Acceptable internal resistance values on 18650 cells

brumbrum

100 kW
Joined
Oct 30, 2010
Messages
1,383
Location
Wales U.K
As in title, If using laptop used and rescued 18650 cells how would i work out what internal resistance values would still make for a good cell that charges to 4.2v

Thanks.
 
I am not aware of any direct correlation between the IR of a cell and it's ability to charge up to 4.2 v.
IR will influence many parameters, most notably the maximum charge and discharge rates, the voltage sag under load, and the heat generated internally during charge/discharge.
But the peak charged voltage is more a function of the cell chemistry and it's condition .
Typically a "good". 18650 cell would have a discharge IR of between 50 and 100 mohm (0.050 - 0.100 ohm)

EDIT :
I would add that if you are looking for some way of assessing if cells are good or bad, then IR can be very useful, but you will need to do controlled, accurate, discharge tests on each cell to first determine their individual IR, then sort the results.
Any "extreme" high IR cells should be junked, and other cells sorted into "matched" values for parallel groups.
Extreme low IR values should also be treated with some suspicion and subject to further tests.
Periodic checks of cell IR is a very good way of preventing unexpected cell failure...but this needs you to keep records of each cell's IR and rechecks each few months/weeks, depending on your usage pattern.
Also, IR is only one parameter for cell matching.
Equally important is to match cells with similar total capacity (Ahrs)
 
Thanks for the detailed answer. What if i want to use slightly differing AH cells together. For instance 2.2ah and 2.4ah but treat them as 2.0ah cells when in use? If i run them in series 2.4, 2.2, 2.4,2.2,2.4 ,2.2, 2.4 etc will this have abad effect on the pack as a whole?
 
I tried to make a 18650 pack last year out of old laptop cells. I tested the cells to have whatever the half cycle life internal resistance was, can't remember what value it was. Anyways I made a 7s 15ah pack and it was garbage. It sagged 5v+ and would not reach more then 15mph when my good lipo pack would make it to 25mph.

But if you want to try you can run whatever amp size cells you want in the parallel strings, just keep the total amp draw in the 2c rating and make sure each series string matches amps. I used 2.2ah and 2.4ah cells on my pack.
 
I have made a number of 18650 packs for my ebikes. From 54v 8ah to 54v18ah. I have just come into the possession of a few hundred 18650 cells which i am currently charging in pairs at 0.6a. I want to take the best cells that keep their voltage after some standing time and also have the least IR. I find that i sometimes build packs from laptop cells that get hot spot corners on the packs which lead to voltage sags when used, which i guess is IR in some of the cells in the pack.

The packs are run on a controller set at 25amp peak which pulls over 1000w on take off and on steep hills, which then steadies to a out 14amps.

I have a number of differing cells from differing manufacturers from 2.0ah to 3ah , i have just ordered a battery impedance meter with the hope of finding which cells i have will give the least resistance for building a new pack as in some instances i only have 20 cells of the same brands and ah's. I need 78 cells to make a 54v 12ah pack ( if all cells are considered as 2.0ah and 2C peak current rating).

Andrew
 
Good luck. I vowed to just purchase new cells when I do make a 18650 pack and use what I have for a future solar project. If you can keep your pack cheap more power to you! That ir tool sounds handy, where did you get it? I just used a resistor and wiring from a online guide, probably didn't give a good reading.
 
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.
 
brumbrum said:
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 realise that know from looking up youtube clips, but hey ho, the meter should give me a little help choosing cells,... Will it?
 
brumbrum said:
I realise that know from looking up youtube clips, but hey ho, the meter should give me a little help choosing cells,... Will it?

Maybe, maybe not? I do know measuring voltage sag under known Amp load will provide the data you need.
 
I believe i may have tried the easy option without thinking it through. Impedance is more for AC current, and Internal resistance is for DC current. They are two differing measurements calculated in completely differing ways. As usual, too many beers and an easy tap of the buy button without any research until afterwards. There should be a breathalyser on internet connection after 8pm for purchasing on the internet.
 
liveforphysics said:
Something like a 1-2 Ohm 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 did some DC internal resistance tests myself with a 3.7 Ohm resistor (it's actually a 1.0 Ohm ±5% 7W resistor wired in series with a 2.7 Ohm ±5% 3 W resistor). I measured voltage sag with el cheapo Mastercrap Multimeter.

Here's How I proceeded :

1) Measure cell's voltage (V1)
2) Measure cell's voltage (V2) with the 3.7 Ohm external resistor (R(ext)) wired across the cell (I wait around 10 sec to be sure to let the voltage sag to it's deepest)
3) Determine voltage sag value (dV) for the cell with this formula :
dV = V1 - V2
For most of the cells I tested (between 5 and 10 years old) : the voltage sag was generaly anything between 0.10 to 0.30 Volt with that 3.7 Ohm as the load.

So using Ohm's Law, I calculated the DC internal resistance with this formula :

R(cell) = [dV x R(ext)] / V1
So for exemple, if the average cell dropped for 4.20 to 4.05 volt, the dV = 0.15 (voltmeter error margin : ± 0.01 V)
R(cell) = (0.15V x 3.7Ohm) / 4.20 volts
R(cell) = 0.13 Ohms
Or around 130 milliOhm (keep in mind the error margin on the measure must be around 10% of the resistance value I guess).

I tested around 200 harvested laptop cells for Internal resistance, remaining capacity and voltage stability over time.

Here are the results in a Excel file if it can give you an idea of what to expect from laptop cells (took me long to measure all this !) : View attachment Li-Ion.xlsx

As you can see, individual laptop cells generally sucks beeing high resistance... between 100 and 300 milliohm per cell.

In practice, I made a pack (14S 9P) with the best cells I could select (see . I equilibrated cells so to have the same resistance and capacity for each parralled string.
I ran this 51.8V 20.7Ah pack with my BBSHD (riding hard ! 30 Wh/km measured with watt meter) around 6 times. Between 29 and 32 km each time (avrg speed 30 km/h top speed 55 km/h).
Every time the pack would get hot.... Above 49-50°C... Once, I've reached 70°C wich can't be good.

Fully charged at 58.7V measured, when I hit the throttle wide open, I'd reach around 1400 Watts on my watt-meter, but the voltage would sag to 49.5Volt !!!
Way too much sag !

After around 200 km use, I dismantelled my battery. I'm retesting each cells now to screen for changes in capacity and resistance (see ).
I think on average, I lost 10-20% of cell's rated capacity from abusing them ! So laptop cells are not long lived at all for my use...
My BBSHD has a 30 amps controller. That means for my 14S 9P battery, It's a max of 3.33A load per cell or aroung 1.5 C.
So my conclusion is.... laptop cells are very weak... they will be short lived for an e-bike.... at least when used at 1.5C peak and around 0.5-1C cruising on a BBSHD.
So if I have to go with laptop batteries again, I think I'd have to go with a 14S20P battery to satisfy my BBSHD's power hunger ! And 14S20P is way too heavy!

I have to admit that my battery build is a bit non conventionnal, being solderless, weld-less... I used springs for connections as I explained in another post see here : https://endless-sphere.com/forums/viewtopic.php?f=14&t=57810&start=150

Anyways, that was my two-cents...
Matador.
 
Just to be thourough, I forgot to say. After these 30 km rides. I stopped riding not because my battery was empty, but because it was getting hot !
After these 6 equivalent 30 km trips , my cells were generelly between 3.72 to 3.80 Volts...So around 52 to 53 volts for the 14S pack... So I figured there was still around 5 to 10 Ah remaining in the battery, but I didn't want to shorten the battery life from the excessive heat, or worst, making it burst into flames... so I stoped riding. The thing is, even when riding less agressively, using PAS at 2/5 (600W), the battery still got to 45-50°C...
For the record, my bike is a 67 pounds "Nakamura Rouen 19inch hybrid" including the battery and the 30A - BBSHD kit. I t has 700C wheels, 46T chainring, rear gears 14T (for high speed) and 34T (for granny speed). I onced hit 62 km/h top speed with is, but generally, on full charge (58.8V), I hit around 58 km/h top speed.

I weight 155 lbs.

For determining individual cells capacity, I used a Foxnovo Digital charger 4S (I used the 1.0A CAP function for discharg, which in reality discharges at 0.5A per cell... so around 0.2C).
For on bicycle measurements (watts, volts, amps), I used a "Yeeco® Digital Multimeter DC6.5-100V 100A Voltage Amperage Power Energy Meter Voltmeter Ammeter DC Voltage Current Volt Amp Meter Tester Gauge Power Monitor Measuring LCD Display with 100A/75mV Shunt" from amazon.
 
For sake of comparison

Can anybody measure the internal DC resistance of a fresh, high-amperage rated 18650 cells and post their results here ??
(like the Samsung 18650 30Q 3000mAh - 15A, the LG 18650 HG2 3000mAh - 20A, the SANYO NCR18650GA 3300mAh - 10A, the Panasonic NCR18650PF 2900 mAh - 10A, the Sony IMR 18650 VTC4 2100 mAh - 30A, the Sony IMR 18650 VTC5 2600 mAh - 20A and many others.)

Feel free to post your values here, as this could serve for a future reference.

(NB : if you measure AC impedance instead of DC resistance, please clearly indicate it)

It's easy to do with the method I described above in this thead.

I always wondered what new and fresh cell's DC resistance Ohm values would be.

I plan to fill my custom battery case with some high amp-rated 18650 in the future.
I don't mind sacrifying a bit of capacity for higher instant amperage capability.
I'm thinking of choosing good, new 30A rated cells!
If I can build a 14S13P pack from 30A cells, the pack would be theoretically be capable of delivering 390 Amps instantenous ... So the 30 amps my BBSHD would pull would be a walk in the park for these cells (even though I have a 80A BMS lol).
The point is, I would like to baby those new cells to prolong their life to the max (and of course charging them only to 80 or 90% of their rated capacity).
And one way to prolong their life by not making them overheat is choosing ultra high-amp rated cells in my opinion. That way, cells wont be stressed out :lol:
 
Thanks for posting your data and findings. I wish i could help you out with the high discharge 18650 cells, but now i use hobbyking lipos and an adaptto controller which actually records each cell discharging and the battery packs milli-ohm resistance. Lucky me having a computer to work it all out :D
 
Search the forum, this has all been done before and the data posted.
Generally a "good" 18650 cell like the Samsung 25R will have a DCIR of 25-30 mohm.
But it will vary depending on many factors like SOC, Temp, etc etc
Also there are independent sites that have lists of performance data for ALL 18650 cells, including capacity, discharge rating, temperature build up, DCIR, cycle life, etc etc.
EG.. http://lygte-info.dk/review/batteries2012/Common18650Summary%20UK.html
There are several others.
 
Hillhater said:
Search the forum, this has all been done before and the data posted.
Generally a "good" 18650 cell like the Samsung 25R will have a DCIR of 25-30 mohm.
But it will vary depending on many factors like SOC, Temp, etc etc
Also there are independent sites that have lists of performance data for ALL 18650 cells, including capacity, discharge rating, temperature build up, DCIR, cycle life, etc etc.
EG.. http://lygte-info.dk/review/batteries2012/Common18650Summary%20UK.html
There are several others.

Wow thanks for that info!
At least on lygte-info.dk , it seem hard to find an 18650 with a DCIR smaller than 45 mOhm.
I'll look up for that thread
 
Heres some good info on battery capacity and discharge temp
http://www.che.sc.edu/faculty/white/2002CapacityFade%20ofsonPart%201%20RamadassHaranWhitePopov.pdf
I keep my latest pack open and not shrinked wrapped or duct tape, it stays atleast 10*C less and cool down much quicker. I also been experiment with cooling fans as temp hit 35*C ipand that seems to help battery pack cool down quicker.
 
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).
 
BeachRider2016 said:
Heres some good info on battery capacity and discharge temp
http://www.che.sc.edu/faculty/white/2002CapacityFade%20ofsonPart%201%20RamadassHaranWhitePopov.pdf
I keep my latest pack open and not shrinked wrapped or duct tape, it stays atleast 10*C less and cool down much quicker. I also been experiment with cooling fans as temp hit 35*C ipand that seems to help battery pack cool down quicker.

Thank you BeachRider ! This scientific publication is spot on with my questionning !!
I just love it when people back their comments with reputable reference. Indeed ES is the place to go for questions !
 
Punx0r said:
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.
 
eTrike said:
Hi. How is your pack constructed? Nickel tabs used for series?

In my case, I'm using all copper bus bars made of flattened copper pipes.
See my post for photos : https://endless-sphere.com/forums/viewtopic.php?f=14&t=57810&start=150#p1213147

As you can see I also use brass (65% copper alloy I think) nuts and bolts and plated springsteel springs for contacts.

For the details, see my post on the link above.
 
Back
Top