Bike won't start...

[lewellen]

I worked for 35 years with low voltage, high amperage DC voltage. I can show you a AAA battery cell, the size of a short pencil, that puts out 1.5V and I can show you a cell the size of a 55 gal fish tank that puts out 1.8V. I saw someone drop a pair of pliers across one of those cells once and after the flash and loud bang all that was left was some metal splatters. I don't think that would happen if you shorted out a flashlight battery. But I could be wrong.

It would probably get hot enough you wouldn't want to hold it, and it might rupture from boiling / vaporization of some of the chemicals inside, but not as dramatic as vaporizing a wrench, probably.

Batteries have both a capacity for energy storage, and an internal resistance. That's the resistance the battery itself provides to current flow through it. It's an intrinsic property of the battery, but you can think of it as being in series with the actual load.

Hypothetically, if you have a 12-volt battery with a 12-ohm internal resistance (besides being a really poor battery) the most current the battery could ever source would be 1 amp. Voltage = current * resistance = current * (internal resistance + external load resistance). In this case, even if the external load resistance were zero (i.e. dead short across the terminals), you still have 12V = X amps * 12ohm ... so 12V / 12ohm = 1 amp, maximum.

In the case of the AAA vs. the fishtank, the fishtank probably has a lower internal resistance, and most certainly has higher energy stored. Even a battery with zero internal resistance can't do very much if it doesn't store much energy ... or current * voltage * time.

But ... don't disrespect those little AAA batteries too much. An AAA alkaline cell stores around 1.4 watt-hours ... or roughly 5000 joules.

A typical round from an M-16 rifle has a muzzle energy of around 1800 joules [1]. So, that little AAA battery contains more than two and a half times as much energy as delivered by a shot from an M-16!

Now that I've taken this thread this far off-topic, here's a question:

You have a car battery, some jumper cables, a model train set, and a can of beer. How do you use this to build a railgun? [2]

Good roads,

- Lewellen180

[1] Wikipedia
[2] I asked this to one of our Ph.D. students as part of a qualifying exam question....

[hjsbmw]

[1] Wikipedia
[2] I asked this to one of our Ph.D. students as part of a qualifying exam question....

Where do you teach?

[1] The M16 releases the energy in an instant, i.e. a small amount of time, while the battery does so usually over a larger time. The M16 has extremely high power, the battery small power. I think looking at the power brings things more into perspective. P = dE / dt

[2] I would assemble a segment of straight rails, use the jumper cable to hook up the battery's + to the right rail, drink the beer to make the can lighter, put the empty can across the rails, then connect - to the left rail. The can should feel a forward Lorentz force F = I x B, also Wikipedia.

[lewellen]

Where do you teach?[/quote]

Naval Postgraduate School, in Monterey.

I find I can get through to the students a little better if I use examples like the M16 comparison...

[1] The M16 releases the energy in an instant, i.e. a small amount of time, while the battery does so usually over a larger time. The M16 has extremely high power, the battery small power. I think looking at the power brings things more into perspective. P = dE / dt

True. What matters is both how much energy you can store and how fast you can release it.

If you want to keep going, you can compare the energy stored in a AAA battery with, say, the same volume of gasoline .... and just how fast you can get that mix to release with an appropriate dispersal and fuel/air ratio. :-)

[2] I would assemble a segment of straight rails, use the jumper cable to hook up the battery's + to the right rail, drink the beer to make the can lighter, put the empty can across the rails, then connect - to the left rail. The can should feel a forward Lorentz force F = I x B, also Wikipedia.

Exactly. Put the can down on the end where you connect the leads; it will work either way but this way you get a longer interaction.

There will be a spark; you might also want to put a rail on top to keep the can from hopping off the drive rails.

If you leave the can full, you will get more momentum transfer (good) but it's then a waste of beer (bad).

Best,

- Lewellen

[ka5ysy]

I am pleased to announce another Instant Battery Death. My RR's second Exide (warranty replacement for first one!) died today, happily in my driveway right on time at precisely 24 months almost to the day. It did actually announce an impending death by a couple of no-starts then allowing the starter to pull in when I pushed the starter today. I ran directly back to the house and shut down in the driveway. There are no more starts in the battery. Happily, I will have a new Odyssey PC535 Battery here for when I return from Durham on Thursday afternoon.