Fuses and thermal fuses

[Arbol]

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!

KarlJ, thanks for your detailed answer.

It is interesting the fact that em3ev has no output fuse. Instead, em3ev has balance wire fuses. So then, probably the balance wire fuses are there to avoid problems in a parallel "brick" contaminate the rest of bricks. And the BMS will act as a fuse if there is a catastrophic event affecting all or most bricks.

I want to use 18650 cells, clearly no lipo. Lipo is a no-no for me, at least for normal usage (probably OK for fast testing).

I want to put my battery inside my triangle, either with a polycarbonate box, or ideally, creating a metal box on purpose. The fact I want to build my own battery is basically due to this.

I am based in Europe, so I will limit myself to 250W motors. Mostly urban usage, max legal speed of 25km/h (so max, max in real terms around 30km/h). And yes, I know that for these levels of power and speed, I would rationally be better served with a "normal, pre-packaged" 36V 10Ah / 15Ah battery. But I want to build my own battery, anyway.

I want to have a hydraulic disc brake front and a mechanical disc brake rear (and a HWBS inside the triangle box).

And I am trying to find a nice donor bike with rear suspension.

So, yes, all your comments fit very well with my design plans.

Mmm then, which are good (resettable) balance wire fuses? Is there a model / brand somebody could recommend? Clearly, these fuses should be for 3.7V, but I am not sure for which amperage. A main fuse amperage probably has to be slightly above the max amperage of the controller. But I have no idea of which is the max amperage a balance wire usually withstands. I guess that this max amperage at the balance wire is significantly less than the max amperage of the controller, but I do not know how much.

 

[agniusm]

I dont think you understand correctly. If em3ev does not put fuses does not mean that they are not necessary because of bms. It is a must, period! When you buy car lead battery, does it come with a fuse?
I think it is not a good idea on building a battery pack for someone with little knowledge. Looks like you have done no research at all before these questions. I woud advice to start at the bottom and do some reading

 

[Arbol]

Well, this is exactly what I am trying to do with this thread: learn!

Life has taught me the best way to learn is to ask those who know. And this is what I am trying to do here.

If every time we do not know about something, we should abandon, there would be no research. By definition, every new "thing" started when the creator did not know much about it. Only after thinking, and especially, after talking to others who may know interesting things, one may start to learn.

 

[Skalabala]

My personal experience in the automotive industry with thermal fuses is not positive.
They get old and start to struggle to flow the currant it is suppose to. Starting to arc etc.
Its a pain to strip open a cars harness to find the reason why there is a power drop :x And then you find a bad thermal fuse

 

[Arbol]

My personal experience in the automotive industry with thermal fuses is not positive.
They get old and start to struggle to flow the currant it is suppose to. Starting to arc etc.
Its a pain to strip open a cars harness to find the reason why there is a power drop :x And then you find a bad thermal fuse

Thanks, Skalabala, a quality statement out of practical experience.

 

[Punx0r]

Definitely worth knowing that, thanks

 

[nukezero]

I don't understand why Paul (em3ev) uses balance wire pptc fuses. IT sounds like he wants to protect the BMS from getting a short/dead short. That's in his description. But even if a cell gets a dead short, there's nothing in there to help mitigate it other than whatever protection circuitry is built into the battery itself.

I think the PTPC fuse Paul is putting is just designed to protect the BMS from shorting out. As we know, replacing BMS is costly.

 

[Arbol]

I don't understand why Paul (em3ev) uses balance wire pptc fuses. IT sounds like he wants to protect the BMS from getting a short/dead short. That's in his description. But even if a cell gets a dead short, there's nothing in there to help mitigate it other than whatever protection circuitry is built into the battery itself.

I think the PTPC fuse Paul is putting is just designed to protect the BMS from shorting out. As we know, replacing BMS is costly.

Yes, that is a relevant question, thanks. I hope somebody can give us an answer.

 

[gwhy!1]

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

This is my reasoning also , but always have a main fuse protecting the main battery supply and as near to the battery as possible.

 

[999zip999]

Very good info. Clear.

 

[wb9k]

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

Whether or not this is acceptable depends on two things, in my mind.

1) How easy is it to repair the balance wire if it burns up? If it's a major PITA, adding a fuse can save major hassles.
2) If a balance wire does burn up, what else might go up with it? This is really the more important question. It may well be a fire safety issue if other pack materials are flammable. God forbid your balance wires get hot enough to trigger a thermal runaway event, either directly or indirectly.

 

[cell_man]

We fit PPTC fuses on the balance wires to protect against a short to other points in the pack, it is not to protect the BMS and I doubt it would offer any protection to the BMS, it is for added safety only. There are many small cables running all over the battery. We fit an insulator under these cables and group them together, but I can't see any negative point to also adding a PPTC at the cell to add further protection. PPTC are operating at a fraction of their rating in normal circumstances, they only come into play in a fault condition, it is very unlikely they would fail and that would result in the BMS tripping anyway, so it would be a fail safe condition. Self fusing on the cables is all well and good, but the potential between the wire and the cells underneath could be anything from 1S, 3.something volts up to the pack voltage. Cable length is unknown, so cable resistance is also unknown. It is therefore a lucky dip if the cable is self fusing or a very hot heat generator.

Balance wires are treated with some caution in our packs, with additional insulation, PPTC etc. Main Cables are treated with even more caution, with typically 2 layers of insulation between and main cables or pack with different potential. Compare that to a LiPo brick with a long balance wire hanging out the side with little or no protection with not a fuse or BMS in sight, or a pack with cables randomly crossing the terminations underneath with no PPTC protection.

Safety is a big concern for me personally and we are always trying to improve things whilst also having to be realistic considering our capabilities and the quantity being produced. At the end of the day it can't be a big mess of cables and it should also be reliable. It's easy to introduce possible reliability issues whilst adding safety features and you don't get much thanks when the pack stops working because that extra connection you made in the name of safety melted and then took out the BMS. There is a lot to consider and I don't see everybody queuing up to only buy the single perfect battery that fits every situation, so you can get that perfect design. I also don't see much bigger companies than our humble little operation doing much more or TBH, even half as much as we do to try and make the product as safe and reliable as possible. It's not perfect, we can only do our best to improve and make improvements as we go.

Thanks
Paul

 

[Arbol]

Thanks Paul. Very interesting comment indeed.