how do I run a 8.4v bike/head light off a 12v battery?

[brmccall]

I recently bought a bike/head light that comes with a 4 * 18650 battery pack, but the output (and the wall wart charger output) is 8.4v. I still have not figured out how they get to the 8.4v. I want to plug into the battery on my buggy and also on my airboat with the light mounted as a head light.
Can somebody please tell me how I can do this?

Thank you
Bryant

 

[poguy]

The battery pack is 2 sets of parallel cells in series. If you aren't concerned with wasted energy, a 3-terminal linear regulator is simple enough. I have used 7808's before. A switching regulator is more complicated and beyond most DIY people.

Perhaps better is a LED regulator that works off 12 volts. Not many of them support multiple modes.

 

[brmccall]

I thought about the 2 parallel sets in series, but (unless I am mistaken) the 18650 is a 3.7 volt battery, and 2 in series would be 7.4v, not 8.4v? I have never done anything like this before, but figure it cant be too much to it, after all, for a couple of bucks you can get a usb charger (5v out, and 12v-14v in).

Could you explane the linear regulator (as well as where to buy one) and how to hook it up.

I would also like to know more about the 12v LED regulator (how to use it, were to buy it, etc..).
I don't care about multi modes when running it on 12v, as I would not have to worry about running down batteries as it would be hooked up to the battery on the boat or buggy being constantly charged by the alternator. But i would like to keep the high/low mode when I am using the small battery pack on my bicycle.

Thanks for any help you can offer.
Bryant

 

[fivemega]

2 in series would be 7.4v, not 8.4v?

2 Li-ion in series would be NOMINALLY 7.4 volts and hot off the charger WITHOUT LOAD 8.4 volts.

 

[hiuintahs]

I thought about the 2 parallel sets in series, but (unless I am mistaken) the 18650 is a 3.7 volt battery, and 2 in series would be 7.4v, not 8.4v? I have never done anything like this before, but figure it cant be too much to it, after all, for a couple of bucks you can get a usb charger (5v out, and 12v-14v in).

Could you explane the linear regulator (as well as where to buy one) and how to hook it up.

I would also like to know more about the 12v LED regulator (how to use it, were to buy it, etc..).
I don't care about multi modes when running it on 12v, as I would not have to worry about running down batteries as it would be hooked up to the battery on the boat or buggy being constantly charged by the alternator. But i would like to keep the high/low mode when I am using the small battery pack on my bicycle.

Thanks for any help you can offer.
Bryant

Like poguy said, the simplest is to use a linear voltage regulator. If you want to set the voltage precisely to an exact voltage, then you could use the LM317 adjustable output regulator.
See this for an example.
http://www.national.com/mpf/LM/LM317.html#Overview

Now the problem with the linear approach vs. the switching power supply approach is that they are way less efficient. But maybe this doesn't matter. You have to run the numbers to know. Switchers are typically always 85 to 90% efficient. With your example of 12 volts input and 8.4 volts output, then 3.6 volts is being dropped across the LM317 regulator............which is in essence wasted power. That would equate to a (12-8.4)/12 = 30% power loss and so you are talking only a 70% efficient design.

The other issue is depending on how much power that is, the LM317 might need to be heat sinked to keep it from over heating. So it really boils down to if the 12v power loss going into the linear voltage regulator (LM317) isn't a concern. For portable battery equipment, normally you shoot for the most efficient design which would be a buck (step-down) switching power supply. It's not hard to do and I could sketch up the design but you would have to know how to solder up a small printed circuit board.

A simple buck switcher could be built with an LM2679. I think you can even buy an evaluation board for $30 and then adjust it to your 8.4v output.
http://www.ti.com/sitesearch/docs/universalsearch.tsp?searchTerm=lm2679&Submit=&linkId=1

Question remains: how much current does your 8.4 volt device draw? This helps in sizing things up.

 

[poguy]

Magicshine knock-off's draw 8-10 watts on high, or about 1 amp. A 7808 regulator with a good heatsink should be good for 1.5 amps leaving enough of a margin. The light will work OK on 8 volts. They probably work on 9 volts, too (7809) and that means reduced current and lower regulator losses. You need to look up the chips used on the light's constant current regulator or ask in the forums here.

The 3-terminal 78xx linear regulators are easy to use. Left pin is 'in', middle is ground, right pin is 'out'. Put a 10-50 uf capacitors across the input and output pins to keep them stable. Use the biggest piece of heatsink you can find or a piece of heavy sheet aluminum + heatsink grease. You can never have too much heatsink...

DX and other places have LED regulators that run from 6-16 volts. Almost all are 1-mode. I would go with a linear voltage regulator as the simplest and cheapest option. A 7808/7809 should be easy to find for less than $1.

 

[brmccall]

Thanks for advice you all supplied. I will see if I can't figure this out and get it up and running. WARNING: I may be back with stupid questions.....

 

[VegasF6]

What's your electronics experience level? Can you assemble a simple circuit and do you own a soldering iron and meter and such? You can purchase a ready-made adjustable voltage regulator, switching or linear for under 10 bucks. Make sure it can handle at least 1 amp or 8.4 watts. A little more would be better.
I have seen these for sale on ebay, satistronics, sure electronics etc.