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I Help with Experiment on Batteries

  1. Jun 14, 2016 #1
    Hi guys!! I am currently having Physics Internal Assessment in my school and I want to do an experiment to calculate the different times it takes for a 9V cell to to discharge (reach a 0.1V) in different temperatures. The problem I am having right now is to efficiently discharge my cell. How can I do this so that I can collect reliable data and it does not take very long to do this?

    Appreciate the help :)

  2. jcsd
  3. Jun 14, 2016 #2


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    Staff: Mentor

    What are your ideas on how to discharge the batteries?
  4. Jun 14, 2016 #3
    Currently, I am testing two circuits to see which one is faster.
    One is where I have a 9V cell connected to a 1.2 Kilo Ohm resistor and a Voltmeter in Parallel. The discharge in this one is pretty slow I must say.

    The other one is a circuit with a 9V cell where current is passing through a tiny copper wire and back to the cell with a voltmeter parallel.
  5. Jun 14, 2016 #4


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    Do not do this! Without a resistor the current is limited solely by the internal resistance of the battery and can be much higher than safe levels. You are risking your battery overheating and exploding or catching fire, both of which can seriously injure someone or burn down a house despite the small size of a 9V battery. Never short circuit a battery!
  6. Jun 14, 2016 #5


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    Working into a zero resistance load (short piece of wire) is just not a good way of assessing the battery's ability to supply energy (which is what we use them for). No energy at all is delivered into an ideal short circuit because the Voltage across it is Zero. Better to choose a low value of resistor that would represent the sort of resistance that a typical 'heavy load' for the battery could be. For a PP3 style battery, that would be no more than 1A and probably more like 100mA. Work out those resistance values for yourself! :wink: Your 1.2kΩ is fine as a low load but you could go for an even higher resistance (say 12kΩ) to represent the sort of load that represents a lot of battery operated devices.
    This is an interesting project and can tell you a lot about how suitable different batteries are for different applications.
    Incidentally, if you want to measure the lifetime of your charged batteries and you don't want to sit next to the battery all day, waiting for it to fail, it is not too hard to use a simple analogue, battery clock to monitor the battery automatically. A clock takes a minuscule amount of current and you can give it the right supply volts by splitting your load resistance into two resistors in series in the ratio 5:1 (approx). This could be 200 and 1k, for your arrangement. Connecting the clock across the lower resistance will give it 1.5V from a fully charged battery. The clock will stop when the battery fails and (like Sherlock Holmes) you can tell when it stopped, even when you are home and tucked up in bed. Look at the clock twice a day and you can identify which half day it stopped. Modern batteries tend to maintain their output volts until right at the end so your clock should keep going over the battery's useful life.
  7. Jun 14, 2016 #6
    Your expectation for the battery to reach 0.1 V is not justified in general.
    For many batteries the voltage drops just a little when they are "dead".
    Are talking about rechargeable batteries, alkaline batteries or some other type?
    For example, some "dead" Duracell batteries (1.5 V) that I have on my desk right now show 1.2 V.
  8. Jun 14, 2016 #7


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    Presumably, however, the indicated voltage would drop to near zero if there were a modest load in parallel with the voltmeter?
  9. Jun 14, 2016 #8
    No doubt. If you short it, the voltages drops. True for a brand new battery as well.
    What is your point?
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