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#5 of 6 TITLE:  Calculate How Much Bicycle Generator Pedal Power is Needed To Power A  Fun Movie 

SUMMARY:  Students learn to use simple alternative solar pedal power energy calculations  to predict how long a DVD movie will play when powered by a Bicycle Generator Pedal Powered lead acid deep cycle battery Powerpack

2009 Pedal Power Generator LLC  - All rights reserved




Buy a $15 power meter like this one

Wall socket AC Watt Meter for wall plugs measureing Watt Hours

Have the students plug a DVD player and a TV into this power meter through a power strip so they can measure how many Watts it takes to run these items.  For example if your television requires 60 Watts of power and your DVD player is running at 10 Watts, then your total power from the power meter should be 70 Watts.


3 Now using the formula Watts = Volts X Amps  have the students solve for Amps.  Which would be Amps = Watts / Volts


4 Now apply the formula for Amps to this situation so that students can determine how many amps will be coming from the sealed lead acid battery or Powerpack.  Assuming you have a  Powerpack that runs off of a 12V DC battery, then  divide your measured Watts by 12.   So in this example if your TV and DVD require 70 Watts of power to run, then your lead acid battery powerpack would be putting out about  70 Watts / 12 Volts = 5.83 Amps of current.


5 Now you have to make an adjustment to your Amperage calculations taking into account the energy loss from converting the 12Volts into 110 Volts AC through the AC  Inverter .  Typically an inverter is about 85% to 95% efficient.  So worst case you will loose 15% of your power across the Inverter.  Now taking  the worst case of 15% energy loss multiply your existing current calculation by 1.15 and you get 6.7 Amps of required current consumption. 



FIGURING OUT HOW LONG THE LEAD ACID BATTERY or POWERPACK WILL LAST:  Now go to the power sonic web site to find the discharge rate of your deep cycle  sealed lead acid battery  or Powerpack.    If you click on the datasheet for the their SLA 18 AH battery you will see this graph displaying the rate of discharge characteristics.   In this case you will see that if the lead acid battery or powerpack is delivering 6.7 Amps of current it will be completely DEAD after about two hours.   (They technical way to say "DEAD"  is that its "State of Charge is 0%). 




DEPTH OF DISCHARGE LIFE CYCLE CONSIDERATIONS: Now explain to the students that your lead acid battery in the Powerpack will last about 3 times longer if you only discharge it half way instead of all the way.  The other way to say this is that if you want your battery to power your TV and DVD player 600 times through a movie instead of 200 times (See graph below)  then the most your should discharge your Powerpack down to is 50%

 AGM deep cycle battery life time calculations characteristics for 18 AH Battery


8 ADJUST THE CALCULATED AMOUNT OF TIME THE BATTERY WILL LAST: Now that you have decided to only discharge your Powerpack to a 50% state of charge to make  it last 3 times longer, then you would take the duration you figured out in the step above and multiply it by 0.5 and get approx ~1hour of battery life while watching a movie with your TV and DVD player plugged into Powerpack that has an 18 AH battery. 




Now get some good popcorn & pizza and sit down to watch the movie while it is plugged into the Powerpack



GRAPH SOC ON THE WHITE BOARD:  Stop the movie periodically to plot depth of discharge  versus Time on the white board.    Assuming your students have calculated it will take 1 hour to get to 50% depth of discharge, then stop the movie at 15 minute intervals.  


HOW TO CALCULATE DEPTH OF DISCHARGE:   Do this by shutting down the TV and DVD and un plug the power cords from the Powerpack.   This is done on the Duracell powerpack by first waiting 3 to 5  minutes for the battery to settle.  Then push the red button labeled "display function" and reading the charge status that pops up telling you a percentage between 0% and 100% which is what you call the S.O.C.  (State of charge).   The depth of discharge is calculated by subtracting the SOC from 100.  So if you read a state of charge of 75%, then your depth of discharge is 25%.     


Have the students compare their graph data to that shown on the discharge characteristics chart shown above.



There are three possible outcomes to this power energy classroom student-run lab experiment:

  1. The calculations made by the students where accurate in predicting the amount of time it would take for the lead acid battery Powerpack to reach a 50% depth of discharge.  In this scenario students should celebrate!
  2. It took longer for the battery Powerpack to get to a 50% depth of discharge than the students predicted.  In this case you could explain that maybe the AC inverter used inside of the Powerpack is more efficient than first thought.  Or that variability in the sealed lead acid battery manufacturing process may have caused the 18 Amp Hour battery to have a little more capacity than the manufacture's specifications.   Like 20 Amp Hours instead of 18 Amp Hours.
  3. The calculations fell short and the battery Powerpack reached it's 50% state of charge much earlier then predicted.  If this happens then bring up the fact that the sealed lead acid battery inside of the Powerpack has possibly seen too many charge / discharge cycles which corresponds to the life cycle characteristics chart above.  Or show them the Sealed Lead Acid Battery Shelf Life & Storage chart below and explain that maybe the Powerpack sat on the shelf too long without float charged or cycled.    long.

Shelf life storage chart graph showing deep cycle battery capacity to retain vs months


  The next step is to do the next lab that involves doing more of the same electrical power engineering calculations to implement alternative solar, wind, or pedal power energies while watching powering a TV and DVD player to watch a movie.   



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2009 Pedal Power Generator LLC  - All rights reserved