How to Calculate Flashlight Battery Life

How to Calculate Flashlight Battery Life


Determining potential battery life of a flashlight can be done one of two ways – you can either turn the light on and stand around until it turns off, or you can calculate it based on the power draw of the light and the capacity of the battery. From personal experience I can tell you that the latter is the easier, quicker and less boring.

Math is hard, or at least for some of us it is. Finding accurate information online is even more difficult. I found both of these statements to be true while researching ways to estimate the battery life for my reviews. After reading about 20 different articles across the web and, unfortunately, running a flashlight for 4 hours until the battery ran out, I think I have got the math down for calculating the battery life pretty accurately.

What Info You Will Need:

Flashlight Amperage – Amperage is simply defined as the strength of an electrical current and is measured in “amps”. Amperage is measured with a multimeter by connecting the probes within the circuit. This is done by setting the multimeter to read amperage then placing the black probe on the negative pole of the battery and the red pole closes the circuit (with most flashlights this is done by touching the probe to the inside/outside of the flashlight casing). Under power, the multilmeter will read the current amperage draw of the flashlight.
Battery Capacity – Capacity is a measure of stored power over time, usually documented as amp-hours (mAh or Ah). Depending on the battery type (Alkaline, lead Acid, NiMH, etc), the amp/hour capacity may change and batteries of the same size and type may have different capacities. Most rechargeable batteries have the amp/hour rating listed on the battery itself while other’s require some digging to locate the capacity (locating the capacity of most household AA brands is a bit tough).
Rate of Discharge – (This only applies to Alkaline batteries) Amp/hour ratings on a battery are based on the number of hours the battery is run. For instance, a battery rated at 20 aH over a 20 hour period, would have been tested with a 1 amp for 20 hours (1 amp x 20 hours = 20 amp/hours). Peukert’s Law states that this math is not constant but actually changes based on the rate of discharge. Using this same example, if you used a 5 amp light you would expect 4 hours of battery life, however this is not accurate as the rate (5 amp vs 1 amp) is higher. In fact, you should expect closer to 2.5 hours of battery life. Another problem with determining the appropriate Peukert constant is that it depends on the battery. For the purpose of my calculations, I used data from to determine an average constant for the math below. Based on the data, I use .6 as my rate of discharge adjustment which is an average of the rates for the most common amperage draws (.1, .5 and 1 amps). This means that this formula is most accurate when your amperage draw is around 500 mAh.
A Calculator – When the math is simple (like 1 amp/hr in 3000mAh battery) then this is not necessarily needed, but when it’s more complex a calculator is definitely a good idea.

Flashlight Battery Life Formula

Alkaline Batteries:

Alkaline Battery Life Formula

Rechargeable (NiMH):

NiMH Battery Life Formula

How to Calculate Estimated Battery Life

The following steps demonstrate how you can estimate your flashlight battery life. Follow the images or follow the instructions, the choice is yours.

Flashlight Battery Life

1. Prepare the flashlight for testing. Remove the tail cap to expose the negative pole of the battery. The battery must be remain in the flashlight at the light needs to turn on in order to test the amperage that it draws.
2. Touch the black probe to the negative pole of the battery and the red probe to the flashlight frame (plastic flashlights will have a metal flat wire inside the frame which you can connect to).
3. Connect the red probe to amperage reading, and black in the ground. Turn the dial to read Amperage. Once all connected, the amperage will show on the display. Let the reading settle and write down the amperage draw of the flashlight.
4. Find the amp hour (mAh) rating of your battery.
5. Do the math to using the formulas above to calculate your estimated battery life.

Other Resources

Powerstream Technologies – How to calculate battery run-time

Ncalculators – mAh Battery Life Calculation

Stackexchange – How to calculate battery life

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