From my tests, the Fusion dropin needs above 4.8V to give of its brightest. Below that it isn't as powerful. That said, I have a 3D at the back door with 3 8000mAh LSD(ish) Ds in it. It works for large black dog spotting in the backyard and doubles as supplemental lighting for that end of the kitchen which does get a bit gloomy at night. Even with LSD cells, self-discharge is more of an issue than current draw.
Sofar out of the 4 ive got ive only givn them a quick charge and discharge but im getting around 4500 mAh average and im sure they will pick up a bit more with a few more cycles but i dont think they will hit the 8000 mAh that is claimed.
Consider that a 10,000 mAh Accurevolution LSD D cell costs about 12 dollars each, minus discounts, bulk, etc.
4 Eneloop AAs (8,000 mAh) costs about 10 dollars each. 5 (10,000 mah) cost 12.50 then. (plus the adapter cost)
So you really aren't saving any money with this approach. You are just going through a bit of extra trouble, introducing an extra point of failure, and effectively using lower capacity cells.
I would recommend against doing this UNLESS you have a pretty good charger that is AA/AAA only and don't want to spring for a universal smart charger. Which is probably a fairly common scenario actually.
That's why I thought the cheap D cells I posted where ok I plan using them with a TK60 when I've saved up enough.
The AA adaptors are good if you already have nimh or LSD batteries and or a hoby charger that can charge up to 16 at a time and form charge, or you could use cheap alkaline batteries but that's not always recommended and they could leak under high loads, but nimh can handle decent current.
The tenergy 10 000 mAh D cells would be nice but shipping to Australia is was to much and alot if sellers will not ship internationally.
I ordered a bunch of the Tenergy AA both LSD and standard, and then some of the D cells, and shipping was quite good, they calculated it after I bought and it was nothing like what Ebay was suggesting.
Well, as several people were alluding to, nobody can FIND the damned adapters! I thought of that a while ago when I bought my TK70. But I could only find a few 2 AA adapters for fairly expensive and looked cheap. The 3xAA DO exist...um...somewhere. All I could find was Serial connections instead of parallel which is what you want. (serial means youd have a 3.6v battery instead of a 1.2 and things would go poorly)
The 3AA ones might be fatter than a normal D battery so nobody really knows how well they would fit.
What about just getting the 3AA serial ones from KD and converting them to parallel like dthrckt did (see above posts 10,13, & 15). This sounds perfect. I'm sure dthrckt can chime in on the diameter of them compared to a normal D cell.
I did purchase some 2AA -> D adapters from DX, but I haven't used them yet.
So if I manage to find 3AA to D adapters/converters those would be applicable for the TK70 as well?
I'm just worried about fryingthe light since it's not cheap. What would be the criteria for watching out for what voltage to avoid frying the thing then?
If you're placing the cells so they are in parallel inside the adapter (all +'s making contact together, and all -'s contacting each other) than you won't be increasing voltage - only increasing capacity (mAh).
the KD 3xAA are slightly narrower than tenergy Ds and they're the same length. The HF 109 led light has a 4AA carrier and it is ~0.070" wider than a D cell (with alkaline AAs in it).
Back about 6-8 years ago an online company called Aventrade offered a decent parallel 4AA-to-1D battery adapter. It was larger in diameter than a standard D cell (as Lennart's Post picture demonstrates), but some flashlights - think plastic body cheap Rayovacs and the like - could fit them. On the maglites I used them in I had to take one of the 4 AAs - leaving 3 in parallel, and they fit snug, but workable.
Custom parallel 4AA-to-1D and 3AA-to-1D battery adapters are currently available, but pricey: