What resistor should i use in my 7 Cree beast?

[gav6280]

I am running 7 cree q5's on a blue shark they have a total Fv of 25.2V at 900mah. My Lithuim polymer battery pack fully charged puts out 25.38V, (6Cells x 4.23).

This is no good for the blue shark as it is a boost only driver. The above calculation would mean that the battery hot of the charger is putting out 0.18 of a volt to much.

So i will need to use a resistor....(No room for another LED)!!

Do i put the resistor in between the battery and the shark OR in series with the Led string?

To be on the safe side i think i should try to dissapate / lose arbout 0.5 of a Volt, how do i work out what resistor i need use?

Thanks..

 

[Aircraft800]

Can you set your charger to charge the Batts to 4.1v? It would help get a few more cycles out of them anyway, and may solve your overvoltage problem, or simply use a dummy cell.

 

[gav6280]

Im developing this item to hopeful be easy to use. I would prefer using the battery pack as its meant to be. Hence my resistor question.

 

[spencer]

The Blue Shark will just go into direct drive when your pack is full charged. The rise of 0.18 volts over 7 LED's will only bring the current up a little bit. Maybe only as much as 20mA. I would let it go but someone with more knowledge may correct me.

 

[Greg G]

I think I'd add another sense resistor to the Shark to raise the current to 1.2 amp or so. That will also raise the Vf enough so that you're always in a boost situation. I'd also add a Remora to the Shark so you have two lower levels to run the light at. You're not going to want to run that light on high for long periods because it will get hot.

You might not even have a problem a 900Ma, as the battery will sag under a load. But I'd ask Wayne about that.