I've recently bought a 95 led 3XD cell flashlight. I would say its only giving in the region of around 150 lumens. But for something like $7.00 its still a bargain I think.
Then I tested its ampdraw. It was 0.49A. That is with the set of 3D alkalines included with the light. The batteries gives 4.68V after I used the light for something like 3 to 4 minutes.
Then I tested it with 3X Nimh AA's. The AA's were not fully charged and gave me 3.91V. But what interrested me was that when testing the ampdraw with the AA's it was 0.61A
Now for my question. The leds are now basically being overdriven by about 20%. How much will the use of Nimh'a shorthen its lifespan?
Hi light. Many 3 cell LED lights are direct driven (no actual driver), and work on the principle that alkaline cells drop significantly in voltage when under load (and thus offer less available current). As you have discovered on your own, NiMh (and NiCd as well) cells hold voltage under load much better than alkaline cells. It's not uncommon that even AA NiMh cells in good condition, will hold a higher voltage under load than D cell alkalines.
As for how this affects LED life, it all depends on just how much they are over driven. In most cases though, when substituting NiMh cells for alkalines, sooner or later in multiple 5mm LED lights, some of the the LEDs will start to flicker, and then finally burn out completely. The First ones to go are the LEDs with the lowest Vf. Then, as more burn out, the available current is divided up equally among fewer LEDs, and eventually they will all burn out.
The only direct driven 3 cell multiple 5mm LED lights that I have from the "old days", that still work on NiMh cells, are those that have individual resistors added for each LED. Even these lights are starting to burn out, as they just weren't meant to be driven at the higher current levels that NiMh cells supply.