Lighting the Way: Increase Your Flashlight’s Run Time

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How You Can Increase Your Flashlight's Run Time?

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 the actual run time ended up being over 360 hours).

Yes, it is another long-run flashlight hack. This one is easier than my other one. All you need is 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.

If you go to the link above you can read my review of this product. Mine is the third review.

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 milliamps) 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 ensure 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 instructions to make a long run-time flashlight are 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 holds the clear plastic cover on the front of the flashlight. Then remove the assembly shown in the picture above. Install the battery and reassemble the flashlight.

Step 1: Un-solder | Lighting the Way: Increase Your Flashlight's Run Time

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 its 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

Step 2: Solder The Resistor | Lighting the Way: Increase Your Flashlight's Run Time
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 (camera on zoom).

Step 3: Testing | Lighting the Way: Increase Your Flashlight's Run Time

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.

Update 11-10-12

I started a test to see how long the batteries would last running 24 hours a day. The test started on the morning of 10-26-12. I was expecting a 200-hour run time. After 15 days (360 hours). The flashlight still puts out a usable amount of light.

I can still read with it without any problem. I also got the opinion of a fellow flashlight enthusiast who thinks the flashlight is still useful for its intended purpose which is for the user to be able to easily see their way around the house in the dark and be able to easily read.

The flashlight is noticeably dimmer than it was at the beginning of the test so I would conclude that this is a 360-hour flashlight. It has exceeded my expectations because the run time has been increased 5.5x.

If I was using this flashlight 6 hours a day the test would have taken 60 days to get to this point. If this had been a real emergency I would gladly use this flashlight several more days.

UPDATE: 3-31-13

If you want a long-run-time flashlight that is a little brighter, try this one.

It’s another easy 10-minute modification.

Step 4:


Eveready recently re-designed the 5109LS flashlight. The original flashlight had three round low-power LEDs. The new version has one square high-power LED.

There are no exposed wires so one of the metal strips that connects the switch needs to be cut. The resistor of your choice is soldered across the cut. The current of the unmodified flashlight was in excess of 160 milliamps. I tested 3 resistors:

50 ohms = 50 milliamps

68 ohms = 44 milliamps

100 ohms = 32 milliamps

I like this version better than the original. It is an easier modification.


To make it easy to find your modified flashlight in the dark you can add a second resistor. This resistor is electrically across the switch so that when the switch is open it is really a little bit closed. Pictured is a 100,000 (100K) ohm resistor.

The current in the circuit with the flashlight “off” is 0.04 milliamps or 4 hundredths of a milliamp. This produces enough light to put a dim but visible spot on the ceiling in total darkness.

This will run the battery down eventually but at a rate of less than 1000th the current when on (used with a 68 ohm resistor), The locator light should run for several years. I also tried a 200,000 ohm resistor which uses half the power.

Courtesy of

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Editor’s Note: This post was originally published on January 12, 2016, and has been updated for quality and relevancy.


7 Responses to :
Lighting the Way: Increase Your Flashlight’s Run Time

  1. Sandy the Swede says:

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

    1. Lux says:

      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 says:

    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 says:

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

    1. MVS says:

      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…

      1. Lux says:

        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.

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