Hello BLFers,
I hope everyone of you had a nice weekend. I made some interesting observations i.a.w. low drain batteries.
To be more precise, I have been using two Sofirn 18650 3.000mAh batteries (while in adapter sleeves) in a Sofirn SP70 flashlight.
This sample here is a so called customer return from Amazon that was kindly sent to me by Sofirn and was probably sent back to Amazon for reasons of a clouded/milky reflector or glas lens. Apart from this imperfection it worked like a charm. It came without any batteries.
Coming to my observations:
Since my 26650 batteries were still charging I thought I try two of those Sofirn 18650 3.000mAh batteries that were part of the SP36 flashlight. Since both flashlights are FET-driven I did not expect much of a problem by using them in the SP70. While the SP36 runs their cells in 3P-configuration the SP70 runs them in 2S configuration. I expected the SP70 to run with less output for these cells are known to have higher internal resistance and deliver less current than high drain cells like Samsung 30Q or Sony VTC6. Both cells had a levelled resting voltage of 4.02V when I put them into the SP70. I ran the SP70 about 10-12min with maximum power. The flashlight became considerably warm on the head section, warmer than I expected it to be with these low drain cells. Afterwards, I unscrewed the SP70's tailcap and took out both batteries. They were moderately warm (but still touchable) as a result of the heat distribution during operation. Both cells were depleted down to 3.64V. Now, comes the interesting part...
While checking the voltage I sensed a very uncommon, unpleasant odor on the positive terminal of the front battery (that was in contact with the driver spring). It smelled very sour and pungent to my nose, like burned chemicals. Since I have not yet experienced any cell leakages, thermal runaways or ventings I cannot judge if this odor has such a serious root cause. The warm battery cooled down and so did the unpleasant smell disappear after about 5 minutes. For reasons of safety I stored the battery outside at a safe place in case it should explode. The next morning I decided to unwrap it. Here's what I have seen so far...
Until here, everything seemed to look rather unsuspicious. This battery is not made by Sofirn but from a third-party manufacturer like DLG. The coding however does not look like it's a DLG battery. We see a tyical joint-welded button top with a disintegrated joint (probably due to bad welding)...nothing unusual for this process technology that is known to cause significant additional contact resistance and doubtful electric contact between the very terminal and the attached button plate. So, I decided to strip the cell and remove the blue sealing ring to check what's underneath...
An even closer look revealed something odd...
Now, actually I don't know what to make of this but to my lay opinion this doesn't look right i.a.w. the chemical odor I sensed the day before. It looks like the containment has failed and some of the cell's content broke out. Should my observation be right, I am happy that it didn't end any worse. I will dispose this battery (with sealed terminals) right away. I hope to receiving some feedback here if I am right or wrong with my assumptions.