Fun With Batteries

OK – from the outset, can I just point out that I seriously don’t recommend that you attempt to replicate the “experiments” I’m about to describe. I should also point out that while I didn’t plan on doing either of these things, it doesn’t take a genius to work out that it’s possible.
Experiment 1 – Electrocuting yourself with a disposable camera
I love taking apart disposable cameras. Last weekend I was finishing off a camera that I’d kicking around as we sat down for a meal. It was one of those models with a flash, and I’d been trying to get the thing to charge for ages by holding down the “charge” button. But the light was resolutely refusing to come on. After about a minute of this, I suddenly realised that I was looking in the wrong place, and the camera was charged. Photo was duly taken and I finished the roll.
If you haven’t been quite as inquisitive as me, then you may not have taken a disposable camera apart before. As an aside, you should note that I was the kind of boy who took apart his first calculator in around 1982 when they were quite expensive and had those glowing red numbers. It made a lovely “Book” for my primary school fancy dress party, where I came as Arthur Dent dressed in a dressing gown from Hitchhikers.
Anyway, once you’ve removed the cardboard outer, you have to prise open the plastic case. Inside you’ll find a AA battery which makes it worth opening alone. The film is quite safe, since the act of winding the film on, returns the unravelled roll back into a standard 35mm film which you can remove for processing. The rest of the camera is made up of various plastic bits, and a circuitboard. I was playing with said circuitry when I manageed to electrocute myself. Nothing too powerful, but something similar to touching an electric fence on a farm.
Once we’d finished electrocuting ourselves a couple more times, we’d worked out the contacts that shorted the circuit. We insulated ourselves with a handy paper napkin and crossed the contacts with a 2p coin. This resulted in a massive flash and close examination of the coin revealed small burn marks that were immovable.
The reason? Well getting the flash to work, means that you have to charge a fairly decent sized capacitor, and the action of me holding down the charge button for such a long time left it pretty full. We managed a couple more sparks before it flattened.
It’s worth noting that the strong charge was generated by a 1.5V battery, but unfortunately I didn’t examine the capacitor closely. But the charge (Q) is directly related to the battery’s voltage (V) and the capacitor’s capacitance (C): Q = C x V.
For the record we used an Agfa camera, but I really don’t recommend repeating this experiment.
Experiment 2 – Electrocuting yourself with a AA battery
For reasons that aren’t important here, I’ve spent the last few days wandering around with a pair of fully re-charged AA batteries in my trouser pockets.
Earlier today, I sat down after a meeting and suddenly felt a massive burning sensation in my leg. I put my hand in my pocket and quickly pulled out the full contents of batteries, mobile, keys and loose change. One of the batteries was really hot, as were my keys and several coins. I’d completely shorted one of the batteries with my keys.
What I was staggered by was the amount of heat the battery was able to create. The battery is a Ni-MH Uniross 1300mAh. In small lettering on the battery, it warns you not to short circuit the battery. Now we know why.