Lighting the way: Simple 3 step flashlight modification that extends flashlight battery over 5x

By on February 21, 2013
Long Run Flashlight Mod

In any emergency you need to have a source of light. Unfortunately most people who buy flashlights will leave them in a drawer somewhere for months or years and when the time comes to use it, the batteries are dead.

This is a huge problem!

Fortunately, one member of our community has made a fantastic instructable that shows you how to get hundreds  more hours out of  any long run time flashlight with a simple 3 step modification.

Check it out below:
Intro: Easier Flashlight Mod Increases Run Time 3.6X

We are going to take a long run time flashlight (65 hours) and turn it into a longer run time flashlight (Update: The title of this Instructable says 3.6x run time but actual run time ended up being over 360 hours).

Yes it is another long run time flashlight hack. This one is easier than my other one. All you need a resistor, a soldering iron and this flashlight.

This Eveready flashlight (model number 5109LSH7 or 5109LS) costs $4.97 at Lowes:

(This is a great long run time flashlight to have even if you don’t do the modification.)

Buy it HERE.

I just picked up another one at my local Lowes. I like this flashlight because as is it has a long run time before the battery needs to be replaced (65 hours). It is bright.

It is cheap and it is easy to hack. You can do this hack in 10 or 15 minutes and most of that time will be waiting for your soldering iron to heat up.

The only thing about this flashlight that is less than optimum is that the 3 LEDS have a little too much current going through them. The LEDS appear to be “straw hat” type (20 milliamp) LEDS. Each LED has about 60 milliamps running through it.

Here is a data sheet for a straw hat LED.

The LEDS in this flashlight may be specially made to handle more current but in case they are not, this modification will bring the current down to a level that will insure tens of thousands of hours of LED life.

I chose to add a 56 ohm resistor to the circuit so each LED runs at about 16.5 milliamps. This will increase the run time by 3.6 times.

The other step by step instructable to make a long run time flashlight is HERE.

Either one will be good to have in an extended power outage.

Step 1: Un-solder

You will need to unscrew the black ring that hold the clear plastic cover on the front of the flashlight. Then remove the assembly shown in the picture above. Install the battery and re-assemble the flashlight. Test the flashlight to make sure it works. Remove the rind and assembly shown in the picture above. Un-solder one of the two red wires from it’s metal contact. I chose the one on the left.

Step 2: Solder The Resistor

Now lets select a resistor. I chose a 56 ohm resistor which reduced the current in the circuit going to the 3 LEDS to 50 milliamps (it was 180 milliamps before the modification). Here are some other resistor values and the currents I measured:
44 ohms (two 22 ohm in series) = 56 milliamps

You may want to try out some other resistors for longer run time or higher brightness. I was going for 200+ hours of run time which is about a month of use if used 6 hours a day.

100 ohms = 31 milliamps

Solder one end of the resistor to the metal contact and the other end of the resistor to the wire that was originally attached to the metal contact.

Step 3: Testing
The first picture shows the light from an unmodified flashlight on the left and a modified flashlight on the right.

Note that the light on the right is produced using 3.6 times less power or about 28% of the power to produce the light on the left. The picture of the warning sign was taken using only the light from the modified flashlight at a distance of 18 feet.

Long run time flashlights make great gifts. Make several for the people on your gift list. They are also great for emergencies. This one is good for a 3.6 x bigger emergency

Click here to view the original instructable.

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'Above Average' Joe

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  1. Sandy the Swede

    Would you have a recommendation for this type of mod to a lantern?

    • Lux

      Hello Sandy,

      I unfortunately do not have a recommendation for a mod to a lantern. For this instructable and the other one I did (500 hour run time) I went with 3 criteria: Inexpensive, readily available and modify-able. This one really fit the bill being available at Lowes and Target, it was also easier to hack than the first one I did and is still under 6 dollars.

      The next one (in the works) is from WalMart , is just as easy to mod and is still under $6 and last of all, much brighter (150 hours +).

      If you or anyone else has a lantern in mind I would love to look into it. I would hope that it would be Under $10 and run on C cells, D cells or lantern batteries. I looked a while back but I didn’t find one that I was personally interested in.

      If you are looking to light up a room and do away with the spot, you may want to try this:

      1. Tape a white card in the front of the flashlight angles at about 45 degrees. The reflected off the card goes off in all directions.

      2. Tape a translucent disposable drinking cup to the end of the flash light to diffuse the light.

      3. I just point mine at the ceiling.


  2. Lux

    oops. Typos:

    1. Tape a white card on the front of the flashlight angled at about 45 degrees. The light reflected off the card goes off in all directions.

  3. Lux

    Here is the latest flashlight mod which is significantly brighter and still with a 150 hour run time and under $6:

    • MVS

      This is awesome. With the derecho that hit us, we were without power for five days, and longevity trumped brightness.
      How ’bout a spdt switch that allows high and low settings, not too much cost or difficulty. I like switches, choices…

      • Lux

        Hello MVS,

        Interesting that you mentioned adding a switch. The flashlight in this instructable did not have a lot of room for a switch. The 10 led flashlight has plenty of room for the switch and in the next week or two I hope to add that as an optional step. It will be the best of both worlds.

        That addition to the existing modification would be easy. The switch is wired across the resistor.

  4. I use a small led light with a head strap attached to a gallon water jug sometimes.point the light at it and strap it on. Works really well at diffusing the light like a lantern.

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