Calc for runtime of Malkoff M60 with two 3500mAh "C" cells

[DHart]

Can anyone show the calculations for run time using a Malkoff M60 lamp assembly with two "C" size 4.2v 25500 li-ions rated at 3500 mAh apiece?

Thanks, in advance, for your help.

 

[Black Rose]

With the two cells in series, they only have 3500 mAh capacity.
When in a parallel configuration, they would have 7000 mAh capacity.

There are several M60 drop-ins, with different current draws.

I think you simply take the capacity of the cell and divide it by the documented current draw to get a guesstimate on runtime.

 

[DHart]

That's what I though might be the case... does anyone know the "documented" current draw of the standard Malkoff M60 (Q5 I presume) with 8.4v?

The Malkoff website says 750 mA, but that's at 6v, so then, runtime would be 4.7 hours with 6v... but I'm giving mine 8.4v.

Tired :tired: tonight... anyone have any estimate of the draw at 8.4v? Guess I can answer my own question... I'll fire up the DMM.

 

[matt0]

0.75A x 6V = 4.5W

4.5W / 8.4V = 0.53A

3.5Ah / 0.53A = 6.6h

Since it runs in regulation from like 5V to 9V, it should draw the same power anywhere within that range. I have heard things about drivers running more efficiently at a higher voltage but that should be a rough estimate

 

[Bullzeyebill]

DHart, you are familiar with MrGman's work. Go to this thread and see post 4 and scroll down. http://www.candlepowerforums.com/vb/showthread.php?t=229135

Bill

 

[Bullzeyebill]

You are also probably familiar with Big Waffles runtimes. Go to this thread and scroll down to post 178, for runtime with M60 in a 6P and 9P. http://www.candlepowerforums.com/vb/showthread.php?t=216438&page=4

Bill

 

[DHart]

0.75A x 6V = 4.5W

4.5W / 8.4V = 0.53A

3.5Ah / 0.53A = 6.6h

Since it runs in regulation from like 5V to 9V, it should draw the same power anywhere within that range. I have heard things about drivers running more efficiently at a higher voltage but that should be a rough estimate

Thanks everyone for jogging my brain on this stuff!

Bullseyebill... thanks for those links... great info there!

Just did a draw measurement at the tailcap... The M60 Q5 draws about 550 mA from two "C" size li-ions.

So... with the cells rated at 3500 mAh, and 550 mA draw, 3500/550 = 6.36 hour runtime.

Using your equations, Matt, .55A x 8.4v = 4.62W

~also~ 4.62W / 8.4V = .55

3.5Ah / .55A = 6.36 hours.

 

[Justin Case]

Thanks everyone for jogging my brain on this stuff!

Bullseyebill... thanks for those links... great info there!

Just did a draw measurement at the tailcap... The M60 Q5 draws about 550 mA from two "C" size li-ions.

So... with the cells rated at 3500 mAh, and 550 mA draw, 3500/550 = 6.36 hour runtime.

Using your equations, Matt, .55A x 8.4v = 4.62W

~also~ 4.62W / 8.4V = .55

3.5Ah / .55A = 6.36 hours.

Well of course you are going to get the same 6.36h answer using Matt0's approach. You are calculating the power draw for 2xLi-ion based on a measured current draw and an assumed 2xLi-ion voltage. Then you are taking that same power draw and dividing by the same assumed voltage again to get the same current draw value. So of course, when you divide that into your cell capacity, you will get the same 6.36h result.

He was assuming that the M60 runs in regulation and thus draw essentially constant power. In the example, he calculated that level of constant power based on the 2xLi primary (6V) data: 0.75A * 6V = 4.5W. Then, assuming constant power draw, he derived the current draw for 2xLi-ion (ignoring voltage sag) by dividing 4.5W by 8.4V to get 0.53A. The 2xLi-ion current draw was an unknown value, and this was a method to estimate it. Knowing the current draw for 2xLi-ion and the capacity of the Li-ion cells (3.5Ah), run time then becomes 3.5Ah/0.53A = 6.6h.

 

[DHart]

Justin... yes... you are so right....

And it's nice to have a light with such runtime on an M60.

 

[DHart]

duplicate post in error....

 

[Kestrel]

Cool, I'm happy to hear that you are finding a use for that FM 2xC body. 6 hours of M60 goodness in a light pretty much the same length as a SF 9P is a wonderful thing.:thumbsup:

Edit: Hey, so no connection issues that had occured previously, right? Ever figure out if it was that cell spacer or your M30 drop-in or whatever?

 

[DHart]

Cool, I'm happy to hear that you are finding a use for that FM 2xC body. 6 hours of M60 goodness in a light pretty much the same length as a SF 9P is a wonderful thing.:thumbsup:

Edit: Hey, so no connection issues that had occured previously, right? Ever figure out if it was that cell spacer or your M30 drop-in or whatever?

Interestingly, no connection issues whatsoever. No idea why I had connection problems before and none now... I do have a wrap of aluminum foil around the base of the lamp module as I find useful with most lights I put Malkoff lamp assemblies into.