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E-Go Kart 18650 Pack - Cell fuses or nickel strips?
#1
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  from  a go kart could cause the fuses to break loose. Any opinions or suggestions would be greatly appreciated.   Huh
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#2
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.
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#3
(04-03-2018, 02:18 AM)Korishan Wrote: 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 now now but why not ask the experts?  Big Grin
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#4
Ahhh, yeah. I missed the 2x packs part Tongue 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. Wink

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.
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#5
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 motor’s 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?
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#6
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! Smile What motor are you using?
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#7
"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.   Smile

"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.
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#8
(04-03-2018, 02:46 PM)garolittle Wrote: "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.   Smile

Hey now, I don't need a mad wife coming over and beating down my door because you bought more batteries Tongue
Proceed with caution. Knowledge is Power! Literally! Cool 
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#9
Lol. I have to pin the blame on someone .... other than me.
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#10
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 welding experience 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 internal resistance from using the 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 I spot weld nickel strips or use individual fuses.  Specifically, I am now planning to use two separate packs of 20S10P configured 18650 cells 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.  

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.
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