Runtime, battery, math question

[GPB]

I'm in the middle of a run time test with the multi-mode mini mag LED on low mode. So far it's run for about 28 hours. Since I also have the same LED in a 2D Mag I am curious how long it would run. Is there a way to extrapolate the run time on 2D's if I know the run time on 2AA's ??

 

[Marduke]

Short answer: No with "but"...
Long answer: Yes with a "however"...

Medium answer: It would be MUCH easier if you simply measured the current draw in the desired setup instead. Applying the measured current draw and voltage, a rough estimate of runtime can then easily be calculated.

 

[GPB]

I've never had any luck measuring current draw. I take off the tailcap and put one lead on the bottom of the battery and one on the case, and I always get 0.00.... what am I missing ?

 

[Marduke]

I've never had any luck measuring current draw. I take off the tailcap and put one lead on the bottom of the battery and one on the case, and I always get 0.00.... what am I missing ?

Probably the correct DMM setting, or a fuse....

 

[GPB]

I copied this out of the thread about the new minimag. does that help ?



On fresh Duracells, I measure 480mA on high and 110mA on low.

Edit: And on rested Eneloops (ZTS: 80%, MM: 2.576V), I get 390mA on high and 80mA on low.

 

[Marduke]

~180 hours on name brand D cells.

 

[Light Sabre]

It is my understanding that the difference in maH between a AA cell alkaline and a D cell alkaline was about 7X. YMMV.

 

[Marduke]

It is my understanding that the difference in maH between a AA cell alkaline and a D cell alkaline was about 7X. YMMV.

At low current draw, yes. ~2900ish vs ~20000ish

 

[GPB]

Thanks guys !! I don't know how long the mini mag is going to continue to run, but if it's still going in the morning, I know the 2D version is going to run for a REALLY long time.

thanks again.

 

[yellow]

when doing such test, You should really measure current all the time + notice changes.

There will be quite a difference if the driver behaves direct-drivish or regulates
(and without comparing - to a 2nd identical light on the same setting but only used for short "comparison bursts", f.e. - the difference in brightness wont be too noticeable)

 

[GPB]

Since the low setting has been estimated to be around 10 lumens, it can't really dim much and still be useful, so I am hoping that I'll be able to "eyeball" it. The Mini-mag was still going this morning so that makes about 37 hours of run time, and counting. I was hoping the 2D version would go for a week and it seems like it would. It's nice to have that kind of brightness and uber long run time in one light.

 

[jankj]

Since the low setting has been estimated to be around 10 lumens, it can't really dim much and still be useful

Yes, it can be much dimmer and still be VERY useful. Depends on the environment and how dark adapted your eyes are. In total darkness just the tiniest amount of light is incredible useful. You won't see far ahead, but you will be able see what's in front of you. Just think about it: How else could people have survived using incan minimags for all these years...

With dark adjusted eyes I find 10 lumens too much if I just need a little light to search through my bag and similar simple close-hand tasks.

But I definitively agree that in a light polluted setting 10 lumens does not really cut it for illuminating those dark shadows.

 

[GPB]

I finally figured out how to measure current at the tail cap of a light !!! ( my cheapie meter just doesn't seem to register anything, but my better meter does....go figure ! )

So I had 2 mini mags. 1 with the multi mode module, and one with the single mode module from a 2D Mag.

Single Mode/NiMH = 1.54 amps
Single Mode/Alks = 1.04

Multi Mode/NiMH = .40
Multi Mode/Alks = .50

( not sure why the Alkalines would give a higher reading in the multi-mode.....can anyone offer an explanation ? )

If anyone cares, the multi mode in the 2D light was drawing .44 on high and .10 on low. Since Mag calls the low 25% power...those readings seemed realistic.

So....can anyone extrapolate lumens from those numbers, if the led is a rebel 90 ??

 

[yellow]

2 cell light, 3 cells "needed" (for voltage),
so its 2/3 of the cells needed.
1.5 A * 2 / 3 = 1 A at system voltage,
deduct 20 % loss of driver (which is a very nice assumption),
makes ~ 800 mA to the led

lux rebel 90 has a datasheet value of 170 lm @ 700 mA, so - with another losses added to be on the safe side - its those 170 lm at least, or a bit more.



did You measure how long these 1.5 A are drawn?
Heatsinking is pretty bad and thus the driver should reduce power relatively soon, within some mins.

 

[GPB]

I don't follow what you mean by "3 cells needed" for voltage. It's a 2 cell mini mag running an LED from a 2D cell Mag LED.

I don't think there's 170 lumens coming out of the light, but it is pretty bright.

I found it pretty interesting how big the difference was between the single mode and multi-mode modules. I assume it's because the added features add complexity to the circuit, thereby reducing the voltage that passes through it. There is a noticeable difference in their brightness when you hold them side by side.

 

[yellow]

thats an assumtion to come from current measured at battery, to current to the led.

the white led runs on 3.7 V, which equals three cells in direct drive.
So with a direct driven light, what one measures at battery is the same current than the one at the led.

when input voltage is different, the driver has to cover up for that,
which means the draw at battery is different from the one to the led.
the cell number "calculation" is a very rugged way to fight that:
get the number of cells, compare them with the number "needed", insert current at battery.

another example:
series driven quad led, running from a 2-cell Li-Ion (= 7.2 V), draw at battery: 2 A
--> 2 cell Li-Ion = 6 Ni-Mh cells
--> 2 A * 6 / 3 / 4 = 1
the whole series wired quad runs with 1 A getting throug each of the the individual emitters
( "/4" because of the series wired quad)

very rugged, but also easy to do and working with easy to get data (everyone has a meter to get the draw at battery)

PS: I agree with Your findings to the Minimag,
but with that (initial ?) draw and the datasheet value, thats what should be.