I’ve never had much interest in Li-Ion powered lights. But a friend of a friend of mine is selling a TK75. And let’s just say the price is right. So I’m considering buying it. I actually have a few A123 LiFePO4 18650 batteries laying around. And I was wondering if these would run a TK75. I know amperage is not an issue, as these can kick out more current than just about anything. But what about voltage? On one hand, it’s never as high as LiCo. But on the other hand, at the typical current draw, those batteries can easily hold a steady 3.2-3.25 volts until they are pretty much depleted. Would this be an issue?
I was thinking about this today as well. Are the cells on a TK75 series or parallel? If they are series, I would give it a shot if I had one, as a buck driver should be able to deal with a slightly lower overall voltage.
Edit: from images it looks like the cells are in series so the driver may be able to handle other chemistries. I was thinking of the possibility of running one of these big triples on LiFePo4s, instead of LiCos as well…
tk75 uses 4 batteries… i think two sets of batteries in parallel with 2 batteries in series in both sets… so it works with 2 batteries also…
Im out of my league here but very curious as to what others with more knowledge might have to say…
As I understand it, the TK75 uses four batteries but can run on two. This suggests a 2s2p arrangement. Also, since even two LiCo cells are only going to give a peak of 8.4V fully charged (and 7.4V nominal), the light can’t possibly be driving the emitters in series. So it would have to be driving them in parallel with a buck driver. This would DEFINITELY work with the dead-flat 6.4-6.5V discharge curve you get with two LiFePO4 cells. The only unknown that could possibly ruin it all is an overzealous low voltage protection. If it cuts off at 2.8V per cell, it’s not a problem since LiFePO4 is dead at that point. But if it cuts off well above 3.0V, it’s a no-go.
I have a TK75 and know for a fact that it will run on Two cells. I tried this one time on trustfire flames and it turned on on Turbo but kicked down to high after a few seconds I repeated this about three times with the same results. I put 2 of my unprotected Panasonics in there and it turned on in turbo and stayed on for at least a minute or so until I turned the light off. I don’t know if this helps any as far as the LiPo’s go though. It must require a fairly high amp cell to run on two cells.
And, as far as current, A123 cells are as good as it gets. I just have to wonder at what point (voltage-wise), the light cuts back output. Also, I know you can get the runtime kits for this light. If this places the battery holders in series, it just might be the way to go. It would also add length for better balance.