My Fatman Liveth

[Mark_Paulus]

As it was starting to get dark on my commute, I finally got off my duff and put my fatman powered 3x3 S*** bike light together.

I used a Craftsman 2D task light (an older style, with an all aluminum case and bezel/head, instead of the newer partly plastic head). I managed to use a modamag Perfect Tri Sink (the thick one) with 3 S*** LEDs. Put a Fatman @ 900 ma, and am running it off of 2x 18650 2400 Mah Li-Ions. Works great as a bike light, as I get a pretty good punch/hotspot so I don't worry about outrunning it, but I also get a nice sidespill, so I'm seeing up close enough to not feel "blind". It's a nice trade-off.

I have a question about this setup, tho. When I use the cells freshly charged, the fatman is pulling about 1.5 amps @ 8.4V. When the cells run down a bit to 7.6V, the fatman is pulling about 1.9A, or a bit more. Does this seem reasonable? Also seems to give around 1 hour of runtime or so, which is enough for 1 days commute (about 45 minutes). I do carry a spare set of cells, and try to charge them every day.

It was a bit in the making, but I'm happy with the results, and find it a very useful setup.

 

[chimo]

Are you measuring the battery voltage under load (while the light is on)? You will have to take both measurements under load for a good comparison.

1.5 Amps should drop the fresh battery voltage below 8.4V

Pin = Vin x Iin

If Vin goes down, Iin must go up to supply the converter the same power level.

If the converter efficiency was constant over the whole battery range, then V1 x I1 would be equal to V2 x I2

However, converter efficiencies are not constant over all voltages so power input will not be constant over the different battery voltages. HTH

Paul

 

[Icarus]

You can sleep on both ears. The figures you are mentioning are very reasonable. :thumbsup:

 

[modamag]

Congrats Mark!
Those number are pretty typical for a K-bin LuxIII.
As the Vin drops the fatman have to work a bit harder and becomes a little less efficient.

Did you know that if you dial your fatman (external pot) all the way down you could get 50hrs plus runtime ;)

 

[Mark_Paulus]

Congrats Mark!
Did you know that if you dial your fatman (external pot) all the way down you could get 50hrs plus runtime ;)

You know, I did some further experimenting, and you couldn't believe the runtime I was getting when I just left it off....... But, those darn holes in the trail kept making me smack into trees, which would somehow manage to get a branch to brush the switch, and then, there I was again, back to burning the batteries :hairpull:.