I made this simple model to predict the resulting current that will flow in a direct drive setup given the battery voltage, cell and circuit resistance, and forward voltage curve of the LED (s). This has helped me understand and plan mods, so I thought it might be useful to some of you modders.

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This is the strategy. Plot two curves: one is the voltage across the LED as a function of current. This is just a downward sloping line that represents the voltage dropping linearly with current. The open circuit voltage of the cell and the different contributions to the circuit resistance are parameters that you input/change for different setups. In other words the voltage at the LED will be the open circuit voltage minus the voltage drop in the rest of the circuit, which is IR.

The other curve is the forward voltage of the LED. This value will increase with current. The curve in the link above is a fit to the forward voltage measurement that djozz did of the XPL V6 2C.

Then the point where the two curves intersect is the value of the current that will flow in the setup. It is an easy graphical way to do a bit more complex calculation. Only the first intersection is physically meaningful because the curve for the forward voltage is only valid in the range of 3-4V.

Here is a run-through of how to use it:

Say you want to know the current that will flow through your single XPL V6 2C light with a FET driver and fully charged 30Q cell. In the expression for the first curve, input ~4.17V for the open circuit voltage of the cell. You could also put in 3.8V for a partially discharged cell. 0.025 ohms is the approximate IR of the cell. 0.01 ohms is the approximate circuit resistance. This resistance includes the FET on-resistance, emitter wire resistance and other current path resistances. The third resistance value in the expression I have as the tailcap resistance. This way you can enter in your approximate tailcap resistance (0.015 ohms for example), or your ammeter resistance.

The forward voltage curve is more difficult to customize to your setup. You would need to make an expression for the forward voltage of the specific emitter you are using if its not the XPL V6 2C. You can however easily alter this expression for a triple or quad XPL setup. In the two factors (x/1), replace the 1 with however many emitters you have, so (x/3) for a triple.

The model predicts 5.85A to flow with a single XPL and 11.5A to flow with a triple. Remember the x-axis is the current in amps and the y-axis is the voltage in volts. I have found the model useful for predicting performance changes from different cells and circuit resistance mods.

Hope this helps someone!