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.