Re: so I got a Cateye EL300.. & general comment on LED headlight runtime claim(long)


ITSME.ULTIMATE wrote:
I lost my bike light and I haven't got around to getting one until
I got pulled over the other day for not having a light. So, I purchased
a Cateye HL-EL300, featuring five LEDs powered by 4 AAs.

The package claims "up to 110 hours" runtime and "400 + candlepower",
but don't take this claim as "provides 400+ candlepower for anything
close to 110 hours"

The advantage of LED lighting is in nearly constant efficiency as the
battery wears down. The incandescent lamp has an inversely exponential
voltage vs efficiency curve. As the voltage of battery drops, the
current draw drops at a lesser rate and once the power drawn by the lamp
is less than about 50%, the lamp would not give any useful light.

LEDs on the other hand maintains a relatively flat efficiency but
current drops in an inversely exponential pattern, but continues to
provide useful amount of light with much lower power. There are some
lights that features an electronic regulating circuit that maintains
constant current through LEDs as the batteries deplete and maintains
nearly maximum output for a good part of batteries' life, but batteries
would not last long.

Cateye HL-EL300 draws 0.224A with brand new batteries and the light
output should be greater than or equal to claimed output. The LED array
is driven through a 10 ohm series resistor, a cheap design.

Energizer engineering datasheet suggests that useful capacity of their
AA alkaline batteries at 0.25A is around 2Ah. If the battery held a
constant voltage throughout the capacity and Cateye light maintained
0.224A pull, the battery would be completely flat in 8.9 hours. It is
the inversely exponential reduction in current draw that allows "110
hour" life.

Here are some measurements I've taken:

4 brand new alkaline batteries:

*0.224A at 5.85v battery voltage(due to internal resistance, voltage
drop is normal)
3.584v at LED, 2.24v across resistor, which implies 38% of power is
wasted there.

4 NiMH batteries:
*0.173A@ 5.22v
3.49v at LED, 1.73v across resistor, 33% wasted in resistor.

What matters to light output is the power going into LEDs. 11% drop in
battery voltage resulted in 25% drop in actual LED power. The ratio of
power wasted in resistor and power used by device do not change in
purely resistive circuit, but LED does not follow ohm's law and its
"resistance" is dynamic with current.

Voltage across regular alkaline battery drops almost linearly with
depletion where as NiMH and NiCd holds relatively flat as it depletes
and plummets down near the end of useful capacity.

Here's my advise for LED headlights without a regulator. Use NiMH
batteries if you want a nearly constant 75% output that last a few
hours. Use Alkaline if you want continuously dimming light that ends its
useful life at high double digit hours giving only a few percent light
output.

.

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