FitBit Experiment: Measuring Battery Life from Low-To-Empty

Yesterday, I performed a little experiment with my FitBit Flex. When I arrived at the office, I received an email alert that my FitBit Flex battery was low. My charger was at home, and I certainly didn’t want my Flex to miss counting any of my steps, but I decided that this was an opportunity for an interesting trial. I would see how many steps (and how many hours) the Flex would last before giving up the rest of its stored power and shutting down. I received my email “low battery” alert at 7:59 am and had just 1,275 steps so far that day.

FitBit Battery 1

I decided that I would not alter my routine at all, but go through my normal process, getting my daily walks and periodically checking to see how the battery was doing. And that is exactly what I did.

Just before 3 pm, 7 hours after the low battery notice, the FitBit app started showing my battery as “empty.” At this point, I had just about 9,000 steps total.

FitBit Battery 2

I resigned myself to eventually losing some steps, but figured it was worth the sacrifice in the name of science in order to find out just how long the battery lasted after the initial “low battery” email notification. So I continued with my day.

By the time I finally went to bed last night, my Flex had not yet died. The battery still showed up as “empty” despite the fact that I had put a total of 17,450 steps in.

FitBit Battery 3

That was enough for me. Rather than put it into “sleep” mode only to have the device quit on my sometime in the night, I decided to charge it overnight. I had enough data to answer the important question.

The results of my little experiment can be summed up as follows:

  • Time from low-battery message to “empty” battery indicator: <= 7 hours
  • Steps from low-battery message to “empty” battery indicator: ~7,200
  • Time from initial “empty” battery indicator to when I decided to charge: ~7 hours
  • Steps from initial “empty” battery indicator to when I decided to charge: ~8,000

And here is the answer to the question that I was really trying to get in the first place: If I receive a low battery message, how long can I use my Flex before it will lose power?

The answer, based on yesterday’s experiment, is

  • At least 14 hours
  • At least 15,500 steps

Seems to me that is useful information. Next time I get one of those alerts, I know I don’t have to rush back home to grab my charger. I can go virtually all day and the Flex will continue to work, despite showing the “empty” battery level on the iPhone app.

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  1. Thank you for this information. As someone who goes into a panic anytime I receive the low battery alert, this is a huge help!

  2. This reminds me of the low fuel warning light on my car. It says low fuel, the needle is pointing at E, yet I still have gone an additional 50 miles. I’m too chicken to drive it until it dies.

    Good info on the fitbit, thank you!

  3. I have the Fitbit One and I was traveling to Atlanta when I got my low battery message and I realized I didn’t have my charging dingle with me. As it turns out, it kept on tracking steps for four full days which gave me enought time to actually get home. I had logged quite a few miles on that trip, but fortunately, I didn’t lose anything.

  4. In the name of science, and in the name of a Fitbit One user, hoping that the conclusion might be close enough, thanks for sharing!

  5. Oddly enough my Flex also ran out of battery today but I had a different experience than you did. I got the notification e-mail at 2:32PM and by 7:35 it was completely dead. I’ve only owned it for a month and this is the first time the battery has drained completely so it may not be conditioned yet.


  6. Maybe a stupid question: I understand that the FitBit counts you steps. But is the device able to distinguish between distances you walk (steps), you bike (limited speed) and you ride (higher speed), as the app “Moves” does?

    Or is FitBit only based on vibrations generated by your steps?…

    1. Michel, I think FitBit uses an accelerometer, but it is fine-tuned for walking. It can distinguish between sedentary, lightly active, active, and very active, but I’ve never tried it with a bike. It is definitely *not* GPS-based.


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