IMR newbie mess-up...?

[Brizzler]

Hi - got a bit of a problem, might have messed-up...

So I got myself a pair of AW IMR16340 that I've since tried out with various LAs - my introduction to rechargable incans. To the best of my knowledge I have treated them as I should have and they are still on their first charge.

Then, a few moments ago, I flicked on the my C2 running a P90 off the cells for a few moments and concluded the beam was yellowing and dimmer than before - time to recharge the cells.
Pulled the cells and checked them with my multimeter: 3.22V for first cell - not a major probem but definitely time to recharge, and 2.63V for second - happend there?!?
So:

1. How did that happen, bearing in mind the two cells started off at about the same voltage, having been fully charged, and have always been used in a pair??
2. What do I do now? :candle: Do I have to bin the second cell, well, in fact bin both and start afresh given that I need a matching pair of cells?? :mecry: Or can I recharge them..?

Any help and advice much appreciated.

 

[SilverFox]

Hello Brizzler,

Recharge them and let them sit for a few days, on a shelf and not in the light. Then measure the voltage of each cell.

If they still show full charge voltage, the cells are OK. This then means that there may be a slight drain on the light when it is turned off. That may be a little harder to find, but you can start by turning the light off, installing the batteries, then trying to measure any current draw across the tail cap.

If the cells drop substantially in voltage under no load, they are crap.

Tom

 

[Mr Happy]

It is not that strange actually. When you run two cells in series you will tend to see different end point voltages -- it is because no two cells are exactly alike. If you examine the typical discharge curve of a lithium ion cell you will see a very steep drop in voltage after the "knee" of the curve. This means that tiny differences in cell capacity can produce large differences in voltage, as you saw.

What you need to do when running more than one cell in series is stop and recharge earlier rather than later, so you reduce the risk of getting into this region.

For now, I would just recharge the cells and be careful not to discharge them so much next time. I doubt they have suffered any harm.

To see what I am getting at, look at this data sheet for a typical lithium ion cell: http://www.eneloop.info/fileadmin/EDITORS/BATTERIES/industrial/datasheets/li-ion_li-po/UR18650A.pdf

Look in particular at the 0.2C discharge curve, and see how it drops almost vertically at the 2200 mAh point. In that case, a difference in discharge of 2190 mAh to 2210 mAh would produce difference in voltage of 3.2 V to 2.8 V.

If you were running two of those cells in series you would want to stop discharging at about the 1800 mAh point, before they reach the knee of the discharge curve.

 

[VidPro]

nothing happened, it seems like a completly normal situation, where one battery is slightly lower than the other, especially if its at that end (of the discharge), where changes at the end of available capacity for the seires Cells could show even larger fluxuations in voltage than that.

if you thought it was running way shorter then it should, then i might check it for self discharge, and capacity.
pull it anyways as soon as it dims badly like that, as that assumes that at least one cell has reached the end of its discharge.

like they ^ already said.
if you had the same situation with a set of primaries, you would throw them away, and put in new ones. Welcome to recharging, where you just charge it up and do it again.

if i wanted some sort of perfection at the end better'n that, i would match the cells with some perfectly matched capacity, and have low cycled both ends, the same, and pulse charged it . . . but then i dont have a real life

think about: if it was 3x1.2 volt set of cells instead of having the 3.6V cell, like in nimhys, one could be at neer 0 while others are still reading 1v (when under a load), even if they all read very closly when midway through the discharge. same idea happens all the time, prefer to pull a little earlier than that, but thats life.

 

[Brizzler]

MANY THANKS for the words of advice and ressurance! :sweat:

Charging cells now, will follow advice given - resting cells, checking voltage and prevent future deep discharge...

 

[Brizzler]

Just a quick update: following a few days' rest (outside of a light) after charging the no load voltages of the two cells are 4.05V and 4.08V (in that order). So they appear to be fine! :sweat:

Thanks again CPF!