#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
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.
Now using the formula Watts = Volts X Amps have the students solve for
Amps. Which would be Amps = Watts / Volts
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
Assuming you have a
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.
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
. Typically an inverter is about 85% to 95%
efficient. So worst case you will loose 15% of your power across the
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
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
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
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%
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
READ & UNDERSTAND THE SAFETY
INSTRUCTIONS FOR YOUR BATTERY POWERPACK.
Now get some good popcorn &
pizza and sit down to watch the movie while it is plugged into the
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.
TO CALCULATE DEPTH OF DISCHARGE:
Do this by shutting down the TV and DVD and un plug the power cords from the
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
There are three possible outcomes to this power energy
classroom student-run lab experiment:
The calculations made by the students where
accurate in predicting the amount of time it would take for the lead acid
Powerpack to reach a
50% depth of discharge. In this scenario students should celebrate!
It took longer for the
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.
The calculations fell
short and the battery
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
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.
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.