Tuesday, June 11, 2013

Experiment 2

Again, dear reader, I refer you to the HO:ME blog for the details of experiment 2.  In short (pun not intended) the point of this experiment is to short out a battery.  Feel the heat, baby!

So I shorted out the 1.5 v AA battery.  Nothing to see, really, so I'll post a random picture here to distract you:

Nice, eh?

The next phase was to short out a fuse.  I bought the collection of automotive fuses from RadioShack, as that was the only one that had a 3 amp fuse.  Unfortunately, 1.62 volts/.8 = 2.02 amps, which I would not predict would blow the 3 amp fuse.  So I just turned right to the 9v, presuming that my calculated 12 amps should take care of business.

Hooked it up.  Watched.  Waited.  No fire, no energy, no nothing.  What?

Checked the battery.  Produced less than 7 volts.  Double what?

This was the same battery I had used for the earlier tests, and at one point I had run the battery through a very small resistor and then through the multimeter, showing 2 amps current.  Left it like that for a while.  Didn't think much of it.  Until I picked that stupid battery up and found it was--what else? hot as blazes.  I got lucky, I did.  Hot batteries have been known to cause some problems.

Dreamliner grounded due to battery fire

Turns out that the above lead to some reading on the difference between battery capacity, current, amp-hours (or watt-hours) and why folks use a stack of 1.5 v batteries to power their remote controlled cars instead of a single 9v.

Voltage for a battery is set by the number of cells, etc internal to the battery.  Capacity is how much energy it can store--like the size of the gas tank in the car.  A battery can then produce a certain number of amp-hours, which is number of amps for a certain amount of time.  A 9v rated at 2 amp-hours can produce 2 amps for 1 hour, 0.5 amps for 4 hours, etc.  This is analogous to the range in your car.

However, just like in the car, speed makes a difference.  The faster the car goes, the more inefficient it is (air resistance increases as a square of the velocity).  Same for batteries.  Running batteries at high current depletes it faster than the rated amp-hours would suggest.

A typical 9v is rated at 0.55 amp-hours, or Ah.  A 1.5 volt AA has 2.4 Ah, almost 4x as much.  So a stack of 6 AAs in series that add up to 9v will last much longer, at the cost of increased weight.

My 9v would only be expected to run 2 amps for about 15 minutes.  But that's a high current for a 9V, so probably even less.  Its ability to put out the number of amps required to burn out the fuse was clearly compromised.

So experiment 2 was a bust (or not, depending on how you view the fuse).  But good learning.

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