External battery packs for small headlamp

[Moondog55]

I just realised that the pictures of this little project never got posted and I can't find the thread the idea was in.
The greatest downfall of the cheap headlamps using the Cree XL-M chip is that almost all of them use 3*AAA batteries and the chip can use a lot more voltage and current than those can provide
So I got a couple of 4*AA battery carriers and added a mini-molle connector. Added a parallel hot and return wire to the headband battery pack and sewed up a little pouch from some scrap goretex

Before I head up the mountain I'll sew on a short cord so it can hang in my armpit where it is warmest but usually I just put it in the pocket of my shirt

With the 4 batteries giving that extra 1.5V and I don't know how much extra current I get a much brighter light and longer run time. Changing batteries is a simple matter of swapping over a pack and 2 packs can fit inside the pouch with a little force

[Mark F]

Disclaimer - I have not tested this so if you pop anything it isn't my fault. I need to buy a cheap 3 cell headlamp to test this as I am not willing to fry my Petzl Tikka XP2 although it does run with Petzl's Li rechargeable CORE battery.

The Cree unit you mention has maximum voltage input of 3.5 volts so I wouldn't be upping the voltage. You get more light by upping the amperage (3A max). You can go a bit higher on the voltage as there will be losses in the circuitry. Just upgrading to 3 x AA batteries will increase the run time by 250+%.

Another option could be to attach a single Li rechargeable battery in place of the 3 x AAA. Standard AA/AAA batteries voltage vary from 1.5v to about 1 volt as they discharge. The different chemistries (Li, NiMh, Alkaline) have slightly different voltage ranges but basically fall in this range. Once they are producing less than about 0.8 to 1 volts the battery is "dead". Three cells in series deliver from 4.5 volts to 2.4 volts. A single lithium rechargeable Li cell such as a 18650 produces from about 4.6 volts down to 3 volts as it discharges. The auto cutoff circuitry usually kicks in at 2.75 to 3 volts in protected cells.

On weights - A high quality 18650 battery produces 2.6 to 3.4 Ah and weighs a bit under 50 grams. Based on Eneloop X series batteries 3 x AAA NiMh batteries weigh 36 grams and produce 0.9Ah - 27% to 35% of the 18650. Moving to 3 x AA NiMh batteries (2.5Ah) the weight rises to 90 grams for 75% to 95% of the 18650 power.

You could also play with electronics to create a regulated setup which will adjust the output voltage to a fixed level and set the amperage.

[Moondog55]

Hmm When I checked the Cree website it gave a voltage range for 3 to 8V but I haven't fried a driver or an LED unit yet and this is the second set of these I've done
Still I've been warned so I'll put a separate unit or three in the bags just in case
I've always intended to change over to the 18*65 batteries but one thing at a time
The driver board in these units uses micro SMD components and is smaller than half a postage stamp so I have no idea what the driver componentry is
I seem to remember the Cree site saying something about maximum life Vs voltage/current tho

[Moondog55]

I just re-read the data sheet, 8V is the forward voltage so the driver must therefore include a voltage regulator and the extra light is due to the greater available current. This is fine with me and so much lighter on my head and shoulders than the old Petzl Mega with either 3*C cells or the little 4.5V sealed lead Acid battery on the belt

[Mark F]

What I was taking the 3 volts from was the specs for the actual led - not the driver board. I assume the driver board must have a buck boost circuit on it that will control the voltage and convert extra voltage to extra amps.