Kodak voltage depression mystery

[Mr Happy]

I have a set of Kodak Pre-Charged AA cells that I have been testing to see how they perform.

In order to measure their maximum capacity I gave them a normal charge and then topped them off at 400 mA until the cell voltages had stabilized at the maximum of about 1.48 V for a while, and then rested them for an hour or two.

When I started the discharge test at 400 mA, cells 1, 3 and 4 began showing a steady reading of 1.33 V, slowly decreasing; but strangely cell 2 was showing 1.16 V. Puzzled, I wondered if I had forgotten to properly charge that cell or something? So I pulled it from discharge and put it back on the 400 mA charge. The charge voltage began at 1.44 V and within 10 minutes or so it had climbed back to 1.48 V. Even more puzzled, I returned it to discharging and this time it sustained 1.33 V and stayed there, slowly decreasing, just like the other cells.

The discharge went smoothly after that and I got readings of 2051, 2087, 2044, 2056 mAh. Cell 2 performed in line with the other three.

Now the low voltage of cell 2 reminded me of what happened when I tested the cells straight out of the pack. At that time I put them on 500 mA discharge and all four cell voltages dropped right down below 1.20 V at the outset and decreased from there. At the time I was surprised about those low voltage readings.

So there is a puzzle. These Kodak Pre-Charged cells almost behave as if they have a "high/low" voltage switch. Sometimes the voltage is high, and sometimes it kicks down and stays low.

(For reference, the discharge out of the pack gave readings of about 1750, so the low voltage didn't necessarily mean a loss of charge.)

 

[r-s]

I remember reading somewhere (don't recall where, or precisely when -- probably within past six months or so) that LSD batteries for the USA market are only charged to about 80% of full charge before packaged, but cells for other (Asian?) markets were given a full 100% charge.

I can vaguely recall talk (either informed, or speculative, I cannot recall) that the reason for this was the worry that people (in this country?) would sometimes forget (or not believe?) that they really were pre-charged, and would give them an initial charge before use. If the batteries already *had* a full charge, this would start them out in life with a bit of a disadvantage, i.e., a major kick to the shins via that bake-out before first use. With a less than full charge, an initial charge would not be as harmful. (Perhaps the cells are designed to endure a full "over"-charge if they are at 80% or less when the cycle begins? This would be nice to know in any event!)

 

[Mr Happy]

Yes, it's certainly true that the LSD cells you buy in the USA have an initial charge that is about 80% of a full charge.

This seems to be something different though. I had a cell that I had personally charged to 100% beforehand, yet on discharge it started reading 1.16 V and holding. I pumped barely another 50 mAh into it, and then on discharge it read 1.33 V and holding. It sustained the 1.33 V with a gradual decline from then onwards without dropping back down to 1.16 V until much later in the discharge.

It's this "two voltages" thing that puzzles me.

 

[UnknownVT]

I had a cell that I had personally charged to 100% beforehand, yet on discharge it started reading 1.16 V and holding. I pumped barely another 50 mAh into it, and then on discharge it read 1.33 V and holding. It sustained the 1.33 V with a gradual decline from then onwards without dropping back down to 1.16 V until much later in the discharge.
It's this "two voltages" thing that puzzles me.

I am not able to do this kind of testing - but I do have two sets of the Kodak Pre-Charged -
I realize that open-circuit voltages are different from under-load -
but FWIW - my second set (bought Sept/2007 coded 7F) measured straight from the packet 1.309, 1.308, 1.308, 1.309V open-circuit
(I wasn't aware enough to have measured my first set, June/2007 coded 7E).

I got 8x eneloop AAs Nov/2007 (Costco pack) - they all measured 1.31V open-circuit out of the package.

Since I had read that eneloops (and Uniross Hybrios) seem to maintain higher voltage - I have subsequently been taking o-c voltage readings of both the Kodak P-C and eneloops to see if there was a difference.

The most significant, as I reported elsewhere, was from the same second set of Kodak P-C in direct comparison with a set of eneloops - which both had been charged about 3 months ago - all open-circuit voltages

Kodak P-C
1.33V FA=9.2A --> 1.317V
1.33V FA=8.6A --> 1.318V
1.33V FA=9.1A --> 1.319V
1.33V FA=8.6A --> 1.317V

eneloop
1.329V FA=9.9A --> 1.322V
1.328V FA=10.1A --> 1.318V
1.328V FA=10.1A --> 1.320V
1.327V FA=10.13A --> 1.315V

FA = "Flash Amps" - sequence of measurements was initial voltage readings for all 8 batteries - then "Flash Amps" - then in the same order another voltage reading (marked as the -->).

So one can see at least in open-circuit voltage readings the Kodak P-C and eneloops seem about the same - the Flash Amps shows that the eneloops can deliver more current which indicates lower internal resistance - but the subsequent/following recovery voltage readings seems to show the batteries are about the same, even after the short but harsh treatment of being more or less shorted.

 

[Power Me Up]

Yes, it's certainly true that the LSD cells you buy in the USA have an initial charge that is about 80% of a full charge.

This seems to be something different though. I had a cell that I had personally charged to 100% beforehand, yet on discharge it started reading 1.16 V and holding. I pumped barely another 50 mAh into it, and then on discharge it read 1.33 V and holding. It sustained the 1.33 V with a gradual decline from then onwards without dropping back down to 1.16 V until much later in the discharge.

It's this "two voltages" thing that puzzles me.

Maybe there was some dirt, etc on the terminals of the cell/battery analyzer causing a high contact resistance?

When you took the cells out for a second charge, it could have been that you got a better contact when you reinserted the cell and it had nothing to do with the extra charge...

 

[Mr Happy]

Maybe there was some dirt, etc on the terminals of the cell/battery analyzer causing a high contact resistance?

When you took the cells out for a second charge, it could have been that you got a better contact when you reinserted the cell and it had nothing to do with the extra charge...

The simplest explanations are always the best. That would explain all the observations of course.

I'm going to watch and see if anything similar happens in further tests, otherwise I'm going to assume it was something like that.

 

[UnknownVT]

The simplest explanations are always the best. That would explain all the observations of course.
I'm going to watch and see if anything similar happens in further tests, otherwise I'm going to assume it was something like that.

I know exactly what you mean -
if you recall it was you who reminded me that the leads on my cheapo DMM may have had high resistance in my previous "flash amps" reading - it was like a Eureaka moment when I tried a better set of leads - then cleaned the contact plugs - and I had been going for years thinking that there was something not right about my DMM for flash amp readings.

Thinking of this - just give me a depression