Acceptable internal resistance values on 18650 cells

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Matador   10 kW

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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Oct 26 2016 3:20pm

I'm also currently in the process of Nickel electroplating my copper bussbars so they don't oxidise anymore.
In that way, I keep the amazing conductive properties of copper, but with the thin Nickel layer, I wont get copper oxide formation on the surface to create additionnal resistance over time.

Here's som pictures of my artisanal plating method :

There's some youtube videos for the plating method... I made my own stock of Nickel acetate, and use some Nickel coins (canadian one made during 1965-1981 were 99.9% Nickel) as the electrode...

I used 8% distilled acetic acid- Nickel acetate solution, Plating at 3.80 volts CC (from 18650s !) at 20 milliamp for 12 hours (giving around 30-35 microns plating thickness).

EDIT : For future uses here's a reference for how to calculate nickel plating thickness versus time and ampere/surface area to plate. See pages 8 and 9 for the formula and tables : https://www.nickelinstitute.org/~/media ... 41015.ashx
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Plating result (LEFT : original copper, 2000 grit sanded, RIGHT : After plating and polishing with Silvo)
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Plating result (LEFT : original copper, 2000 grit sanded, RIGHT : After plating and polishing with Silvo)
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Matador   10 kW

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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Oct 26 2016 3:54pm

Yes, the springs are in the conductive path. Probably the "weakest link" in my battery build...

I will need to address that issue in the future.

I plan to use copper or brass piece to fit at the end of the springs (probably by soldering), and wire the piece to the bussbar bolt through a thick enough but flexible copper wire. Sort of like in the picture below but with a wire in between... That will be a long term project !
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Matador   10 kW

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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Oct 28 2016 10:50pm

eTrike wrote:Cool, as you likely know that is the main source of your heat and sag. I used a salvage laptop sourced pack for a time while my main pack was being assembled. With a 1kW pack @600W(12A?) I would expect a very minor heat increase even with used laptop cells. Keep us posted :D
Thanks, that's good to know. I'll definitly try to modify my contacts.
Sure thing I'll keep posting my upgrades.

Also, contacts like theses look pretty strong for amp capability:

Ref : http://allaboute-cigarettes.proboards.c ... iew-942940
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Re: Acceptable internal resistance values on 18650 cells

Post by Doctorbass » Oct 29 2016 12:06am

There is some concept to know to understand how the cell max current is calculated and where RI is part of that calculation:

A 18650 cell have a shape that limit his dissipation to a given amount.
also the chemistry of a 18650 cell have a max temp rating
Both of these two last conditions make it having a max wattage in heat loss it can dissipate.
the max current a cell can carry is a matter of that max wattsge it loose in heat and also the max temperature it can reach.

ex 12 milioohms and 30A mean (10.8watts)

So in that case, if 10.8 Watts of heat is the true limit that cell can take to dissipate before reaching the max allowed temp limit by the chemistry

The famous 10 sec burst rating is all a matter of temp increase and temp limit....


few exemples:

new high power 18650 cells typically will have between 12 to 30 miliohms
100 to 150 miliohm is more about lower power 18650 ( ex: LiCO from laptop)
The old Makita packs with SONY 1.6Ah LiMn 18650V developped 10 years ago had about 45-50 miliohms
meanwhile the A123 LiFePO4 18650 of 1.1Ah had 18 miliohms and was rated 30C cont and 60C burst...

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Re: Acceptable internal resistance values on 18650 cells

Post by BeachRider2016 » Oct 29 2016 12:41am

Matador wrote:
Punx0r wrote:It's tempting to say that if a pack is making a lot of heat during normal discharge use then the cells are inappropriate for the application. Active cooling will help keep them cool but doesn't stop the waste of stored energy being converted to heat in the first place. When a high energy density cell is leaned on too hard the delivered capacity plummets and you could well end up delivering no more than a more power-orientated cell (just without so much heat).
Absolutely

In fact, just with the resistance of the whole pack I can measure the loss in power via heat dissipation with the ohm's law

Power = V x I, or
Power = R x I^2

In my case, at full throttle, I see an enormous voltage sag : at 30 amp, I see 9.5 to 10 volts sag..... Hence, I'm probably loosing around 300 Watts in heat losses with my laptop pack for 1400 Watts in my motor. So losses are in the orders of 18% (300 watts loss for 1700 watts used).

On average, each cells in my laptop pack is 2300 mAh (tested discharge capacity a 0.5A/cell)... So with this in mind, I'd rather just used higher power cells with less capacity... Like 1800 mAh but 30A/cell. At least they would'nt heat up which could prolong their life.
9-10V sag is way too much.

My 13s20p is overkill for 1000w hub. It only sags 2-4volts while drawing 40-60 amp ~ 2800w full throttle on startups !! I doubt my battery generate any significant heat since the thermal probe is on the BMS heatsink, but the fan sure cool it down very quickly. Fan has its own 12v source so its not drawing from cells. Temp rises if I go full throttle or draw 30amp continuously due to BMS. Its rated 20A continuous so it overloads the bms and not the pack thus generates heat.
I should do some modding on the chinese EBB side cover like ducted intakes for air cooling since its a fully enclosed case with 2 small holes for wirings on the bottom and 2 more toward the seat mount :shock:

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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Oct 29 2016 10:55am

BeachRider2016 wrote: 9-10V sag is way too much.

My 13s20p is overkill for 1000w hub. It only sags 2-4volts while drawing 40-60 amp ~ 2800w full throttle on startups !! I doubt my battery generate any significant heat since the thermal probe is on the BMS heatsink, but the fan sure cool it down very quickly. Fan has its own 12v source so its not drawing from cells. Temp rises if I go full throttle or draw 30amp continuously due to BMS. Its rated 20A continuous so it overloads the bms and not the pack thus generates heat.
I should do some modding on the chinese EBB side cover like ducted intakes for air cooling since its a fully enclosed case with 2 small holes for wirings on the bottom and 2 more toward the seat mount :shock:
Yes, 9-10V is way too much sag ... I still believe there's an efficient to avoid that sag, but without soldering....

Are you using ol laptop cells or fresh new high amp-rated 18650s ?

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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Oct 29 2016 1:47pm

I think I'm going to try to copper-plate the springs... Gotta make copper acetate first though. But even if I do 30 microns thickness plating, i'm not sure It'll make a such big difference in conductivity on thoses springs (the springsteel nickel 0.9 millimeter diameter and 19 cm long).

According to my calculations, if I electroplate a 28 microns thickness layer of copper, It's like adding the equivalent of a suplemmentary 28 AWG copper wire to the original conductivity of the spring. Like that :

28 AWG - 28 microns electroplating thickness
26 AWG - 44 microns electroplating thickness
24 AWG - 68 microns electroplating thickness
22 AWG - 104 microns electroplating thickness
20 AWG - 156 microns electroplating thickness
18 AWG - 232 microns electroplating thickness
16 AWG - 337 microns electroplating thickness
14 AWG - 480 microns electroplating thickness
12 AWG - 671 microns electroplating thickness
10 AWG - 921 microns electroplating thickness

See my excel file for calculations :
Spring Cu-plate.xlsx
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In the future, I'd like to be able to pull 30AMPS per 18650 cell. What do you think is the minimum thickness of plating I need ???

To be honest, I'm not sure if it's possible to plate more than 50 microns. I'll have to check that.
In you opinion, my spring shoud be equivalent to how much AWG of copper wire ?
I plan to use my battery on my 30 BBSHD. Maybe on day going to upgrade controller to 50AMP. My battery will be 14A13P.

The thing is, we need to define a max temperature rating in order to determine the ampacity of a wire righ ? So what's the max safe temperature that the steel core of the springsteels cans tolerate before losing their elastic recoil ?

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Matador   10 kW

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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Oct 29 2016 3:36pm

Checked on the internet... Seems like electroplating of copper has been done to up to 1/4 inch thickness in the past (6350 microns) in the past http://www.finishing.com/240/62.shtml

6350 microns would make my springsteel wire go from 0.9 mm diameter thick to 12.7 mm thick LOL (equivalent of 0000 AWG wire !), but that wont be necessary. I think 500 microns would already be amazing (equivalent of 14AWG, but springsteel wire would now be 1.9mm thick!). Don't know if i'll be possible in practice, I'm going to need a very low constant (zero ripple) CC voltage to get good results.

Already started to cook some copper acetate electrolyte... It's starting to turn copper blue !
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Re: Acceptable internal resistance values on 18650 cells

Post by Hillhater » Oct 29 2016 4:54pm

I suspect a 0.5 mm copper layer on that 0.9 mm spring wire may just restrict the spring action more than a little ?
Have you considered using some short pieces of heavy copper braid inside the spring ? Maybe soldered each end ?
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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Oct 29 2016 5:28pm

Hillhater wrote:I suspect a 0.5 mm copper layer on that 0.9 mm spring wire may just restrict the spring action more than a little ?
Have you considered using some short pieces of heavy copper braid inside the spring ? Maybe soldered each end ?
Your're right. 0.5 thickness plating would make the wire 1.9 mm diameter instead of 0.9mm. Maybe I can plate 0.1 to 0.2 mm thick (100 to 200 microns), that would give the equivalent of 22 to 19 AWG gauge in copper. Should handle 7 to 14 amps per springs (Max amps for chassis wiring rating) but only 1-2 amps according to "Max amp for power transmission" rating (which is ultra ultra conservative).

Copper braid could be interesting... I though of that but it doesnt look clean LOL : here's some pics (click on last one) : https://endless-sphere.com/forums/viewt ... 5#p1236287
For some reason the image is not displayed you have to click on it

Well here's one picture
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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Oct 29 2016 5:47pm

BeachRider2016 wrote:My 13s20p is overkill for 1000w hub. It only sags 2-4volts while drawing 40-60 amp ~ 2800w full throttle on startups !! I doubt my battery generate any significant heat since the thermal probe is on the BMS heatsink, but the fan sure cool it down very quickly. Fan has its own 12v source so its not drawing from cells. Temp rises if I go full throttle or draw 30amp continuously due to BMS. Its rated 20A continuous so it overloads the bms and not the pack thus generates heat.
I should do some modding on the chinese EBB side cover like ducted intakes for air cooling since its a fully enclosed case with 2 small holes for wirings on the bottom and 2 more toward the seat mount :shock:
I'd like to see pics of that massive 260 cells battery !

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Re: Acceptable internal resistance values on 18650 cells

Post by BeachRider2016 » Oct 29 2016 8:15pm

I did get lucky on a few batches of used hp laptop batteries. They were most likely unused, overstock replacement batteries. Most came with atleast 9v in a 3p config, but some were 3-6v however they took charge and held on for over a month w/o changes. The packs manufactured in 2010 though, 6 yrs deteriorations...so not quite sure how great they still are but so far at the 20p configure 2600mah capacity, with theoretical 2C max discharge, they could max out at over 100amp. So with all my full throttles on the 1kw hub, the only thing get quite hot is the motor.

I got the pack/build pics on the "chinese ebb frame build" on my signature. Kinda cruel work but the frame hides everything :mrgreen:

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Matador   10 kW

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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Oct 29 2016 8:52pm

BeachRider2016 wrote:I did get lucky on a few batches of used hp laptop batteries. They were most likely unused, overstock replacement batteries. Most came with atleast 9v in a 3p config, but some were 3-6v however they took charge and held on for over a month w/o changes. The packs manufactured in 2010 though, 6 yrs deteriorations...so not quite sure how great they still are but so far at the 20p configure 2600mah capacity, with theoretical 2C max discharge, they could max out at over 100amp. So with all my full throttles on the 1kw hub, the only thing get quite hot is the motor.

I got the pack/build pics on the "chinese ebb frame build" on my signature. Kinda cruel work but the frame hides everything :mrgreen:
That's a sweet build! Seriously ? 800 bucks ? :D
I saw your pack, 260 cells.... of which 240 samsung 26F cells and 20 sony.... That's a 2.5 kWh 13S20P with 104 max AMPS at 2C (5678W usable power at a 54.6V full charge !)!

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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Oct 29 2016 9:03pm

The Cu(OAc)2 solution in aqueous acetic acid (5% distilled vinegar) is starting to look more and more concentrated. I'll let it reduce overnignt in order to be even more concentrated.
More concentration mean more ability to plate at higher amps withot risking buble formation by water decomposition (2 H2O --> 2 H2 + 02).

The blue is getting stronger
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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Oct 30 2016 7:34pm

So I started the experiment of copper electroplating springs.... I'm going at very low voltage. That means low current densities (0.1211 A/dm2) since I'm using a weak acid solution of copper acetate and I want the most regular plating possible.

According to calculations (ref : http://polynet.dk/ingpro/surface/elecomk.htm), plating those pieces to 100 microns thickness with a 1.5 AA battery should give good results, but it's going to take a while to plate.

Anyways, it's starting to change color already !
After a good Copper-plating procedure to increase conductivity (to 100microns = 22 AWG, so good for 7 amps per springs according to my calculations :
Spring Cu-plate.xlsx
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I might add a protective layer of nickel, once again, by plating, but this time with nickel acetate instead of copper acetate like I did here : https://endless-sphere.com/forums/viewt ... 5#p1235640 .

Here's preliminary results after one hour of low-current plating (11.3 milliamps of current with one AA battery, plating the spring surface which is 0.054 dm2).
See photos :

My goal is to plate a THICK COAT (min 100 microns... maybe 250 microns if I can but I doubt it) of copper... and then finish if off with a protective layer of nickel.
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Re: Acceptable internal resistance values on 18650 cells

Post by BeachRider2016 » Oct 30 2016 11:07pm

Matador wrote:
BeachRider2016 wrote:I did get lucky on a few batches of used hp laptop batteries. They were most likely unused, overstock replacement batteries. Most came with atleast 9v in a 3p config, but some were 3-6v however they took charge and held on for over a month w/o changes. The packs manufactured in 2010 though, 6 yrs deteriorations...so not quite sure how great they still are but so far at the 20p configure 2600mah capacity, with theoretical 2C max discharge, they could max out at over 100amp. So with all my full throttles on the 1kw hub, the only thing get quite hot is the motor.

I got the pack/build pics on the "chinese ebb frame build" on my signature. Kinda cruel work but the frame hides everything :mrgreen:
That's a sweet build! Seriously ? 800 bucks ? :D
I saw your pack, 260 cells.... of which 240 samsung 26F cells and 20 sony.... That's a 2.5 kWh 13S20P with 104 max AMPS at 2C (5678W usable power at a 54.6V full charge !)!
Thanks for the positive vibe! I was going a bit under the true numbers given its "used" cells. Its now going over $1000 with the new pack and other nicer upgrades-still not too bad.
Anywho, if you want to see a really nice 480cells pack its on this thread : https://endless-sphere.com/forums/viewt ... =6&t=79612
I supposed hes a builder so eventually things get sold so he prob has an endless budget.

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Matador   10 kW

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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Nov 02 2016 9:40pm

Hillhater wrote:I suspect a 0.5 mm copper layer on that 0.9 mm spring wire may just restrict the spring action more than a little ?
Have you considered using some short pieces of heavy copper braid inside the spring ? Maybe soldered each end ?
My attemps to copper-plate the springs (nickel plated springtseel) have failed miserably.
It would plate alright, but then as soon as I would try and compress the springs, the copper plating would just brake and flake off...

Now, as you have suggested, I think I will go with the "Copper Braid route". Seems flexible enough to adapt to springs action...

I really want to design a battery case that can support very high amps for futur uses (to me, the battery is way more important than the ebike motor :D ), nothing like those commercialy available battery cases.
So even though I'm still using some shitty, albeigt "extensively" tested (mAh & IR), laptop cells, I plan to buy some expensive quality high-amp 18650 cells in the future (rated 30A or 20A per cell) and fit them in my custom case. So for now, laptop cells are perfect for troubleshooting my custom battery case, as destroying these recycled cells is not as heartbraking.

What I want is to really troubleshoot is the overall resistance problem and reduce the milliOhms values to the minimum possible (If possible, to the point where internal resistance of the whole battery is almost only due to resistance of the cells themselves). So for that matter, I've already used copper busbar that are equivalent to 5AWG copper wires... Now is the time to fix the springs problems.

Another important feature for me is leaving the pack solderless, with individual springs pressing on each individual cell... I want to be able to troubleshoot individual cells for the futur.

I'm thinking of using 0.25 inch wide (6.3 mm) flat copper braid (rated for 32 amps according to an ebay seller).

The thing is, most sellers that sell this size and shape (I want the flat braid, not the tubular one) will sell the tinned version of the copper braid. Oh, and It's generally pretty expensive.
Does is matter if it's tinned ?

I think I'm about to buy this : http://www.ebay.ca/itm/50-Ft-Flat-Braid ... SwU-pXqIpM
What do you think ? Tinned good enough ? Gauge good enough (1 braid per cell) ? Will Tin raise the resistance of copper braid much ?

I made a prototype (mock-uo) using 0.100 inch solder wick... It looks like this (at 4 amp, a strand of that too small gauge solder wick gets slightly warm to touch though) :
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My 14 parallel string racks (for 14S13P) stacked in series.... Since I'm working on it, I've removed the series connection mini copper busbar shown in another thread (another solderless DIY pack by Snath, page 7).

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Re: Acceptable internal resistance values on 18650 cells

Post by JGaspar » Mar 08 2017 11:14pm

I was wondering if instead of the copper wick, one could use a coil of copper wire that could double as a fuse. The spring would need to be isolated from the circuit, humm ...another problem

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Re: Acceptable internal resistance values on 18650 cells

Post by Matador » Mar 30 2017 9:22pm

liveforphysics wrote:
brumbrum wrote:The impedance meter is an ebay purchase. It is just for voltage and ompedance for measuring batteries. I proably could have don it with a resistor.

AC Impedance meter's have there place in taking relative measurements of the same type of cell, but the value they give is not indicative of the DC load internal resistance, it's mostly a measurement of the ESR of the capacitance a cell inherently has.

Something like a 1-2Ohm resistor across the cell while measuring voltage sag (with a real DMM, at least 4.5digits) after 5 seconds is a much more meaningful value with respect to selecting them and matching them for building a pack.
I second that... Tried it with 5 resistors (0.5 Ohm, 1.0 Ohm, 2.0 Ohm, 2.2 Ohm and 4 Ohms) on VTC4 cells. I plot a graph. Get the slope and voila, the great of the slope is -r, where r is the internal resistance of the cell. Here my results in practice : https://endless-sphere.com/forums/viewt ... 5#p1276187

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