12.5V 600mA LED strip: boost, buck or LDO linear regulator?

[ratkins]

Hi guys, I've been looking everywhere for information about constant current drivers and finally found CPF, this post in particular was great but now of course I have more questions .

The project is for "ricer"-style blue underlighting for my bicycle at Burning Man this year. I've gone and bought some blue LED strips from DX, rated at 12-12.5Vf, 600mA, 8W and 80 lumens (can't link them right now because of DX's website upgrade).

So, I need these to run overnight on a rechargeable battery I can easily strap to a bike. I also want a relatively constant brightness over the life of the batteries.

My options thus far seem to be:

* Small 12V sealed lead acid battery + boost regulator
* Two 7.2V NiMH remote control car batteries + buck regulator
* Two 7.2V NiMH remote control car batteries + current limiting resistor
* ??? something else?

What's currently (pun intended!) confusing me is battery discharge curves. I gather from the two 7.2V NiMH packs, I'll get somewhere between 16V and 11V over the course of one discharge. If the Vf of my LED strip is 12-12.5V (I haven't hooked them up and measured it yet) there will be a point relatively early on where the battery voltage will drop below the Vf of the LED strip and it won't light.

Given that I'm looking for maximum runtime (I need six or eight hours out of this, really I want it to last all night) will a 12V battery and a boost regulator be the most efficient way to go?

Also, from how far away will a blue 80 lumen strip be visible? I've got two if I need them, if one will be bright enough I'll make one for another bike.

Cheers (and thanks for your help!), Robert.

 

[Mr Happy]

I suspect you could try the 14.4 V NiMH pack and a series resistor. The useful capacity of NiMh is delivered from about 1.3 V per cell down to 1.1 V per cell when discharged under a moderate load. So the 14.4 V pack when nearly empty will still be providing about 13.2 V.

If you want to run a 600 mA load for 8 hours or more, you will need a capacity of at least 5000 mAh, maybe 6000 mAh.

How will you charge the battery? A 6000 mAh, 14.4 V pack is going to need a lot of charging.

Maybe it would be more practical to run the strips at a lower current like 200 mA?

 

[ratkins]

But won't the series resistor route give me wildly varying light output over the discharge cycle of the battery, in addition to wasting a lot of power?

I don't mind putting the batteries on to charge every day (I'll charge them off some 12V source, either a car cigarette lighter adapter or a deep-cycle marine battery).

I don't want to run the LED strips at 1/3 their rated current, I'm worried they won't be bright enough as it is!

Cheers, Robert.