1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Energy from bike riding used to charge phones/laptops

  1. Feb 15, 2015 #1
    I've been thinking up a project on harnessing the kinetic energy to use for charging or powering small deviecs around the house firstly my laptop and smart phone. I have a basic idea of what I'm going to use and have some of the things needed.
    My idea was to use a small brushless or maybe bushed alternator to charge a lipo battery then to power or power the charger of my phone or laptop. I have two 5000mah 7.4v lipo batteries to store the power then to convert or direct use (the laptop uses a 7.4 li-ion battery) but need to investigate the alternator and something to control the charge going to the batteries from the alternator. I'm aware of the process of charging a lipo battery but am unsure if such device designed to use with an alternator is available (like an alternator for a motorbike using a lipo in place of a lead acid battery).
    Thanks for any feedback,
  2. jcsd
  3. Feb 15, 2015 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF.
    There are lots of online examples of using bike power for all kinds of things, and the manufacturer usually publishes the specs gor thrir batteries. Where have you looked?
  4. Feb 15, 2015 #3
    Thank you,
    there is though majority of them consist of people setting up the bike stationary to power things straight off the alternator which is very different to what I want to do.
    I guess what I'm asking is if there is a way to charge a 5000mah 7.4 lipo (not my phone or my laptop battery) through a smallish alternator (brushless perhaps) on a moving bike to use the power from the onboard batteries afterwards?
    I understand the charging process of lipo's and their sensitivity btw.
    The use of a charger on the bike would work though a consistent flow of 12V to the charger seems a little tricky from an alternator on a bike. How would I do this?
    Thanks for your reply,
    Last edited: Feb 15, 2015
  5. Feb 25, 2015 #4
    What you are trying to do comes under the general idea of regenerative braking. A lot of semiconductor manufacturers have solutions for this.
    You basically need to load the alternator in a controlled manner to ensure you don't spend too much effort on cycling. You can also have a control where you use the braking from your alternator when you are going downhill.
  6. Feb 25, 2015 #5


    User Avatar
    Science Advisor

    You should spend some time asking yourself how hard you are planning on working and how quickly you are expecting to charge the batteries. To charge 2 7.4V 5000 mah batteries requires about 266,000 Joules. So if you generated 100 Watts (pretty good power output, and remember the generator and charger are less than 100% efficient), it will take about 45 minutes to charge the batteries. How much power are you expecting to generate on your bicycle? Is it a stationary bike, where you are just charging the batteries, or are you expecting it to charge up while you ride? Are you expecting to be working hard, with lungs pumping and sweat rolling down your face (this might be 250 Watts), or are you talking a leisurely ride (maybe 25-50 Watts)? How long are you expecting the charge to take? All of these things will determine whether your project will give the results you expect.
  7. Feb 26, 2015 #6
    I usually ride for about an hour or so and its not really an easy ride. Just though it would be cool to use some of this power for something other than distance on a bike.
    It sounds from your rough guide that it could be feasible .

  8. Feb 26, 2015 #7
    Thank you for that link. It looks a basically what i need. Maybe running a charger of the power coming from this could be the solution.
    or maybe a set of nicads or nimh to keep to the power consitant to power a charger to charge the lipos properly.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook