E-Go Kart 18650 Pack - Cell fuses or nickel strips?

garolittle

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Hello everyone. I have been asking several sources for their opinions on this question. For an electric go kart project, my plan is to use the following:


  • Brand new Samsung INR18650-25R cells (20Amps maximum continuous discharge)
    Battery packs: Using the Samsung cells, I plan to have two separate 20S5P packs connected in parallel
    Motor: 72V Motenergy Brushless Motor (ME0201014201). Continuous current - approximately 50 - 60 amps at 72V
    Charge Controller: Sabvoton (svmc72150) DC Current Limit: 100A

The main objective is speed so driving range is secondary. With that in mind, would you recommend that I make the parallel and series connections using a spot welder and the .15mm x .70mm pure nickel strips or should I individually fuse the cells? To avoid excessive heat, I have been practicing using a spot welder to connect thin fuse wire (which blows at about 5 amps) to the cells instead of soldering. I would think that spot welding the fuses improves safety (since thermal runaway would be less likely) but I also want to avoid adding too much resistance into the circuit. I was also not sure if the vibrations froma go kart could cause the fuses to break loose.Any opinions or suggestions would be greatly appreciated. :huh:
 
Using brand new cells, you could probably get away with not using cell level fuses. As you know the starting of the cells.
However, if you choose cell level, then I'd spot weld like you mentioned. Use a fuse that'll pop at 10A instead, though. You have to account for surge current. If you make a 5P, and the motor runs at 60A continuous, then about double that under startup. So, you need to be able to handle about 120A surge. Maybe even 150A surge. So, with 5p, at 150A, that'd be 30A per cell.

Actually, with those numbers, you probably should considering 10p if you can get away with it.
To know for certain, find out what the surge current of the motor is under load (not unloaded as that current isn't realworld in this case)

As far as the fuses breaking, you could always just hot glue the fuses afterwards. That'd keep them from moving around. Also don't make then tight. Put a slight curve in them.
 
Korishan said:
Actually, with those numbers, you probably should considering 10p if you can get away with it.

Ok. This is where I show my ignorance again.I plan to have two separate 20S5P packs connected in parallel. Since the two packs of 5P will be connected in parallel, I thought that would give me a 10P battery pack. :huh:I can hear the internet snicking at me nownow but why not ask the experts? :D
 
Ahhh, yeah. I missed the 2x packs part :p my mistake.

So yeah, electrically that would be 10p overall. Even if the packs are system voltage packs (meaning they each have their own series connections internally), the two in parallel on their outputs would make it electrically 10p. So that'll work out. ;)

Would it be possible to go to 20s6p or 20s7p at all? The extra 1 or 2 in parallel would help a lot if you can manage it.
 
The go kart will have space for one 20S7P pack on each side of the kart so I will see if the budget can support it. I was not able to find Max Amp under load for I did find this from the motors manufacturer:

Power: 6 Hp continuous, 19 peak Hp
Voltage: 24-72V DC
Voltage Constant: 50 RPM/VDC.
Torque Constant: 1.20" lbs/amp.
Max Motor Current: 300 A/1min.

Thanks for the suggestion regarding spot welding nickel strips. I could always spot weld two strips on top of each other for all series /parallel connections to ensure sufficient amp capacity. Any thoughts on that idea?
 
I doubt you'd need "that" much nickle strips. 1 layer about 1cm wide should be fine. I would just make more than the standard 2 spot connections. In other words, hit it with 3 shots so you have 6 points of contacts for each strip (I'm assuming here that you are talking about using the strips and not using the fusing wire)

For the bus bar, again, just 1 strip would suffice. But, it might need to be too wide. If that is case, I wouldn't worry about spot welding, I'd see about soldering the two strips together. That gets rid of any resistance between the strips plus gets rid of any possible movement. You could check out floyd's build where he's using a resistance soldering setup.

300A peak. That's a lot of Amp draw. So that basically is what you need to be prepared for. With those numbers, if you stick with 10p, that means each cell could get to 30A for a prolonged period. This is why I was stating about going with 6 or 7p per pack to give you 12/14p total. That would be 25A/22A respectively. With seeing 300A, it might be better to make each pack 10p instead. You don't want to blow fuses, but you also don't want to overtax your cells drastically shortening their life.

Or, instead of setting up 2 packs in parallel, go with 4 packs. This might be the easier route, depending on what you plan on doing with them outside the cart and your mobility requirements.


Wow, 1.2 lb/amp. sheesh. That thing can deliver over 300lbs of torque. That's decent! :) What motor are you using?
 
"In other words, hit it with 3 shots so you have 6 points of contacts for each strip ..... Will do. This is my usual practice.

"I'm assuming here that you are talking about using the strips and not using the fusing wire"..... Correct. Will use pure nickel strips.

"... check out floyd's build where he's using a resistance soldering setup." Will do. I love learning from others.

"This is why I was stating about going with 6 or 7p per pack to give you 12/14p total...." Will do. My wife will kill me for spending so much money on batteries but I will tell her that you said I had to. :)

"What motor are you using?" 72V Motenergy Brushless Motor (ME0201014201) It weighs 22 pounds and will be mounted next to the go kart seat. The battery pack(s) will be in front of the driver in an effort to distribute the weight.

Thanks for the advice. I am going to take my time with this build (probably several months). There is nothing I enjoy more than learning from this forum. You are a true expert and I appreciate the time you take to share your knowledge with others.
 
garolittle said:
"This is why I was stating about going with 6 or 7p per pack to give you 12/14p total...." Will do. My wife will kill me for spending so much money on batteries but I will tell her that you said I had to. :)

Hey now, I don't need a mad wife coming over and beating down my door because you bought more batteries :p
 
Ok I have been giving this a lot of thought lately. I like the idea of simply spot welding pure nickel strips to the brand new 18650 cells (without individual fuses) for two reasons:1) I have spot weldingexperience so it would be easier and 2) it would be much faster. However, since my motor and charge controller could pull a max of 300 amps, I am concerned that the internalresistance from usingthe nickel could become an issue. I have read that resistance within nickel becomes an issue the higher the amps pulled. Also, I agree that battery pack size needs to be increased to avoid excessive amps on individual cells regardless of whether Ispot weldnickel strips or use individual fuses. Specifically, I am now planning to use two separate packs of 20S10P configured 18650cells connected in parallel. This would reduce the max amps for each cell to 1.33 amps(400 cells divided by 300 max amps).

That said, I am re-considering the use of individual cells fuses. The individual fuses would be soldered onto a copper bus bar which would run the length of each parallel connection. The fuses would be spot welded to each cell to avoid the heat from soldering directly on to the cells. I created a very small photo type as you can see in the picture below.
image_puervl.jpg


I am willing to put in the extra time to individually fuse each of the 400 cells that would comprise my battery pack but my question is (assuming fuses that blow at 5 amps) would this be better from a "resistance" standpoint than using spot welded nickel strips? Thanks for any advice that you can offer. This forum is great.
 
As a thought, why has nobody come up with spot welded nickel strip fuses ?

Stamp out a pattern with the two pads each end and then a thin section to act as the fuse... would make pack setup a lot more easier and less damage to the cells internally, via soldering.

Pattern : O-O
 
They are made. I've seen them before, but I can't remember where I saw them now. Nor what they are called. A little googling might turn something up.

They kind of look like a bandage that you'd put across your nose, or like an exaggerated " I " shape.
 
That is a pretty cool idea. I keep debating (with myself) on whether I should 1) Stick with pure nickel strips and a spot welderor 2) go the Tesla route and use individually fused cells (also spot welded)
 
After a bit of reading it may be due to the melting point. The nickel strip will heat up a lot more and stay conductive in a small overload situation, fire risk... fuse wire has a lot lower melting point and better for smaller overloads. Shorting protection - nickel would work but a prolonged overload no. Learnt something new...

Interesting one... "A123 sizes their fuses to be 3.6 mm wide and .007" thick nickel sheet. They rate there fuses to blow at 2100amps in .1 seconds." http://mit-evt.blogspot.co.uk/2009/06/
 
Remember, you can show your gratitude by donating 18650 cells ;)
 
Korishan said:
Remember, you can show your gratitude by donating 18650 cells ;)

Well I was about to send you $1000 check but since you only prefer used 18640 cellswith two voltseach I can send that instead :p
 
hahah :p
 
OK this may be a dumb question but how would I replace a cell that has a blown fuse if I have a large battery pack? For example if I have a 20 S 10 P configuration and one of the cells in the middle has a blown fuse how do I replace that particular cell when the rest of the pack is connected with those Cell holders such as this?

YEHAM 4x5 Cell Spacer 18650 Lithium Battery Plastic Holder Bracket (10pc) https://www.amazon.com/dp/B06XHHM28S/ref=cm_sw_r_cp_api_-VD2AbCSFVMH2

Maybe Im missing something obvious but it seems like I would have to disconnect the fuses on all of the cells, pull off the brackets, and then refuse each of the cells.
 
Desolder all connections to that cell, break the little dog ears that hold the cell in the holder, slide it out, slide new one in, resolder the connections adding new fuse wire.

This is why I plan on going with solderless cell holders. That way I can just pop a cell out without needing to de/re-solder anything
 
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