Author Topic: tailcap measurements?  (Read 5236 times)

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Offline scrivener

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tailcap measurements?
« on: October 08, 2013, 10:59:30 PM »
New newbie. I don't know how to take tailcap measurements.  Have old needle-type VOM.  Pls help?

Offline MRsDNF

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Re: tailcap measurements?
« Reply #1 on: October 09, 2013, 12:07:59 AM »
Hi and welcome scrivener. Does you meter have an amp reading? If you could post what model you have or a picture we could possibly be more helpful.
When the big picture is to big, look for a point you can start at and build from there.

Offline JohnnyMac

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Re: tailcap measurements?
« Reply #2 on: October 13, 2013, 02:45:34 AM »
Like MRsDNF said...a pic of your meter would help us in telling how to perform a tail reading of amps. 

As for the reading itself, once you know how to set your VOM/DMM to read amperage it really easy.  All you do is remove the tailcap while the cells are in the light, press the black lead from your meter to the negative end of the exposed cell while touching the red lead to the exposed aluminum of the battery tube end.  This will light the light and the meter will read the amperage.  Just remember that when your light uses more than one cell in series that you need to multiply the tailcap reading by the number of cells to get the full amperage being drawn by the driver.

Offline Gazeddy

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Re: tailcap measurements?
« Reply #3 on: August 21, 2014, 11:47:16 PM »
  Just remember that when your light uses more than one cell in series that you need to multiply the tailcap reading by the number of cells to get the full amperage being drawn by the driver.

when the cap is removed you now have an open circuit when you put the meter in it re closes the circuit. the current displayed is the current the driver is receiving.
im aware im new around here but in series the voltage is increased not directly the current.

Offline JohnnyMac

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Re: tailcap measurements?
« Reply #4 on: August 22, 2014, 02:49:32 PM »
  Just remember that when your light uses more than one cell in series that you need to multiply the tailcap reading by the number of cells to get the full amperage being drawn by the driver.

when the cap is removed you now have an open circuit when you put the meter in it re closes the circuit. the current displayed is the current the driver is receiving.
im aware im new around here but in series the voltage is increased not directly the current.
Yeah you are new but you are wrong on this one :P   Don't feel bad though, most people have a problem wrapping their head around this.

In series voltage is increased, correct.  The overall amperage provided by the cells themselves though stays the same as one of the cells.  You are correct in that thinking as well.  Three 2500mAh cells in series only provide 2500mAh total.


Where you are mistaken is in the draw of the driver based upon the number of cells.  If a driver draws 3A from your cell (or cells) it draws 3A regardless of the number of cells.  That said, in this 3A torch, if you measure tailcap draw as we are talking about then with a single cell you will see 3A being drawn.  If you are using 2 cells in this light then each cell will only see 1.5A at the tailcap.  3 cells will show only 1A at the tailcap.  This is because while voltage is multiplied by the number of cells, the amperage drawn through them is shared.

Let's say you have two lights.  Both have 3A  single mode drivers and both output 850 lumens.  One is a single cell light and the other uses 3 cells.  Measure the tailcap draw on each using the prescribed method.  The single cell light will show 3A.  The triple cell light will show only 1A.  According to your thinking the driver is putting out the same lumens while only using 1A.  Seriously?

By the same token, if you were correct in your thinking, you should see 3A measured at the tail in this 3-cell light.  Go ahead and try.  You won't see 3A I guarantee it.  You will see only 1A.  If you had a 2 cell light with the same performance specs you would see 1.5A at the tail.  Why? because the current load is being shared by all the cells.  A single cell bears the current draw all by itself.  Additional cells split the load among them because they are in series.

Conversely, if you have a soup can light like a Courui that had a 3A driver that uses 3 cells but in parallel, not series, the light sees them as one giant 4.2v cell.  If you could measure tail amps on this light you would see 3A draw no matter how many cells were in it.  One cell, 2 cells, 12 cells, it wouldn't matter because the cells are in parallel.


Parallel cells: Voltage stays the same but total mAh is multiplied.  Think one large cell and tail draw is as one cell.
                     4.2v 2500mAh cell x 3 = 4.2v 7500mAh
Series cells: Voltage is multiplied but mAh is the same as one cell.  Think many cells and tail draw is divided among all cells.
                     4.2v 2500mAh cell x 3 = 12.6v 2500mAh
« Last Edit: August 22, 2014, 02:54:08 PM by JohnnyMac »

Offline Calmaja

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Re: tailcap measurements?
« Reply #5 on: January 25, 2015, 06:36:25 PM »