| The radiated power is proportional to T^4, so it ges really very steep with temperature.
That means the temperature does not need to rise that much in order to radiate the extra power.
So 125% voltage means about 112.5% temperature, so about 3420K, so very likely still within the design limits.
It would mean extra halogen evaporation, but I would guess the halogen cycle would take care of that.
The recrystallization wont be that much problem, because the filament is rather thick and short, so does not have to be that strong as with the mains ones. Plus given the battery internal impedance and the thick heavy wire, it wont exhibit the overheating hotspots (as the mains do) either.
Don't forget these lamps tend to be designed for few barely 100h life or so.
And the batteries have quite significant internal resistance, so even when they may exhibit about 1.6V/cell open circuit, when loaded it quickly falls down to 1.4..1.3V, soon after to about 1.2V for such a heavy load the 0.5A means for an LR6.
This voltage droop is, why all incandescents for dry cells are designed for 1.3V/cell for lower current fed from larger cells (e.g. 0.3A from R20 or so) and 1.2V/cell for the higher currents from smaller cells (0.3A from R6 or even this 0.5A from LR6).
In fact the single use cells having higher voltage "1.5V" is a myth, it could be the case only for some extreme light loads (<<100uA for LR6, so like digital clocks or thermometers; where the internal corrosion eats the batteries more than the actual discharge; and even not valid for analog ones, as the motor pulse consumption draws way higher current, so the voltage actually falls down to 0.8V/cell during these pulses and the oscillator and counters still have to work).
If you look on the charts, the voltage range of "1.5" cells for the rated capacity is 1.6..0.8V (some rate the runtimes assuming even 0.7V/cell, mainly for the high current intermittent loads), for the Ni based rechargeables it is 1.25..0.9V per cell, so actually higher on the minimum side (going lower there wont help the runtime, because the characteristic there is rather steep; and it would make larger risk to damage some cell by reversing one of its electrodes in a multicell battery, given the cells not being exactly equal so some may reach the dangerous 0.6V level while the complete pack may not seem to be that much below 0.9V/cell)...
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