New eneloop, charge and discharge tests at 1A

I have a 4-pack of new eneloops that I recently bought.
I just ran 1A charge and 1A discharge cycles on them all, using my Turnigy Accucell 8150 Hobby charger. I then corrected the data to account for the measured readout error the Hobby charger has in current and to correct for voltage drop through the wire I use. And here are the results in chart form:

Charge:


Discharge:


Musings on the results:
When charging, I have the -ΔV cutoff set to 5mV.
I also have the Temp max set to 40°C.
Only eneloop #2 (green) terminated based on -ΔV in this test. In all the other cases, termination was by max temp, although you can clearly see that the cells were fully charged by the curve. There just wasn't enough of a -ΔV to trigger the 5mV termination. This shows why it is so important to have a max temp, or even better, a ΔT/Δt (delta of temperature over delta of time) cutoff. As well as ideally a max mAh and a max time cutoff too.
I also had set a max mAh of 2500mAh, and a max time of 150 minutes, but neither of these termination methods was reached in the the tests, because in the case of eneloop #2 -ΔV activated, and in the cases of eneloops #1, 3, and 4; the max temp cutoff activated.
I think I would be better to tweak the max mAh to something like 2200mAh, looking at the chart. Once the curve flattens out after the -ΔV, any further charging is fairly pointless and possibly just damaging to the cell.
I might try the same tests again but using a 2A rate. Maybe there will be a bigger -ΔV signal at that rate and they might all terminate using that method at that rate, or maybe even at 1.5A.

samgab said:

Only eneloop #2 (green) terminated based on -ΔV in this test. In all the other cases, termination was by max temp, although you can clearly see that the cells were fully charged by the curve. There just wasn't enough of a -ΔV to trigger the 5mV termination.

Click to expand...

I don't understand this from your chart. The data appears to show the blue, red and purple cells having a -ΔV of -40 mV while the charger continued to charge. For example the blue line peaked above 1.52 V and dropped down to 1.48 V while charging continued and the temperature continued rising.

It seems that either the plotted data is not showing an accurate picture or the Accucel charger is not working properly...?

Mr Happy said:

I don't understand this from your chart. The data appears to show the blue, red and purple cells having a -ΔV of -40 mV while the charger continued to charge. For example the blue line peaked above 1.52 V and dropped down to 1.48 V while charging continued and the temperature continued rising.

It seems that either the plotted data is not showing an accurate picture or the Accucel charger is not working properly...?

Click to expand...

The -ΔV termination of this 8150 is very hit-and-miss, I've found. The charted data is correct.
However, something that is filtered out of the data, is that when charging NiMH cells or batteries, the 8150 pauses charging every few seconds for a moment to take an OCV reading. I think it gets it's -ΔV termination signal from that OCV reading rather than the displayed VUL reading.
I have all the raw data, and can chart it showing all of the OCV read pauses, but it looks very messy.
Like this:

I think what is more likely, is that this 8150 is just not very good at -ΔV termination. Which is why I am glad of the 3 other termination methods.

I've just charged one of the eneloops at 2A, and it still terminated by max temp rather than -ΔV. Again, the -ΔV is clearly visible on the chart.
It seems to need closer to 50mV -ΔV to terminate.
Maybe it would work better using hobbyists battery packs of multiple NiMH cells in series, at a high charge rate, than single eneloop cells.
But for the most part I'll use the C9000 or the C808M for charging the eneloops, I just wanted to see the results of graphing it on the 8150.
I'll post comparison charts of the 2A rates when I have them.

samgab said:

However, something that is filtered out of the data, is that when charging NiMH cells or batteries, the 8150 pauses charging every few seconds for a moment to take an OCV reading. I think it gets it's -ΔV termination signal from that OCV reading rather than the displayed VUL reading.

Click to expand...

I don't think that is a very reliable way of detecting a -ΔV signal. The drop in voltage is caused by the drop in internal resistance of the cell as the temperature rises. Essentially you have:

-ΔV = (I)(-ΔR)

If there is no current flowing the voltage drop will not be visible (unless you measure the voltage really quickly after stopping the current, before it has had time to settle out).

I think perhaps you are right, that the charger is not very good at termination on single cells, but maybe it does work better on packs.

Hi Sam. I've alluded to this before, quite a few times. "Hobby" chargers are not really "electronics hobby chargers", but rather are "R/C hobby chargers". Thus they are primarily designed for charging welded, or soldered "packs" of cells that are in series . For this purpose, they do an excellent job, but when charging, particularly individual NiCd or NiMh cells, they really don't do all that good a job. For Li-Ion cells, it's a bit different, as the "rules" are simpler, so hobby chargers work quite well for these.

Dave

45/70 said:

Hi Sam. I've alluded to this before, quite a few times. "Hobby" chargers are not really "electronics hobby chargers", but rather are "R/C hobby chargers". Thus they are primarily designed for charging welded, or soldered "packs" of cells that are in series . For this purpose, they do an excellent job, but when charging, particularly individual NiCd or NiMh cells, they really don't do all that good a job. For Li-Ion cells, it's a bit different, as the "rules" are simpler, so hobby chargers work quite well for these.

Dave

Click to expand...

Duly noted. I realise, of course that you're correct. However, I have still learnt a lot from running experiments like this on various cells using the hobby charger. I'd really like to get a CBA III, but it's really only to satisfy my curiosity, so I just can't justify the spend. And any proper lab grade test equipment is just inaccessible.

samgab said:

......I'd really like to get a CBA III, but it's really only to satisfy my curiosity, so I just can't justify the spend. And any proper lab grade test equipment is just inaccessible.

Click to expand...

Yeah, I "needed" one about as bad as I needed a bad case of the flu. I will say that after having mine for 4-5 years now, while I don't really use it that much, it shows things that there just isn't any other practical way to see, short of "lab grade" equipment, as you mentioned. In my opinion, for what it does anyway, while usefull, I think it was way overpriced. As I undertand it, many feel that way about the CBA III (mine is a "II"), as well. But, if you just have to have one, they are pretty neat.

Dave

Just for interest's sake, I added the results of a 2A cycle on each cell:

2A charge:


2A discharge:


Combined results:
Charge:


Discharge


Charging at 2A, the max temp terminated the charge each time. I've set a somewhat conservative 40 degree C temp cutout, but it is enough to see the sharp increase in temp when the battery is no longer accepting further charge, and the chemical reaction changes.
I also fine tuned the Voltage drop correction by 0.05V after more testing, measuring the voltage at cell compared to the chargers voltage readout under load.