded ones won't fit.  The leaded ones will be ok for my flip-top mod and Denali as I have room in those mods, but that's about it.

[rrtwister]

Does it really matter if you go several amps over with the fuse rating. I am just wanting to protect against a battery failure. If it fails the amperage will sky rocket quickly and the fuse should protect me... right?

[CraigHB]

Yes, you can load PTC fuses about 50% higher than the hold current without issue.  They don't actually start to trip until to they get to the trip current and then it's very slowly, like in the minutes.  They trip very fast when subjected to the high fault currents of a short circuit.

Conversely, if you were to go way higher on the hold rating than you need to, you could get into a situation where the fuse would not trip fast enough to protect components (like the cells).  But since short circuit fault currents are pretty high for the better performing cells we typically use, you're talking a fuse with a 20 or 30 Amp hold current.

[mamu]

I wrote a summary (a loooooooong summary ) on fuses and hope it is ok to put in this thread.  If you want me to start a new thread with it and you delete this post breaktru, that's aok - just let me know.

Writing this was great for me as I feel I've got it sorted out now.

Some of my knowledge is from working with fuses over this last year, some is from reading tutorials and watching youtube videos, and some is from our awesome resident modder, Craig, who has been most generous in sharing his knowledge. 

Why use a fuse?
A fuse will protect you and the circuit in case of a current fault.  In this day and age where we're pushing higher current loads at higher power as well as using batts with very high discharge ratings, wiring in a fuse during the build is pretty much mandatory and not an option.

What is a PTC fuse and how does it work?
Resettable Fuses

What is a resettable (PPTC) fuse, and how does it work?

How does the resettable fuse work?

Fuse Specifications:
Ihold = Hold current: maximum current that the fuse will allow to pass without tripping.

Vmax = Maximum voltage the fuse can withstand without damage at rated current (Imax).

Rmin = Minimum resistance of the fuse in initial (un-soldered) state.

Time-to-trip: The time it takes for the fuse to trip at a given temperature.

Itrip = Trip current: minimum current at which the fuse will trip.

Imax = Maximum fault current the fuse can withstand without damage at rated voltage (Vmax).

Pd = Power dissipated from the fuse when in the tripped state.

R1max = Maximum resistance of device at 20°C measured one hour after tripping or reflow soldering of 260°C for 20 seconds.

How to choose an appropriate fuse for the converter you're working with:
A.  The first step in choosing a fuse is to read the converter's datasheet and find the max input current.  Choose a fuse with a hold current close to or equal to the converter's max input current. 

If you don't know the max input current or it's not stated in the datasheet, Craig taught us a nifty equation to calculate that max input current: converter's max output power at 80% efficiency divided by min input voltage.

For example, the max output power for the DNA30 is 30W.  At 80% efficiency that's a total of 36W.  Min input voltage for the DNA30 is 3.2v.  Using Craig's equation: 36W / 3.2v = 11.25A.  The datasheet for the DNA30 states max input current is 12A - which is pretty close to the result of Craig's equation.  For the DNA30 then a 12A fuse would be appropriate.

You can choose a hold current slightly under or slightly over the max input current of the converter if you can't find a fuse that is equal to the max current and still have good protection as there is a current cushion between hold current and trip current.  What is important is that you don't want to choose a hold current that is too low else you'll get annoying inadvertent tripping when running the converter at or near max output or from normal operating internal heat (plus each time a fuse trips, the resistance takes days to return to the initial state).  For the most part, we are using fuses to protect us and the circuit from a batt fault or reverse polarity.  Either of those situations will cause a huge current dump from the batt, so no matter if the hold current is a bit high the fuse will trip rather fast.

B.  Next, look at the Vmax rating of the fuse.  Vmax must be higher than the max input voltage for the converter.  A boost converter's max input voltage is 4.2v, while a buck converter's max input voltage is 8.4v.  For a boost converter a fuse's Vmax rating of 6v is appropriate, but that Vmax of 6v is not appropriate for a buck converter as higher than 6v input will damage and/or destroy the fuse when in the tripped state which then will result in loss of protection.

C.  Next, look at the Rmin rating of the fuse.  The lower the resistance the least effect the fuse will have on the circuit - 10mOhms of resistance at 10A wastes 1W and causes an input voltage drop of 0.1v.  So... to have the least effect with adding unwanted resistance to the circuit, we parallel fuses.  Wiring 2x fuses in parallel cuts the fuse's internal resistance in half, BUT it also doubles the hold current and trip current.  If you are going to use 2 fuses in parallel, you must then look for a fuse with a hold current half the max current of the converter.  For example - if the converter's max current is 10A, you will need to choose a fuse with a 5A hold current (2x 5A = 10A).

D.  Next, look at the fuse's trip time - the lower the trip time the faster the fuse will kick in at its trip current to protect the circuit. I try to choose fuses with less than a 5 second trip time.

E.  Next, look at the fuse's Itrip - this should be less than or equal to 2x the Ihold.  For example, if the Ihold is 5A, the Itrip should be around 10A.  If Itrip is higher that 2x Ihold, the fuse may not trip when you need it to.

F.  Finally, you'll need to choose a fuse based on its type - leaded or smd.  For some of us (me  ) our mods are tightly packed and only an smd fuse will fit while some mods with larger enclosures don't have that space constraint, so a leaded fuse would be suitable.  However, keep in mind that smd fuses usually have the lowest rated resistance and lower trip times and are therefore usually the better choice.  But you may not find an smd fuse with the hold current you need.  Lower Ihold fuses are readily available via smd, but higher Ihold fuses are not.  After doing an intense search for the highest Ihold for an smd fuse I've finally found one from Littelfuse that is 7A, the next lower Ihold available via smd is 4.5A.   AND these suckers are teeny tiny - so be prepared to use tweezers and a magnifying light lol.

Note: for series batts, a common fuse on the positive batt is appropriate.  For parallel batts, a fuse for each batt is recommended - especially if you want to protect the circuit and each batt from reverse polarity.  A common fuse will NOT protect the batt that is put in backwards from a meltdown - each batt (when using parallel batts) needs its own fuse for full protection.

If working with parallel batts, the total combined hold current should be equal to or close to the max current of the converter.  For example, if the max current of the converter is 10A, the total combined hold current of all fuses that you've added to the batts should be around 10A.

Various example configurations:

Buck converter with a 10A max input current:
Choose one 10A fuse and wire to the batt+ of the series batts or, more preferably to lower the fuse's resistance, choose two 5A fuses then solder the 2 fuses together in parallel and wire to the batt+ of the series batts.

Boost converter with 12A max input current:
For one batt mods - choose one 12A fuse and wire to the batt+ or, more preferably to lower the fuse's resistance, choose two 6A fuses then solder the 2 fuses together in parallel and wire to the batt+.

For dual parallel batts - choose two 6A fuses and wire one fuse to each batt+, then connect the free end of each fuse with a common wire.  For me, I have a stash of 3A smd fuses lol and will be soldering two sets of 2x 3A fuses in parallel and putting one set on each batt for a total of 12A combined for the DNA30.

Some interesting facts about paralleling fuses:
-Derating is necessary because two fuses in close proximity radiate heat less effectively than single fuses. As a general rule, a minimum reduction in rated current of 10% for 2 fuses in parallel (20% for 3 or 4 fuses in parallel).  Because of the derating factor, you don't want to put more than 2x fuses in parallel.

-The voltage rating of 2 fuses in parallel can be less than the voltage rating of a single fuse. The derating factor can be 15%. Again because of the derating factor, you don't want to use more than 2x fuses in parallel.

-Fuses for parallel connection must be of identical type and rating.

-For a given prospective short circuit current, the duty on parallel fuses is eased, as the current in each fuse is inversely proportional to the number in parallel.

-An ideal parallel arrangement will give equal current sharing, but even if completely balanced paths are not achieved, a small degree of self compensation will occur.

Final note:
When applying solder to a fuse, make it quick and short - heat will increase the fuse's internal resistance and that resistance may take days to lower back to its initial state.

[Breaktru]

Excellent mamu. This is great info for the new modders and those that do not have an electronic background.
Well done girl 

[Haileah]

Thanks Mamu. It was very informative

[CraigHB]

Very nice, hopefully that well be a big help for those with questions. 

[mamu]

Thanks guys!

It was a bit of work organizing my notes and my thoughts on this but I'm glad I took the time to write it.