4 R2's, A Blue Shark, How many LiPo cell's?

[gav6280]

I have got :

Blue shark with pot removed and sense resistor changed to output 1200ma

4 Cree XRE R2 in series

A monster custom made heat sink

And i definitely want to use a Lithium Polymer Pack from an R/C car.

Now i know the blue shark is a boost only converter and in the worst case the LED's could potentially (unlikley) draw as low 3.7 Volts x 4 = 14.8 fv for the string.

A Lithium polymer battery has a theoretical maximum safe voltage of 4.235 V per cell right?

So to keep things as efficient and as heat free as possible i plan to use a 3 cell pack giving a 12.69 maximum voltage output from the battery, meaning that the blue shark will initially only have to raise the output voltage by around 2.11V?

But towards the end of the battery's cycle it's output before cut off would be around 8V meaning the shark has to boost by around 7 - 9v at 1200ma, will it be able to cope ok with this?

Cheers.

 

[steve6690]

Hi Gav,
I'd go for 3 cells. I have exactly the same setup sat on my breakfast bar, hooked up to a bench supply. 4 x xr-e r2's from dx running at 1 Amp. When I first fire it up the Vf is 14v but as it gets hotter the Vf settles at around 13.2v. I've just turned it up to 1.2A and the Vf rose by 0.3v to 13.5V.
So, if the shark was knocking out 14v at 1200ma, from around 8v input, the input current to the shark would around 2.5A or slightly less, which is fine for a blue shark with decent heat sinking.
I'd planned to use a blue shark for mine but now I'm going to test a bFlex running from a 4-cell li-ion.
By the way, I have just completed my first "for sale" prototype dive light. It's a 3s2p li-ion pack, blue shark, but feeding a MC-E M bin WG at 1A. The Vf of the series wired MC-E is lower than the 4 R2's at around 12.8v @1A.
The light ran fine for 4 hours in my bath with no visible problems from overdriving the MC-E.

 

[gav6280]

Cheers Steve appreciated, cant give green on this forum.

I just wanted someone to double check me before i go and blow up some expensive parts.....again.....

Will be showing it of on YD when its all done, we really will have to try meeting up at some point for a splosh in.

 

[steve6690]

Excellent. I'm looking forward to seeing that light. I'm keeping an eye on your midweek divers site for days when I'm off, so we will have a dive soon mate. I can show you my first dive light ( 7 x q5's) and my current project ( 1 x MC-E ).