JeremyChoy said:
JeremyChoy said:
daromer said:
This is how I see it.
On smaller packs where you will be using 1A discharge this could cause issues... But on packs where you perhaps will reach 200mA at most or even only 50mA then this is not considered to be a big issue.
The problem that can arise is that if you calculate 1A at max and you have mixed cells you could get an uneven useage and the ones with lowest iR may reach 1.5A instead...
Have you tested the ir on above cells? Would be nice to see what the difference in a table would be on above 3.9-3.6V compare to IR.
shouldn't take too long to test IR. small project for tonight.
Voltage sag by colour, organized by IR
3.9x = Dark Green
3.8x = Light Green
3.7x = Yellow
3.6x = orangish.
Jeremy,
What I am doing is grouping cells by manufacturer, model number and measured capacity.
If you do this, then your measurements make sense. If you have a 7s pack,you should look for groups of seven with the same battery capacity, manufacturer, and model& internal resistance. When you build your parallel packs, you put one of these cells in each of the parallel packs. If you do this consistently, then each pack should charge and discharge evenly.
I have downloaded the data sheet for a lot of my cells and track the data sheet capacity versus measured capacity. That way, I get an idea of how damaged cell is.
High Drain Batteries
Im maybe a little overboard on this, butI am segregating my high drain batteries to use in high drain packs. I have some projects whereI plan to use the batteries for some high drain intermittent applications.
A high drain battery may only be rated for 1300 mAH , but it can deliver 20A continuous current.
So If I get one of these, the internal resistance is quite low, but in a low current application like .25 amps, it will still only give 1300 mAH.
Measuring Open circuit voltage.
The open circuit voltage may not be a validdatapoint for determining the internal resistance. I was thinking about this myself and found a good article at battery university about how to measure internal resistance. For real world measurement, use the"two tierdc load test".
Here's the battery university link:
http://batteryuniversity.com/learn/article/how_to_measure_internal_resistance
You should apply a small load for a few seconds. 0.10 amps and measure the voltage.
Then measure the load at 1 amp. measured soon after the first measurement.
So in the following example you have an 18650 charged to 4.17 volts.
Drain it at 0.10 amps for a few seconds and read 4.10 Volts.
Then drain it at 1.00 amps for a few seconds and read 3.90 Volts.
In this case the internal resistance is delta V / Delta I or (4.10-3.90) / (1.0- 0.1) = .20V / .9 A = .222 Ohms = 222 mOhms.
IMAX B6 Cable resistance
The other thing to consider when you are trying to read small resistances is the resistance of the test leads and connections.
Take 10 batteries of "known" resistanceand do the same test. Then hook them all in parallel and hook up your test leads.
In theory you should see one tenthof the voltage drop with the one amp test. Whatever the difference between calculatedaverage voltage drop at .1 amps and actual voltage drop at 1 amp is due to the IMAX B6 charger cables.
