A couple of nights ago I was messing with some old NimH AAs, and picked up a (non-eneloop) 2650 mAh Duracell that had been lying around. It tested 1.39 v., and I spontaneously put it in a UT-01 that is (was) afflicted with ‘tail cap disease’. The light had been set aside for future cannibalizing.
Those of you with UT-01s are familiar with it - many need to have the tail cap loosened and retightened after being shut off or they refuse to work.
I put this cell in and was rewarded with a very feeble light and no mode changes, so I stuck it on the charger - which refused it, and flashed a “HIGH” message, indicating high internal resistance….so I chucked the cell.
Then I went back and put a partially discharged Energizer Ultimate in the UT-01 and discovered that it now worked perfectly. The ‘tail cap’ disease that it’s had since almost new, is gone. I’ve used it mercilessly for the past couple of days, cycling it a lot and running it for lengthy periods on all levels, and it behaves flawlessly.
Now admittedly, the light still refuses to start on a absolutely fresh lithium primary (I suspect that at 1.8v it sees it as a depleted 14500) but it works on alkies and Eneloops as well as lithium primaries that are very slightly discharged.
Go figure! But somehow, that messed up Duracell with its high internal resistance fixed something.