I have an old (maybe 15 years old?) 2AA Ultrafire C3, first model, one-mode.
It is still very bright with freshly charged NiMH batteries, but as soon as the input voltage is not optimal, it lights up with a very bright flash but after less than one second goes to a stable dim output.
I already carried out the "solder blob" fix a few years ago because it did not always turn on, now it does but because of the above issue I wonder if there is a way to upgrade it with a better pill and LED.
These are in fact more than ten years old Varta Ready2Use, and one of them was much more discharged than the other when I checked them out of the flashlight. My BT-C2000 charger reported an internal resistance around 300 mΩ for both of them.
I have replaced them with basic non-rechargeable Alkaline and the flashlight shines perfectly bright.
Since I plan to give it to a friend when my next flashlight (Emisar D4v2) arrives, I will not bother replacing the batteries with new rechargeables, nevertheless I'd like to know more about the "newer 1.5V rechargeable Alkalines" you have mentioned. I assume I can find plenty of information (and reviews?) about them here on BLF or on CPF, but I need to know what their commercial name is in order to search for them.