An upgrade for my old Ultrafire C3?

Hi all,

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.

Thanks in advance for your suggestions.

Thanks flashburn, looks you are totally right.

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.

Even though it works fine now with good batteries, I might try and improve the brightness by using better, more recent, LED and driver.

Any suggestion would be welcome for a replacement pill (seems to be 18mm diameter).