Fuses and thermal fuses

From looking at em3ev battery packages, cell_man writes:

PPTC thermal fuses used on all signal wires. If a fault were to ever occur on any of the small balance wires which run around the battery pack, the fuse simply shuts down the connection and keeps everything safe. Once the fault condition is removed, function returns to normal.

http://em3ev.com/store/index.php?route=product/product&path=35&product_id=158

I have googled for thermal fuses (and fuses in general) for batteries, and I do not find many conclusive statements. There are some discussions about the need for fuses for each cell, or only one at the overall battery level. But I do not see many pics / discussions about this issue.

Is there a DIY battery with 18650 cells, with images, including fuses and thermal fuses? I would be interested to see that.

Up with the thread. Comments are welcome!

One question: is it necessary / advisable to have a global fuse, if one has fuses at the balance wires (like em3ev does)?

It seems to me that having the fuses at the balance wires is more efficient, since it may happen only a "parallel brick" fails (and its fuse is activated) and the only thing that happens is voltage goes down (eg, a 13s package becomes a 12s one).

Instead, a global fuse would be an overall on / off switch: if something fails (no matter what fails) the whole system is switched off.

My doubt is whether it is unsecure to leave the rest of the system once there are fuses at the balance wires (so no global fuse), or if it would be good practice to add the global fuse, too.

And, of course, how is best to do it in practice.

Bearing in mind I am no expert:

I see cell level fusing as a last step safety device, so would not use resetable fuses. If a cell-level fuse opens then there is something wrong with the cell or the internal wiring/busbars in the pack, so you will need to get in there and make remedial work before the pack can be operated again.

I would bear in mind that cell-level fusing introduces lot of potential failure points and electrical resistance so needs to be executed with care.

I would still use a pack-level fuse. This seems to be common practice and I would size it to blow before the combined cell-level fuses in the case of a short at the output terminals. Much easier to replace one pack fuse than dozens of cell-level ones

1. There must be appropriately rated main fuse, it is a must. Switch is not a replacement for a fuse.
2. Fuses on balancing wires are not necessary but in case the wires rub against the cells will prevent damage. It does not protect battery from cell internal shorts. For that cell level fuses must be incorporated.
3. Some say that you need main switch/contactor just in case for you to have an option to isolate a battery.

To sum up you for best case scenario you would have:
1. Cell level fusing
2. Series connection fusing
3. Main fuse
4. Contactor
5. If there is a risk for balancing wire short - balancing wire fusing.

agniusm said:

1. There must be appropriately rated main fuse, it is a must. Switch is not a replacement for a fuse.
2. Fuses on balancing wires are not necessary but in case the wires rub against the cells will prevent damage. It does not protect battery from cell internal shorts. For that cell level fuses must be incorporated.
3. Some say that you need main switch/contactor just in case for you to have an option to isolate a battery.

To sum up you for best case scenario you would have:
1. Cell level fusing
2. Series connection fusing
3. Main fuse
4. Contactor
5. If there is a risk for balancing wire short - balancing wire fusing.

Click to expand...

Thanks guys for your informative answers.

Some questions:

1. What is the difference between a series connection fusing and balancing wire fusing? Isn't a series cable going directly to the BMS through the balancing wires?
2. I have seen fuses with "switches" (called toggle fuses, I believe, such as http://www.ebay.co.uk/itm/20-30-50-60-80-100A-Car-Stereo-Audio-Inline-Circuit-Breaker-Reset-Fuse-12V-24V-/310793751700?pt=UK_BOI_CircuitBreakers_RL&var=&hash=item485cc04c94): that would work as a combination of (3) (main fuse) and (4) (contactor), right?
3. Is it usual in e-bike batteries to have cell level fusing? I would like to see how it is done

Series fusing means that there is a fuse inline from positive to negative connection. Balancing wires are sense wires and has nothing to do with series connection.

agniusm said:

Series fusing means that there is a fuse inline from positive to negative connection. Balancing wires are sense wires and has nothing to do with series connection.

Click to expand...

Ah, now I understand, I was confused, thanks.

agniusm said:

1. There must be appropriately rated main fuse, it is a must. Switch is not a replacement for a fuse.
2. Fuses on balancing wires are not necessary but in case the wires rub against the cells will prevent damage. It does not protect battery from cell internal shorts. For that cell level fuses must be incorporated.
3. Some say that you need main switch/contactor just in case for you to have an option to isolate a battery.

To sum up you for best case scenario you would have:
1. Cell level fusing
2. Series connection fusing
3. Main fuse
4. Contactor
5. If there is a risk for balancing wire short - balancing wire fusing.

Click to expand...

Given best case scenario is doing (1) to (5), what is normal scenario? I mean, what is "conventionally" accepted by experts to control most of the risks, at a reasonable cost and building complexity? What do most experts do? (1) to (5) all together, or do they skip some of them?

ANY circuit that connects directly to the battery pack---main power lines AND balance lines--should be fused. The normal method is to fuse the + line side rather than the gnd/return wires. Fuses should be as close to the cells they are connected to as practicable. A well designed pack will also have fuses built in at the individual cell level.

wb9k said:

ANY circuit that connects directly to the battery pack---main power lines AND balance lines--should be fused. The normal method is to fuse the + line side rather than the gnd/return wires. Fuses should be as close to the cells they are connected to as practicable. A well designed pack will also have fuses built in at the individual cell level.

Click to expand...

Thanks, that's helpful. So, you would agree with (1) to (5) except for:

(2) Series connection

Is that correct?

What exactly do you mean by "series connection" fusing? Fuses between series elements in a pack? If so, some very large packs have this feature as well, but I wouldn't think they're necessary in a small bike pack unless the pack is broken up into separate chunks that are connected by wire harnesses. Any harnesses connecting two sub-packs in series should also be fused. Maybe I'm misunderstanding your intent here....

wb9k said:

What exactly do you mean by "series connection" fusing? Fuses between series elements in a pack? If so, some very large packs have this feature as well, but I wouldn't think they're necessary in a small bike pack unless the pack is broken up into separate chunks that are connected by wire harnesses. Any harnesses connecting two sub-packs in series should also be fused. Maybe I'm misunderstanding your intent here....

Click to expand...

Hello wb9k, you are not misunderstanding me, your understanding is spot on.

I am referring to agniusm comment in this thread, a bit above:

"To sum up you for best case scenario you would have:
1. Cell level fusing
2. Series connection fusing
3. Main fuse
4. Contactor
5. If there is a risk for balancing wire short - balancing wire fusing."

If I understand agniusm correctly, he is suggesting (like you) cell level fusing, balance wire fusing and main fuse (as well as a contactor, I guess you also like that), but he was also suggesting, in number (2), series connection fusing (in agniusm words: "Series fusing means that there is a fuse inline from positive to negative connection").

Would you agree with that? Or with cell level fusing, balance wire fusing and main fuse you believe there is enough security for the whole package?

Interesting question. For sub-modules connected in series by a harness, I would say you should always fuse the harness. For cell groups connected by conventional weld straps, it's more of a judgement call. If the pack is well-designed AND you are careful to observe proper protocols when servicing the battery, this should not be necessary. However, as an example, imagine you have a pack on the bench and the weld straps are exposed. Say you drop a wrench on the pack, shorting two adjacent cell groups together. Cell-level fusing should protect you from things getting out of hand, but when it's over you've got a mess that is difficult or impossible to repair. Had there been a fuse between each series cell group, you would have just blown the fuse, which you could then just simply replace. So while that series fuse doesn't necessarily make the pack any safer, it does make it easier to recover from this type of event. It's a judgement call, but I would think the added complexity and connections are not worth the expense, effort, and additional failure opportunities. Agniusm is a sharp guy though....maybe he's thinking of another scenario I haven't considered, but in my experience, this would be an unusual design practice.

wb9k said:

Interesting question. For sub-modules connected in series by a harness, I would say you should always fuse the harness. For cell groups connected by conventional weld straps, it's more of a judgement call. If the pack is well-designed AND you are careful to observe proper protocols when servicing the battery, this should not be necessary. However, as an example, imagine you have a pack on the bench and the weld straps are exposed. Say you drop a wrench on the pack, shorting two adjacent cell groups together. Cell-level fusing should protect you from things getting out of hand, but when it's over you've got a mess that is difficult or impossible to repair. Had there been a fuse between each series cell group, you would have just blown the fuse, which you could then just simply replace. So while that series fuse doesn't necessarily make the pack any safer, it does make it easier to recover from this type of event. It's a judgement call, but I would think the added complexity and connections are not worth the expense, effort, and additional failure opportunities. Agniusm is a sharp guy though....maybe he's thinking of another scenario I haven't considered, but in my experience, this would be an unusual design practice.

Click to expand...

Thanks.

Is there a place where I could see pictures of how good professionals put fuses in practice for e-bike batteries (at cell level, balance wires level and / or main fuse level)?

I have done a bit of search at this forum, but I have been unable to find that. I am not saying this information does not exist in this forum, it is I have been unable to find it (and I have spent some time doing that, even though not a whole day).

I would like to understand what is "standard practice" from good professionals on the matter.

From casual observation of pack builds on this forum most have either a single fuse at the output, or no fusing at all. There have been a couple of recent threads discussing ideas and prototypes for cell fuses.

You may want to look at commercial EV packs for the kind of extensive fusing you've described. I think examples of such practice amongst ebike builders will be rare or non-existent.

makita packs have thermal fuses in every cell. that is one reason why i use them . i'd rather have a safer cell, used, than an unfused cell new. But not everyone can use them, heroic commuters needing a large pack can't get enough watt hours. the fuse takes up space in the cell, and reduces capacity.............
i still use a main fuse, and a fuse between packs, as i'd rather not blow the cell fuse since it ruins the cell
UPDATE: now that i've learned how to use a JB external fuse, i can fuse any cell!

As i mentioned before it was from my view point best case scenario. It does not mean it is correct in all cases. I myself would go with single main fuse and be done with that. Bike battery is nothing to compare with automotive batteries and if you take precautions, design your pack well it should be good enough. There are guys using batteries for several years and that's quiet a few examples without any trouble. Cell level fusing is complicated and def. not for casual DIY'er. Think materials, logistics, custom labor. To be more specific find someone who could supply say A4 sheet of nickel, then send it to you, then you have some CAD time designing your custom tabs and calculating cell fuse rating. After that's done, find someone who could CNC it for you, then you need some spacers on positive side of 18650 can, CAD and CNC time again + sourcing the right material. After more logistics you have all your parts. Now, if you don't have spot welder you need to send your cells and all bits and bobs to someone who does explaining the procedure and if you are lucky, they will do it as you requested. Now that you have sophisticated battery pack you need to case it nice with proper connectors. Now comes the BMS
Its easy when you think about it, it gets harder and expensive when you start thinking how you will accomplish it.
Now for the pictures.

Circuit is doing what you need:




http://endless-sphere.com/forums/viewtopic.php?f=14&t=55870

On first photo you can see cell fusing, small balancing wire solder spot with fused link and main battery fuse.

Here is well known Tesla battery cell level fusing:




Perhaps wb9k forgot to link you to Hymotion packs with cell level and main fusing:

http://endless-sphere.com/forums/download/file.php?id=133278

My A123 battery kit has operational fuses by accident just using traces for balancing:




It was proven practically to evaporate if short occurred:



My current battery build has balancing fusing and main fuse:





My advise would be to buy ready made battery or kit from circuit when it is available, expensive, but cheaper then DIYit yourself
P.S

wb9k said:

... Agniusm is a sharp guy though....

Click to expand...

How should i take that?

Thanks agniusm, very detailed and precise answer.

Just in order to fully understand you: in your post you say:

"My current battery build has balancing fusing and main fuse"

but also, at the beginning of your post you say:

"I myself would go with single main fuse and be done with that"

Can you please explain that? Do you think the balancing fuses add value? I ask this because it seems the balancing fuses add quite a lot of value: both yourself and em3ev have balancing fuses (em3ev in his triangle packs). You, in addition, have a main fuse. em3ev apparently has no main fuse (but I could be completely wrong on that).

OK, I try to answer myself, using the information from the following link:

http://www.allaboutcircuits.com/vol_1/chpt_11/5.html

First I consider the following statement from the link:

"For our series battery bank, one fuse will suffice to protect the wiring from excessive current, since any break in a series circuit stops current through all parts of the circuit"



This statement seems to suggest that having a main fuse is a good idea, since with only a fuse at the discharge cable level is enough to deal with shorts in a series construction. This is probably why many builds have only a main fuse: in this very simple way, builders are able to handle a big array of potential problems.

However, not all problems can be solved by a main fuse:

"With a parallel battery bank, one fuse is adequate for protecting the wiring against load overcurrent (between the parallel-connected batteries and the load), but we have other concerns to protect against as well. 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. To guard against this eventuality, we should protect each and every battery against overcurrent with individual battery fuses, in addition to the load fuse"



So, this statement is suggesting we should have fuses at the cell individual level.

However, I am wondering if having fuses at the balance wire level is also partially protecting the battery from the "parallel battery bank": in reality we should have fuses at the cell level, but due to the complexity of the task, it seems good battery builders, such as agniusm and em3ev add fuses to the balance wires for that task. Is that interpretation correct? (to be precise: my interpretation / guess assumes adding fuses at the balance wire level is akin to having fuses at the cell level with regards to parallel battery protection).

I have balancing fuses because i have my kit which already has it. For my next battery and i'm pretty sure it will be 18650 i will go with main fuse only, perhaps balancing fuses if design will require it.

P.S. I am in no way good battery builder, just a beginner! Just my few cents on the topic.

When you say "perhaps balancing fuses if design will require it": under which conditions will your next build require balance fuses?

Undrer the same as was posted before. If they rub against cells or there is big risk in rubbing through insulation. That should be rare thou. I think you are over concerned.

Most balance wires are so thin and the battery output current so high that they are self-fusing

Mmm yes, probably I am over concerned. But at the design phase, I prefer to be over concerned rather than under concerned

So, to sum up, the bare minimum is to have a fuse at the discharge cable level.

Then, one could add fuses at the balance wire level, but this is not essential (I assume the fuses at the balance wires must be different than the main fuses, with a much lower current, right?).

In order to do the right thing, one should do what circuit is doing: fuses at the cell level. This is complex, and probably should be entertained only when going to relatively big levels of production (unlikely for a DIYer).

Which fuse models do you recommend for a 13s7p configuration (both main fuses and balance wire fuses, which I guess are quite different to each other)?

having purchased a couple of EM3EV packs now, i can tell you they are a very well made pack.

They dont have an output fuse as the BMS takes care of this, ie short it BMS will drop it out anyway.
99% sure the average punter would not be able to buy or if you did it would have no warranty a pack with no BMS (at least from EM3EV)

Pack safety for an ebike should boil down to cell types, Hobbyking Lipo's have a bad rap for setting fire to themselves but i'm sure many failures
of this type of cell are self inflicted by over discharge, incorrect connections when splitting apart to charge or reassembling to ride.
A hobbyking type battery has cell taps for balancing but no on-board BMS.

My point is there is plenty to consider which also affects the requirement for such things as mentioned in above posts,
EG: if your battery is held on the bike by duct-tape it will likely rub and over time and something will turn ugly someday.
there are dozens of things to think about when building a high-powered bike and like everything in life, normally compromises are made.

Having built 2 EM3EV ebikes now, one with the bottle shaped battery and the other with the frame bag idea, i can say that the frame bag is excellent,
makes for a very neat job and a bike build can take under an hour to build.

Step 1 Decide what you are going to use it for.
Assuming whatever the use it will be capable of 50km/hr quite readily.
Whatever category Pick a bike that has well above average brakes (my 1st mistake).

If this is your first ebike build and you intend to ride it on bikepaths and the road, there is little point in having more than about 2000W (which an 8T mac appears to take happily)

A good ebike (or bike) is the sum of ALL its parts.

Me, next bike will be bigger, have at least font suspension, will be purchased with decent discs and calipers, plenty of space for a battery of 14S 18650's ~15AH
Aim will be for a rear drive as i've done front drive and if i need more grunt, i'll add another motor!