Or more like an estimation : Graphing Calculator - GeoGebra

I used this online graphing calculator to graph the LED current-voltage curve and the voltage drop in the circuit and cell/battery, the intersection is the resulting current. To obtain each LED functions I plotted the current-Vf data from community tests (maukka, Texas_Ace, koef3) in excel and obtained a polynomial function which matches reasonably well the ploted curve (3 degrees seems sufficient).

How to use :

Select the LED

Number of LEDs

Cell Voltage

Cell internal resistance

Flashlight resistance

The last one is not something that is usually known unless measured, my PL47G2 for example is around 24mΩ, (incl 4mΩ for each spring), but obviously that will vary between drivers, wires used, spring resistance, bypass…

Some example of cell’s DCIR :

GA/35E/MJ1~35mΩ, VTC6/30Q~20mΩ, P42A/40T~17mΩ, 30T~12mΩ

The intersection of the two curves is the current (clicking on it gives the value), it’s only valid at the current (per LED) tested. (e.g. for E21A it’s only valid between 0.2 and 2A per led)

Stuff that makes this quite approximate :

LED Vf varies with samples, heat.

Cell DCIR varies with samples, heat, state of charge, and it disharges very fast at high current.

Flashlight resistance is generally unknown, varies with heat.

Testing with my PL47G2 with a partially charged P42A, calculated : 12.64A, measured 11.3A (current clamp, tailcap wire bypass). Close enough :partying_face:

Probably not very useful in the end but I liked the visualisation.

Tests used : sst-20 , lh351d , xp-l hi , 219c , 219b , e21a sst-40