18650's and their quirks

[J57ltr]

So for about the last year I have been playing with the 18650 styles of Li cells. Most are from used laptop battery packs. What I usually find is that they will be anywhere between 3S and 6S packs and up to about 4P. Most of the time I will have one set of the series cells that are completely dead. The others seem to charge and discharge fine within their parameters for the particular cell.

Recently I was setting up a set of cells that were from a 4S3P pack one set of parallel cells was dead, but the others seemed ok for my use. I had another 4S2P pack in the same situation (and same cells, brand and PN) so I took them apart to make a 3S5P battery. I ended up having a bad cell in one of the sets so I decided to remove them all mark them and cycle each cell individually.

I set up the test them by charging each one and setting it aside then when I was finally done (there are 15 of them) so they were able to rest for many hours until I started the discharge. I measured the voltage at rest then I discharged each one at 1 amp (1/2C for these cells). Some would only put out about 300 mah before they hit the 3V cutoff a few went into the 1400 mah range and some were less than 100 mah. The IR of each cell is between about 200 to 350 milliohms. The data is all over the board some of the ones with the highest IR performed the best at 1 amp and some with the lowest IR performed the worst and vise versa.

After playing with one cell started using lower and lower loads down to ¼ amp and I would get more than 1000 mah out of them. I will have to post the chart I made but it just doesn’t make a lot of sense to me. I am no battery expert so other than them being used cells I would like to know why they act this way.

Jeff

[BrianG]

Well, being used, they could be at or near the end of their life-cycle. And since manufacturing is not perfect, slight differences in material tolerances could account for the variance in cells for a given pack.

Individual cell life could be dependent on the physical proximity of the cell to a heat source in a laptop. There are certainly hot spots in any laptop and if only part of the battery pack makes contact with those spots, only those cells will be as affected.

[J57ltr]

What gets me is that I can pull almost full capacity out of them but only at pretty low amperages, but others I can get the same at higher amperages, all while having IR's that are very close.

What's even more odd is when I tested them as sets of cells 3P and 2P they all had the same discharge and charge times and mah, but seperate they are wildly different.

[BrianG]

How are you measuring IR? The best way IMO is to measure voltage unloaded, then measure again at X load, then take the voltage difference divided by the load current. Do that over the course of the discharge cycle to get a graph of IR. I think you'll find the IR for the poor performing cells is higher than the others.

When cells are in parallel, you are getting some conflicting results because a good cell is somewhat compensating for the not-so-good cells, and the not-so-good cells are surely affecting the performance of the relatively good cells. Basically, parallel cells masks each cell's true performance. Which incidentally, is one reason I don't like any kind of 2p+ setup.

[J57ltr]

It's a predetermined time in the discharge cycle, it actually unloads measures the Delta/Time and makes the calculation.

I was going to use the Eagle tree but was having problems getting the charger to work properly, it keeps telling me that there is no load. I am going to check the connections again I think I have a bad Dean's.

Also when I say the cells in parallel I am meaning as I pulled them so they should be about as matched as they are going to get.

I skipped a few days while I was out of town and am going to get started messing with them again tonight after I get this 4 wheeler unloaded.

Jeff

[J57ltr]

Ok so I got home and charged a single cell back up it took about 65 mah.

About an hour later I set it to discharge it at .2 amps 4.9 hours ago and it's still at 3.600V and it's at 1200mah. The IR is 189 milliohms.

One thing I have noticed is that at different discharge rates the IR changes.

Jeff

[snellemin]

IR always changes during discharge, up to a point. Same thing happens when charging at a higher rate. Temperature also effects IR.

[J57ltr]

This morning I had 1430mah out of this cell.

Then what is the point of knowing IR. I ahave seen the same cell at 275 mohms at one level and 25 at another level on a good cell.

Jeff

[snellemin]

IR is best taken at 1C charge rate and then will tell you the condition of your cell. Other then that I don't really care for IR and charge away.