That is safe to do, as long as circuitry to protect against
overdischarge is included. This is *usually* included Li-Ion battery
packs.
Yes, it will be a lot dimmer. By a rule that I consider
"1-size-fits-all", "they" like to say that light output is proportional to
applied voltage to the 3.4 or 3.5 power.
There are some variations.
With more severe dimming like that of operating a 120V-rated
incandescent lamp at 36 volts, the exponent gets greater, I would like to
say more like 4.
As for a specific example, I have worked out at least somewhat that a
"USA-usual" 100 watt 120V 750-hour-rated-life 1670-1750-rated-lumens
lamp will typically produce 10 lumens at 36 volts.
I give fair chance that I need to rework this, based on a somewhat
recent minor adjustment/redefinition of the blackbody formula. However,
I still expect that lamp to typically deliver typically deliver somewhere
between 10 and 11 lumens at 36 volts, from about 15.5-15.6 watts of power.
I say more on performance of a few specific 120V incandescents at
various voltages in:
http://members.misty.com/don/incchart.html
And photometric figures for only one, based on working from an older
version of the constants (valid as of 1960's) to plug into the blackbody
formula.
I have yet to hear of a "normal flashlight bulb" that works well at 36
volts.
My experience is that "flashlight lamps" are mostly designed to work
from 2 to 6 cells at nominally 1.2 to 1.25 volts per cell, in my words
"in average condition and after average resistive voltage drops". As in,
mostly designed for 2.4 to 7.5 volts. There are a few designed for "1
cell", around 1.2-1.25 volts.
Adapting these to 36 volts requires, as I like to see best, "switching
buck regulator" circuitry.
Also, keep in mind that most "flashlight lamps" have short design life
expectancy, due in part to "economies of scale" that apply to tungsten
filaments.
Best I can think of for now - use lamps rated for 28 volts - those are
available, likely including from Digi-Key. Use ones rated to last a few
thousand hours at 28 volts.
And I would like to say 37.5 volts rather than 36 for 10 Li-ion cells in
series for projecting life expectancy of a 28-volt-rated incandescent lamp
according to life expectancy being inversely proportional to voltage to
the 12th power, maybe 13th: Figure on 3% of rated life expectancy of a 28
volt incandescent lamp. Despite being low, this often exceeds design life
expectancy of most incandescent lamps designed for flashlights.
As for current and power consumption at 37 volts in lieu of 28 volts:
Since most 28 incandescent lamps achieving "flashlight lamp brightness"
at 36-37.5 volts have a vacuum, I like to figure current being
proportional to voltage to the .57 power, maybe .58 (as opposed to less
for gas-filled tungsten incandescent lamps), and power consumption to be
proportional to the 1.57, maybe 1.58 power.
This means that at 37 volts, current is about 1.175 times that drawn at
28 volts, and power consumption is about 1.55 times that at 28 volts.
Thankfully, light output at 36 volts is slightly over double that of
light output at 28 volts.
I would look into miniature bayonet base T1-3/4 bulb 28V incandescent
lamps rated to typically last at least 1,000 hours at 28V, such as 1819,
1829, 757, 1820, 1864, 1873 and 1822.
If life expectancy at effectively-with-10-cell-Li-ion (probably at least
37.5 volts for this purpose) needs to be 60-plus hours typically, then I
would rule out lamps rated to typically achieve less than 2,000 hours
life expectancy at 28 volts.
Accordingly, the list shortens to:
1819, 757, and 1873.
It appears to me that Digi-Key has in-stock the 757 among these.
It appears to me that the 757 is, at least nominally at 28 volts,
designed to draw .08 amp (2.24 watts) and produce typically .62 MSCP
(7.79 lumens) with nominal design life expectancy of 2500 hours.
With "my extrapolations" of life expectancy at 27.5 volts, current and
power consumption at 37 volts and light output at 36 volts,
with life expectancy inverse proportional to 12th power, light output
proportional to a lowish 3.2 power (lower-than-usual is suitable for
overvoltage and vacuum tungsten incandescent lamps), and current
proportional to a fairly-typical-for-vacuum-tungsten-incandescent
at proportional to voltage raised to the .58 power,
I project that the 757 typically does this with a 10-cell Li-ion
battery:
Light output: 17.4 lumens - fairly typical for 3-cell flashlights
designed to use C or D size cells.
Current and power consumption: I project .094 amp of a 757 lamp to draw
at 37 volts, based on "power rule" of .58. .094 amp times 37 volts is
about 3.48 watts.
17.5 lumens from 3.4 watts is 5.15 lumens/watt, low even for incandescent.
Life expectancy: The 757 is nominally designed to have typical life
expectancy of 7500 hours at 28 volts. At what I consider to be the
life-expectancy-effectively-average 3.75 volts from a 10-series battery of
Li-ion cells, for 37.5 volts, and power of -12 for life expectancy, I
expect (plus/minus plenty) the 757 to typically last 225 hours +/-
plenty - fair chance anywhere in the range of 100 to 500 hours.
- Don Klipstein (
[email protected])
