#6 of 6 TITLE: Calculate Impact of Adding A Solar Panel To A Pedal Power Charging System


SUMMARY:  Students will build upon concepts learned in lab 4 by predicting effects of adding a solar panel to a 12V DC Powerpack  inverter whilst it is delivering power to a DVD movie.

2009 Pedal Power Generator LLC  - All rights reserved




FIGURE OUT MAX CURRENT COMING FROM YOUR SOLAR PANEL:  Buy a 12 Volt DC Solar Panel that comes with a cigarette lighter plug that will plug into your Powerpack. If your 12 Volt DC Solar Panel has a Wattage rating of 30 Watts, then you would use the power equation

Amps = Watts / Volts  

to solve for the maximum amount of current you will expect to see coming from your solar panel.   For a 30 Watt 12V solar panel, you would get Amps = 30W / 12V = 2.5 A


  Read & understand the manufacture's instructions for the solar panel regarding operating, installing, and safety.  
  Setup the the solar panel outside in full day light adjusted so that it is facing the sun.  Set the Powerpack next to it.  If the power pack is too far away to reach the DVD and TV then run a 25' to 100' extension cord from the battery Powerpack to the classroom.    
3 STUDENTS CALCULATE NET CURRENT DRAW FROM BATTERY:  Explain to the students that they must now subtract the Max Solar panel current from the calculated current delivered by the 12V lead Acid battery power pack from lab 1 to get the new current delivered by the 12Volt battery pack. 

Using the example parameters explained lab 1 we assumed that DVD and TV will be drawing about 6.7 Amps of current from the 12 Powerpack internal battery.   Now assuming that you are using a 30W 12V  Solar Panel that can deliver approx. 2.5A into the cigarette lighter plug of the powerpack,  the net current delivered by the battery will now be :

6.7 Amps - 2.5A = 4.2 Amps

Now the students can recalculate battery duration to 50% depth of discharge looking at the discharge characteristics graph below (Explained in lab 1).  Using the example of needing 4.2Amps, we can assume that this falls just to the left of the 3.6 mark shown below on the curve.  That would correspond to a 100% discharge time of approximately 3 hours.  Since we have agreed to only go to 50% depth of discharge (Explained in lab 1), the final approximated discharge time is about 1.5 hours.




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



Stop the movie periodically to plot depth of discharge versus Time on the white board. Assuming your students have calculated it will take 1.5 hours to get to 50% depth of discharge, then stop the movie at 30 minute intervals.

x 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 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 shown above.


  STUDENTS CALCULATE HOW MANY WATTS ARE NEEDED TO POWER THE TV AND DVD MOVIE PLAYER WITH 100% 12V SOLAR ENERGY:   Total power required to run the DVD and TV was calculated in the first step of lab 1.  So if 70 Watts is needed to run the TV and DVD player you solve for required solar panel as follows:

70W x 1.15 (inverter efficiency factor) =  80.5 Watts total solar power

So the students would have to hook up 3 30W Solar Panel  to produce enough power to run a TV & DVD movie player that runs off of 70 Watts of power.


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 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



In order to show the students actual power delivered by the solar panel, you can do some additional wiring and use the RS232 Serial Com Port Pedal Power DC power monitor Watt Meter available here: This will connect with a computer to show students how many Volts, Amps, Watts, and Watt-Hours are being generated.



2009 Pedal Power Generator LLC  - All rights reserved