Yes, that was my greatest concern as well with the new model. (Whether the not too over-engineered thermal design of the LT1 can handle the extra dissipation by the additional 2x2 7135s…)
But I made some calculations, then performed some torture-tests yesterday which made me a bit more relaxed:
The top “cap” of the light has a surface area of 5850 mm2, which as a rule of thumb, should translate to a thermal resistance (Rth) in the neighborhood of 7 °C/W.
Rth (junction to solder point) of the LH351Ds 2.2 °C/W according to the datasheet.
Rth of a typical MCPCB is around 1 °C/W.
Rth of a reasonably applied half-decent thermal paste could be estimated around 0.3 °C/W
I=2450mA at full blast, while Vf of the LEDs (in worst case, i.e. when either extreme of the tint range is set) is 2.8V. Considering an efficiency of 70% that gives a power dissipation of about 4.8W, total.
If you put all this data together, mix some thermal design theory in it, you will receive a predicted heatsink temp of 62.6°C and LED junction temp of 79,4°C at an ambient temperature of 29°C.
The measurements I’ve done afterwards, seem to confirm this theory (heatsink temp of 66.0°C, junction temp 82.8°C, in a non-ventilated area at 29°C ambient):
What I actually measured are the head/heatsink temps. (Simultaneously via non-contact thermometer, and K-type thermocouple, always taking into account the highest reading) The junction temps are just projected from the heatsink temps. There is no way to measure them directly. However if anyone wants to determine their values more accurately, they could measure the MCPCB temp instead of the heatsink and offset the measured values by 4.8W*2.2 °C/W= 10.56°C. This way you could eliminate the potential inaccuracies stemming from the estimated MCPCB and thermal paste Rths.
Now, 82.8°C might sound awful, but actually it is not. The “design temperature” of today’s power LEDs (LH351D included) is 85°C. This is where the manufacturers expect OEMs to run them in their end-devices. This is the temp they are binned at, this is also where their environmental stress tests are performed. It is a perfectly safe temp for our LEDs (the top end of their operating junction temp is 105°C!).
I also measured the temperature of the battery tube. Its 44°C is also not catastrophic, most Li-ion cells today allow a max discharge temp of 60°C. However I am sure it will have some lifetime penalties if the cells are subjected to it on a regular basis: Some researchers say every 10°C above room temp reduces the lifetime cycle rating by ~25%. I can neither confirm nor deny if this statement is accurate though… maybe someone with more practical experience with the quirks of Li-ion cells can.