Newbie with LED headlamp questions

Hi everyone! I am new to this site and the LED world but I do have some basic DC electrical knowledge. I am in to mnt biking and bikepacking which finds me using Lights quite a bit. I recently bought a cheap chines headlight http://www.amazon.com/gp/product/B006QQX3C4/?tag=cpf0b6-20

After one ride under the rain I rained the batter pack, my fault the pack was not water proof and had I looked at it carefully I would have notice it. It's cheap enough I could just buy another but I like tinkering and think this would be a fun project. I want to make a lithium battery pack for it which the original one from the reviews I read is rather bad. First however I need to learn more about Led so here are my questions:


1-Is there a book or preferably something online that I can read to give me basic knowledge on Led torches and how to design a system? I read the post Beginners guide to explain simple LED flashlight but while it was a great start I need more info.
The specs on that system states it's a 8.5v with 4 X 18650 batteries giving a 4400mAh pack. The Led spec is a XM-L T6 LED but after opening the light I found CREE XML-U2 marked on the LED.


2-How can it be 8.5V if it has 4 batteries and each cell is 3.7V? A pack with 4 3.7V can only be configure to give 7.4V or 14.8, so is the driver board stepping the Voltage down or up?


3-In the pack there is a small circuit board, is this the driver board? If not what is it and is the driver board located inside the light housing?


Thanks in advance for ur help

Welcome to CPF, Dieselcustoms! :wave:

Are you sure that battery pack has gone bad? Just soaking it once shouldn't be much of a problem really, as long as you let it dry out thoroughly before using again.

The Led spec is a XM-L T6 LED but after opening the light I found CREE XML-U2 marked on the LED.

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Read as "Cree XM-L, U2 bin" (assuming that LED is what the markings say ). If you check the XM-L's datasheet, you'll find that's simply a slighty-higher-output one vs. an XM-L T6.

2-How can it be 8.5V if it has 4 batteries and each cell is 3.7V? A pack with 4 3.7V can only be configure to give 7.4V or 14.8, so is the driver board stepping the Voltage down or up?

Click to expand...

Probably just marketing... Battery voltage isn't constant under (dis)charge. Li-ion's nominal 3.7V goes up to 4.2V fresh off a charger. Rounded up a bit: 2x 4.25V. So if measured you'd probably see that 8.xx drop to 7.xxV during use. Pack configuration will be either 2 cells parallel (and 2 of such pairs in series), or 2 in series (and 2 of that in parallel). Makes no difference performance- / capacity-wise. Either way output is always above the LED's operating voltage. So keeping efficiency/heat issues in mind, driver will most likely contain a buck regulator.

Note that to obtain 1200 lumen from a single XM-L, you'd need to push ~3.5A through it, at ~3.4V (see datasheet) that would be ~12W going into the LED! With most of that turned into heat, its small housing would heat up fast! Not saying it's impossible, but in his case that "1200 lumen" is likely overinflated.

3-In the pack there is a small circuit board, is this the driver board? If not what is it and is the driver board located inside the light housing?

Click to expand...

Chances are that's a battery protection circuit, that watches individual cell voltages and protects against over-(dis)charge. Driver will be inside the light.

Not sure what you mean by 'tinkering' in this context, other than replacing that battery pack. :thinking: Light is regulated, so chances are it'll take any DC input in 5~9V range or so. Have you tried hooking it up to a plain AC -> DC adapter? Any battery pack with output voltage in that range, and the 'oomph' to power the light on high for long enough, would do then. For example 4-6 AA NiMH's in series might work too (if not running @ high all the time).