# How many cells do you think i need>?



## storx (Nov 24, 2013)

I have been collecting parts for reverse trike EV build in the future, i bought the motor kits... dual ME0913 w/ kelly controllers, i was origionally going to run both to the rear wheel, but recently someone pointed out i could run them to both front wheels using cut gears and cv shafts, anyways i have been collecting 18650 cells to try and build the first battery pack on a budget, so if it all works out as planned, then i can possibly look into new cells in the future...

Right now i have about 500ish 2000mah+ cells, i was aiming to have acquired a lot more, because i started with around 2300 cells after tearing all the laptop batteries apart, but its what it is.. 

I have about 1100 cells that range between 1700-1800mah, being the most popular capacity range it seems on these used cells.. i have been contemplating adding these to the stack of what i would call usable for my battery pack and just save all cells 1700+ for the build... 

Anyways i am aiming to replicate this type of design with EV with some slight changes.. How many cells do you think i need to achieve my 100 mile range?










I plan on building a small attachable trailer later on that will house a generator of some sort to tow behind to supply power to the bike for longer extended rides outside the 100 mile range..


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## bigmotherwhale (Apr 15, 2011)

Have you got an IR meter? could save you alot of time in your cell testing, you can get them for very cheap from hobby stores, and could weed out the weakest cells easily, 
Basing solely on capacity is alright if you have enough of them however i would have though. 

These packs seem like a lot of effort but it should work ok, if done correctly, are you planing on running individual fuses on each cell, tesla style?


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## storx (Nov 24, 2013)

bigmotherwhale said:


> Have you got an IR meter? could save you alot of time in your cell testing, you can get them for very cheap from hobby stores, and could weed out the weakest cells easily,
> Basing solely on capacity is alright if you have enough of them however i would have though.
> 
> These packs seem like a lot of effort but it should work ok, if done correctly, are you planing on running individual fuses on each cell, tesla style?


Can you elaborate on what you said about the IR, im assuming you mean internal resistance... 

I have an Imax B6 Mini, which has an internal resistance test, but i am not sure how accurate it is, because from what i read 18650 cells should be like 30miliohms, but when i tested like 40 cells the lowest one was 118miliohms... all ranged around 118-138miliohms...i was planning on testing the resistance after they are all tested for capacity, the capacity testing isnt taking all that long, i bought 2 of the foxnovo 4s and the company that makes them enjoyed my review i left them on amazon that they sent me a 3rd one for free after reading what im working on...so i can test 12 cells every 2-3hrs...

In regards to the adding a fuse to each one, i actually have been contemplating doing what i did on my ebike battery to the motorcycle, on my ebike battery my initial pack i built out of just the first 130 cells that had over 13.0v, because i really didnt know what i was doing at the time, within the first few times trying to run the bike off of them i had issues with cells shorting out on me draining the voltage of entire string... just so you know i am no electrical engineer, this is all just having fun and learning as i go..., but i kept having to unsolder so much of the pack to get the one bad cell out that i bought the imax b6 mini to start testing cells before assembling in a pack, well i had no way to temp attach the leads to the cells, so i bought some very tiny magnets from hardware store to charge them and test the cells with that i decided i would try connecting the pack together with magnets on my latest ebike battery pack, so what i did was take a small peace of thin wood and drilled holes in the wood in the spacing the batteries would be in the plastic spacers, then i placed it ontop of a peace of steel and placed a magnet in each hole allowing the steel surface to tell me which surface would be correct and then i soldered the wire to all the magnets in the holes, then when i assembled the battery i was able to quickly connect all the magnets to the cells to wire the battery up, to make sure they magnets would stay secured even more i gobbed hot glue on the ends of all the cells to trap the magnet against the end of the cell, to prevent it from jarring loose or falling off during harsh conditions, since testing the cells and doing the magnet connection method the battery has survived over 2000miles on the bicycle thus far without any issues.... i did have one cell go bad and tried to short out that string of cells, but the magnet actually acted like a fuse in a way and melted the metal coating off the magnet, making the cell isolated from the rest of the pack.. not sure what the amp rating is on the magnets, but it worked out really well for connecting the batteries and acting like a fused safety link....the magnets are very cheap, they are only like $5 for 100 of them, 1/8" x 1/16" magnets


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## bigmotherwhale (Apr 15, 2011)

yes i did mean an internal resistance meter and i too have had high readings, i believe its to do with the connections between the meter and the cell,
as a guide compare it against a brand new fully charged cell. you should be looking at about 10m ohm.

It all sounded good until you got to the magnets!
i wouldnt recomend that, use a spot welder or even a low temperature solder if your carefull.


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## Vanquizor (Nov 17, 2009)

Interesting approach-as long as the magnet has sufficient strength and the surfaces are clean there is no reason it can't work. Limitations are based on the conductivity of the magnet's coating and how good the contact is between the magnet and cell as well as between the magnet and wire. If you burned off the coating isolating a cell then obviously there is reasonable resistance and therefore some lost efficiency there. I would try and quantify that before scaling up (measure internal resistance directly on the cell vs through your magnet/wire assembly).

You could remove resistance by placing thin copper sheet with wire soldered to it directly on the cell(s) pinned in place by the magnets- that would surely have better conductivity than the described assembly unless you have some crazy silver coated magnets or the magnetic force is insufficient to clamp effectively over the added thickness. In this case the magnet is just providing a clamping force much like a spring contact or a wing nut on a stud terminal.


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## bigmotherwhale (Apr 15, 2011)

Soldering to the magnets is likely to demagnetize them at least partially 80 - 160 degrees celsius is the operating range of neos.

placing a copper sheet in between the magnet and cell sounds like a good idea but if you think about it its increasing the distance of the magnetic coupling - the law of squares - enough said.

you are asking for trouble if you ask me, it adds resistance to joins could result in heat production that would result in a loss of magnetism - a positive feedback effect that needs to be avoided at all costs. 

I would get plate of aluminium laser cut with holes and solder or spot weld fuse wire to the tops of the batteris and to the plate - just like the tesla modules (infact i would probably just buy some tesla modules if i could to be honest!)


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## Vanquizor (Nov 17, 2009)

bigmotherwhale said:


> Soldering to the magnets is likely to demagnetize them at least partially 80 - 160 degrees celsius is the operating range of neos.
> 
> placing a copper sheet in between the magnet and cell sounds like a good idea but if you think about it its increasing the distance of the magnetic coupling - the law of squares - enough said.
> 
> ...


Agreed this is totally the right way to do it.

Of course if I had a cell that I did the solder a wire to a magnet trick on in front of me like the OP does I would DEFINITELY take a measurement to see how bad this hack really is.


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## Karter2 (Nov 17, 2011)

Re the original question....how many cells for 100 mile range.
It's relatively simple to estimate at a basic level..
Each of your cells should have about 6 Whrs of energy (3.7 v x 1.5 Ahr ),...
...and let's say you need at least 50 Whrs per mile minimum.
Then you would need at least 8-9 cells per mile, or say 900 cells minimum 
...assuming that 50whrs per mile is realistic ?
Obviously, if you are likely to use more Whrs /mile then you will need proportionally more cells.
However,..that is not the full story.
900 cells at 6Whrs each gives you a 5.4 kWhr pack (theoretically) of cells hardly capable of 1C constant discharge.
So you could not expect more than 5.4 kW of power at any one time, and likely much less (2-3 kW) as a constant discharge. Much less than your motors are capable of.
Personally, I suspect you should be looking at nearer 2000 of those cells to be sure and give reasonable performance .....and that's a 100kg+ pack !
And remember, your pack will only be as good as its weakest link...or cell string.
.....no matter which way you assemble them !


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## storx (Nov 24, 2013)

Karter2 said:


> Re the original question....how many cells for 100 mile range.
> It's relatively simple to estimate at a basic level..
> Each of your cells should have about 6 Whrs of energy (3.7 v x 1.5 Ahr ),...
> ...and let's say you need at least 50 Whrs per mile minimum.
> ...


hmmm, i was thinking i would need far less due to the difference of weight of the vehicle compared to lets say the tesla model s with 7000 18650 cells that gets up to 270 miles..


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## storx (Nov 24, 2013)

Vanquizor said:


> Agreed this is totally the right way to do it.
> 
> Of course if I had a cell that I did the solder a wire to a magnet trick on in front of me like the OP does I would DEFINITELY take a measurement to see how bad this hack really is.


i will take some measurements of some of the cells directly, then through the leads soldered to the magnets tomorrow and post them up.


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## Duncan (Dec 8, 2008)

Hi storx

When you are going aerodynamic drag is the main thing - weight is much less important,

The Tesla is very aerodynamic - your thing - less so - you may actually need MORE power than the Tesla to do a steady 70mph


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## Karter2 (Nov 17, 2011)

Tesla cells are good for 3.2 Ahr and so have approx 11 Whrs of energy each...or nearly double the capacity of each of your cells......but it still takes 7000 of them to make that 270 mile range !
As I said, the maths is pretty simple.


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## storx (Nov 24, 2013)

So today i took the time to check the IR of the stack of 100 cells i have that are 2200mah and above, using the imax b6 mini with its leads, the lowest IR reading i got was 66miliohms, out of 100, 93 of them tested between 66-100miliohms, i had 7 that tested between 101-135miliohms. I tested a nearly brand new energizer brand 18650 cell, maybe recharged 3-4 times tops with the imax and it said 68miliohms, so i dont know if my cells are truly in that range or the tester has some sort of internal resistance in the leads being added....

I also took 20 of the cells that ranged between 66-80miliohms and tested them through 2 leads soldered to a single magnet, they averaged 33-41miliohms higher testing through the magnetic leads i made to simulate pack connections.

I also noticed that many of the cells i had to retake the test numerous times because if the leads touched the old spot weld points on the ends, they would read significantly higher IR, for example one cell had like 6 spot welds on the end of it, if i tested the cell putting the leads on old spot welds on both ends i was seeing numbers as high at 228miliohms, but if i moved it away from the center to the outer edge placing it on bare metal i got 80miliohms.

Could someone answer this for me, those 7 cells that read 101-135miliohms, are they cells i should leave out of the pack due to being higher than the rest? or are they ok to have in the pack at that level of resistance, i dont know whats considered bad, to toss in my unusable pile...

I also tested a pair of cells i had that had the oem spot weld, testing the cells directly they were 178 and 206 miliohms, if i test the lower one on one by connecting it directly to the end of the cell, then place the other lead in the middle of the 2 cells on the tab, it went up to 293 miliohms.


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## Karter2 (Nov 17, 2011)

storx said:


> I also tested a pair of cells i had that had the oem spot weld, testing the cells directly they were 178 and 206 miliohms, if i test the lower one on one by connecting it directly to the end of the cell, then place the other lead in the middle of the 2 cells on the tab, it went up to 293 miliohms.


Are you saying those two cells were still welded together as a parallel pair ?
If so, you must realise that your readings will be the combined resistance of the two cells ( as like a pair of resistors in parallel )
You cannot measure the individual cell IR when they are joined in parallel.
I would not bother wasting time with any cell of over 100mohms !


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